Vasantha

Breathing And Exchange Of Gases Introduction

If you hold your breath, even for a few seconds, you will gasp for air. Thus, breathing is an indispensable process in our body.

Through breathing, we take in oxygen. It reaches the cells and oxidises organic food substances to release energy.

This is called respiration also known as cellular respiration. The organ system concerned with breathing and respiration is called the respiratory system.

Breathing

Breathing Definition: The process by which air from the environment enters the body and after an exchange of gases in the tissues, goes out of the body is known as breathing.

Breathing is also known as ventilation.

Breathing Stages: There are two stages of breathing—

1. Inspiration— intake of air from the environment into the body;
2. Expiration— release of air from the body to the environment.

Breathing Importance: Breathing helps in the intake of oxygen from the environment into the body (specifically into the lungs of terrestrial animals including humans), through inspiration.

Within the lungs, the gaseous exchange takes place and carbon dioxide is released from the body into the environment through expiration.

Respiratory Organs In Animals

Respiratory organs have evolved in animals according to their physical requirement and adaptation to the environment. According to their requirement, these organs are of various types. The following table shows various types of respiratory organs in different organisms.

Respiratory Organs In Human Beings

The organ system of the human body through which the exchange of gases occurs between the body and the environment is called the respiratory system.

Components of the respiratory system: The organs and structures of the human respiratory system are—

1. Nostrils and nasal passage,
2. Pharynx
3. Larynx,
4. Trachea,
5. Bronchi and bronchioles
6. Lungs

Nostril And Nasal Passage

Nostril And Nasal Passage Definition: The openings by which the nose opens outward are known as nostrils or external nares and two cavities that extend from the external nares to the pharynx at the posterior end is called nasal passage or nasal cavity

Nostril And Nasal Passage Structural Features:

1. Nostrils or external nares are present as two small openings at the base of the nose.
2. The two openings through which nasal passages are connected to the pharynx are called internal nares.
3. Each nostril leads into a nasal passage. The two nasal passages are separated from each other by a cartilaginous partition called nasal septum.
4. Each nasal passage or cavity is divided into three parts— the lower vestibular region, middle respiratory region and upper olfactory region.
5. The dilated chamber inside the nasal cavity is the vestibule. The inner surface of the vestibular region is made up of stratified squamous epithelium.
6. This surface contains sebaceous glands and nose hairs (vibrissae) which serve to filter out inhaled particulates.
7. The next region is the middle region called the respiratory region. The walls of the respiratory region are lined with respiratory mucosa. This respiratory mucosa is made up of pseudostratified ciliated columnar epithelium containing mucous cells.
8. The mucous cells produce mucus, while the serous cells produce a watery fluid containing an anti-bacterial enzyme.
9. The respiratory region is highly vascular and hence appears reddish. This region helps to condition the inhaled air.
10. The upper region of the nasal chamber is called an olfactory region. The wall of this region is lined by olfactory epithelium.
11. The olfactory region appears yellowish-brown in colour. It helps to detect the odour of the inspired air.

Nostril And Nasal Passage Functions:

1. The nostrils and nasal passages provide a pathway for the air to enter the body.
2. The nose and nasal cavity filter and clean any foreign particles present in the inhaled air.
3. These particles get stuck into the mucus, which prevents them from entering the lungs.

Pharynx

Pharynx Definition: The tube-like structure that connects posterior nasal and oral cavities to the larynx and oesophagus is known as the pharynx.

It extends from the base of the skull to the level of the sixth cervical vertebrae.

Pharynx Structural features:

1. The pharynx is the common part of both the digestive and respiratory systems. Its length is approximately 12-14 cm.
2. Structurally, the pharynx can be divided into three parts according to its location. These three parts are nasopharynx, oropharynx, and laryngopharynx.
3. The nasopharynx is located between the internal nares and the soft palate and lies superior to the oral cavity.
4. The oropharynx is a space present between the soft palate and the root of the tongue that extends inferiorly as far as the hyoid bone. The palatine and lingual tonsils are present in this region.
5. The laryngopharynx is the part of the throat that connects to the oesophagus. It is located between the mouth and sixth cervical vertebrae. The superior boundary of this structure is at the level of the hyoid bone.
6. The ventral face of the pharynx possesses an opening known as the glottis. Through this opening pharynx is connected with the larynx.
7. The anterior part of the glottis has a cartilaginous outgrowth, known as epiglottis. When food is swallowed, epiglottis prevents food from entering the larynx by covering the glottis as a lid.
8. The upper part of the pharynx contains a small muscular structure known as the uvula.

Pharynx Functions:

1. The pharynx serves as a pathway for both the digestive system and respiratory system since both food and air passes through it.
2. The surface of the nasopharynx is covered with pseudostratified ciliated epithelium tissue. It filters the inhaled air.

The Larynx

The Larynx Definition: The cartilaginous structure located between the pharynx and the trachea that helps to produce sound is called the larynx.

Larynx Structural features:

1. The larynx is a cartilaginous triangular box whose apex, known as the Adam’s apple in males, is visible at the front of the neck. The larynx is also known as the voice box. The size of the larynx is larger in men at puberty than in females. For an adult male, it is 4.5 cm but for a female, it is 3.5 cm.
2. The laryngeal skeleton is composed of nine cartilages—three unpaired (thyroid, cricoid, and epiglottis) and three paired (arytenoid, corniculate, and cuneiform) that are connected by membranes and ligaments.
3. The thyroid cartilage is larger in size. It has two parts or laminae. A clear groove is visible between the two laminae, known as the thyroid notch.
4. The upper part of the larynx bears a ‘U’ shaped bone known as hyoid bone. This bone remains connected with the larynx by a flat, extended membrane.
5. Ligaments of the larynx are paired. The upper two pairs are known as ventricular folds (also known as false vocal cords) and the lower two pairs are known as vocal folds.
6. The vocal folds are responsible for producing sound. These are known as vocal cords. During expiration, air comes out through the vocal cord creating vibrations which produce sound.
7. Below vocal folds, the epithelium is of pseudostratified ciliated columnar type. Here, the action of the cilia directs mucus movement upward towards the pharynx. This helps in removing mucus from the lungs.

Larynx Functions:

1. The ciliated mucous lining of the larynx has the ability to remove foreign particles and to warm and moisten the inhaled air.
2. The other important function of the larynx is sound generation (phonation).

Trachea

Trachea Definition: The membranous tube-like structure, surrounded by rings of cartilage and extends from the posterior end of the larynx to the bronchial tubes and conveys air to and from the lungs, is known as the trachea or windpipe.

Trachea Structural Features:

1. The trachea is a rigid tube, about 12 cm long and 2.5 cm in diameter.
2. The trachea is encircled by 16-20, C-shaped cartilaginous rings, made up of hyaline cartilage.
3. Posteriorly, it has a flat band of muscle and elastic connective tissue. This is called the posterior tracheal membrane. It closes the C-shaped rings.
4. The cartilaginous rings hold and support the trachea preventing it from collapsing in the absence of air. However, it also provides some flexibility for any neck movement.
5. The tracheal mucosa consists of pseudostratified ciliated columnar epithelium tissue. The mucous layer has goblet cells that secrete mucus. Its submucosa contains cartilage, smooth muscle and seromucous glands.

Trachea Functions: The ciliated epithelium of the trachea washes out the mucus-containing debris towards the pharynx. This prevents their accumulation in the lungs.

On coughing, the wall of the trachea contracts, narrowing its diameter. Therefore, coughing causes air to move out faster through the trachea, thus expelling mucus and foreign objects.

Bronchi

Bronchi Definition: The two channels, formed by the branching of the trachea and leading to the lungs at the level of the 5th thoracic vertebra, are known as bronchi.

Bronchi Structural features:

1. The two primary bronchi arise from bifurcation of the trachea. The right bronchus is wider (2.5 cm), shorter in length and more vertical than the left bronchus. Its length is approximately 5 cm.
2. Each bronchus continues for 2 to 3 cm and enters the hilum (a slit in the lung) of each lung, respectively. Like the trachea, the primary bronchi are also surrounded by C-shaped hyaline cartilages.
3. The main right bronchus (primary bronchus) is further subdivided into three secondary bronchi while the main left bronchus (primary bronchus) divides into two secondary bronchi.
4. Each of the secondary bronchus again divides into branches forming tertiary bronchi and finally smaller terminal bronchi.
5. The divisions of the respiratory passageways are known as the bronchial tree.
6. The bronchial tree has a substantial amount of elastic connective tissue, which is important in expelling air from the lungs. The inner wall of the bronchus possesses ciliated epithelium.

Functions: Bronchi carry the air from the trachea, all the way to the alveolar air sacs.

Tracheobronchial tree

The structure consisting of the trachea, bronchi, and bronchioles that form the airways in the upper part of the lung is called a tracheobronchial tree. It is referred to as a tree because its branching pattern resembles that of a tree.

Bronchioles

Bronchioles Definition: The numerous branches into which a bronchus divides are known as bronchioles.

Structural features:

The bronchioles are of three types, each becoming progressively smaller in size. These are—

1. Lobular
2. Terminal bronchioles,
3. Respiratory bronchioles.
4. The terminal or transitional bronchioles divide into respiratory bronchioles. Apart from air conduction, they are also involved in gaseous exchange between blood and air.
5. These respiratory bronchioles divide further into smaller branches to form alveolar ducts.
6. The terminal end of these ducts forms a sac-like structure known as alveoli (singular: alveolus).
7. Human lungs contain approximately 300-500 million alveoli.
8. Bronchioles are lined with muscular walls.

Bronchi Functions:

1. The bronchioles conduct air from the bronchi to the alveoli. They also regulate the amount of air that passes through the lungs during inspiration and expiration.
2. The lobular and terminal bronchioles are known as dead space—as no gaseous exchange occurs in these places.
3. The respiratory zone begins with the respiratory bronchioles, leading to the alveoli where the exchange of gases takes place.
4. There are very small alveoli present at certain points on the wall of the respiratory bronchiole. These help to carry out gaseous exchange within the respiratory bronchioles.

Bronchi Conducting and Respiratory zones

The airways beyond the larynx can be divided into two zones. The conducting zone extends from the top of the trachea to the beginning of the respiratory bronchioles.

It contains no alveoli and so, no gaseous exchange with blood occurs. The respiratory zone extends downwards starting from the respiratory bronchioles. It contains alveoli and is the region where gas exchange occurs with the blood.

Lungs

Lungs Definition: The paired, cone-shaped spongy organs required for breathing, present on both sides of the human heart in the thoracic cavity are known as lungs.

Lungs Structural features:

1. Lungs occupy a considerable portion of the thoracic cavity. A muscular partition called the diaphragm separates the thoracic cavity from the abdominal cavity.
2. The lungs do not fill the entire rib cage. Most of the space within the rib cage, inferior to the lungs, is occupied by organs like the liver, stomach and spleen.
3. The right lung is divided by an oblique fissure and a horizontal fissure into three lobes—superior, middle and inferior.
4. The left lung is divided by an oblique fissure into two lobes—superior and inferior. Each lobe is subdivided into lobules, and each lobule contains a bronchiole serving many alveoli.
5. The left lung is slightly smaller than the right lung. The anterior part of the left lung has an indentation called the cardiac notch to accommodate the heart.
6. Lungs are enclosed within a thin, transparent, double-layered membrane called pleura. The space between the two layers of pleurae is called the pleural cavity.
7. The pleural cavity is filled with a film of watery fluid, called the pleural fluid.
8. The space between the pleural sacs of the two lungs is the central compartment of the thoracic cavity. This is called the mediastinum.
9. Bronchus, blood vessels, lymphatic vessels, and nerves enter into the lungs through a slit in the mediastinal surface of the lungs, called the hilum.
10. The outer pleural layer (attached to the chest wall) is called the parietal pleura, while the inner layer (attached to the surface of the lung) is called the visceral pleura.

Functions of pleurae and pleural fluid

Lungs Reduce friction: Pleural fluid acts as a lubricant that enables the lungs to expand and contract with minimal friction.

Creation of pressure gradient: Pressure in the pleural cavity is lower than atmospheric pressure. This creates a pressure gradient, which actually assists in the inflation of the lungs.

Compartmentalisation: Pleurae, mediastinum and pericardium compartmentalise the thoracic organiser This compartment formation prevents the spreading of infections to another organiser

The inner wall of the lungs is lined by numerous alveoli. It lacks any cartilage or smooth muscle. The wall of the alveoli and alveolar duct is lined by a single layer of squamous epithelial tissue.

The cells that remain lining the alveoli are called pneumocytes which include four types of cells, such as—

Type 1 alveolar cell: The walls of alveoli are mainly composed of these cells. These are flat and elongated cells with extremely thin cytoplasm and flattened nuclei. They are mainly involved in gaseous exchange. It occupies around 95% of the surface area of the alveolus,

Type 2 alveolar cells: It occupies around 2% of the surface area. These cells are small, cuboidal cells that are usually found scattered in the squamous epithelium of the alveolus.

Type 2 cells secrete a detergent-like lipoprotein called pulmonary surfactant. This surfactant forms a thin film inside the alveoli and bronchioles. It is also responsible for regeneration of the normal alveolar structure subsequent to injury,

Type 3 alveolar cells: They are also known as brush cells, due to their characteristic appearance under the electron microscope. These cells remain scattered throughout the lungs, closely associated with nerves.

Type 3 cells may function as chemoreceptors.

Macrophage cells: These cells kill pathogens that enter the respiratory system.

Function Of Alveolar Or Pulmonary Surfactant

• Pulmonary surfactant is a lipoprotein complex produced by type II alveolar cells. The surfactant reduces the surface tension of the alveoli.
• As a result, alveoli do not collapse due to surface tension, even though, they are very small in size. The alveoli can, thus, always remain open, to make breathing easier.
• Moreover, due to the reduction in surface tension, fluid cannot enter the alveoli from the surrounding blood capillaries. In this way, surfactant prevents pulmonary oedema.
• The most studied surfactant protein is SP-A, which is mostly found in Clara cells (dome-shaped cells with short microvilli, found in bronchioles) and Type II alveolar cells.

Macrophage cells Functions:

• Lungs are the main components of the respiratory system, as they carry out the gaseous exchange (which takes place within the alveoli).
• Lungs may also release certain useful substances into the blood. Some of these substances play local regulatory roles within the lungs.
• Lungs also act as a “sieve” that traps and dissolves small blood clots generated in the systemic circulation. Thus, the lungs prevent them from reaching the systemic arterial blood.

Mechanism Of Breathing And Its Regulation In Humans

The mechanism of breathing involves oxygen-rich air entering the alveoli within the lungs and carbon dioxide from the alveoli being expelled out of the body.

Bones And Muscles Associated With Breathing

During breathing, the bones of the rib cage that play major roles, are—

1. Sternum, present at the front of the rib cage,
2. 12 thoracic vertebrae and
3. 12 pairs of ribs.

The muscles that are necessary to carry out breathing are called respiratory muscles. The main respiratory muscles are—

1. Diaphragm And
2. Intercostal Muscles

Diaphragm

• Thin, dome-shaped muscle, attached to the lower ribs and vertebral column is the diaphragm. When the diaphragm contracts, it moves downward which increases the vertical dimensions of the chest cavity and inspiration occurs.
• The diameter of the thorax also increases as the ribs are lifted outward.
• During expiration, the diaphragm relaxes and moves upward which decreases the volume of the chest cavity. Generally, in normal breathing, the diaphragm moves about 1 cm.
• It may move up to 10 cm during forced expiration. Paralysis of the diaphragm causes paradoxical movement.
• During this movement, the diaphragm moves up rather than down, with inspiration. This occurs when a person sniffs.
• The phrenic nerve is the nerve associated with the diaphragm, that controls its functioning.

Intercostal muscles

Muscles associated with the rib cage are called intercostal muscles. There are 11 pairs of internal intercostal muscles and 11 pairs of external intercostal muscles, located adjacently, at an angle of 90° to each other.

External Intercostal Muscles: Due to the action of these muscles, ribs move upward and forward at the time of contraction of the muscles during inhalation.

Cough and sneeze

These are a type of reflex action controlled by the nervous system. Generally, a sneeze occurs when the inner surface of nasal passages gets stimulated or irritated and a cough occurs when the inner surface of the trachea gets stimulated.

Hiccup

A hiccup is a sudden inspiration due to the contraction of the diaphragm. This phenomenon is observed when the phrenic nerve of the diaphragm becomes stimulated.

During this sudden inspiration, the air strikes the vocal cord, which produces the sound.

Internal Intercostal Muscles: The action of these muscles pulls the ribs downward and inward (antagonistic function to external intercostal muscles) during Exhalation.

Mechanism Of Breathing

There are two stages of breathing—inspiration and expiration. The mechanism of breathing is discussed on the basis of these two stages.

Inspiration

Inspiration Definition: The active phase of breathing in which O₂ rich atmospheric air enters into alveoli is called inspiration.

Inspiration Mechanism:

The diaphragm is dome-shaped in its relaxed state. During deep inspiration, it contracts and i move downward.

Simultaneously, the external intercostal muscles contract and the rib cage moves upward and outward.

Due to the contraction of the diaphragm and the external intercostal muscles, the volume of the thoracic cavity increases.

The sternum moves upward and forward. This also causes a further increase in the volume of the thoracic cavity.

An increase in the thoracic cavity increases intrathoracic pressure by 3 mm Hg with respect to atmospheric pressure.

The lungs expand as the thoracic volume increases. As a result, a partial vacuum is generated in the alveoli, since the air pressure within the alveoli decreases.

As the alveolar pressure is now less than atmospheric pressure (air pressure outside the lungs), air naturally flows into the Alveoli, Via the respiratory passages.

Expiration

Expiration Definition: The passive phase of breathing, in which CO₂ rich air is expelled from alveoli to the atmosphere is called expiration.

Expiration Mechanism:

1. During expiration, the thoracic wall and lungs recoil due to their elastic properties.
2. The diaphragm and external intercostal muscles move upward and relax. As a result, the rib cage and sternum move down and inward.
3. Contraction of internal intercostal muscles moves the rib cage and sternum more downward and inward.
4. This causes a reduction in the thoracic cavity volume.
5. The intrapulmonary pressure in the alveoli increases by 3 mm Hg with respect to the atmospheric pressure. Then, CO₂ rich air flows through respiratory passages and is expelled out.

Path Of Expiration

CO₂ rich air → alveoli → alveolar duct → bronchiole bronchi → trachea → larynx → open glottis → internal nares pharynx → nasal passage → external nares.

Inspired Air, Expried Air, Alveolar Air

Inspired air

Inspired air Definition: The volume of air taken in the respiratory system under resting conditions, during normal inspiration or forceful inspiration, is called inspired air.

Volume: A person can normally inspire 500 ml of air under resting conditions, but the volume increases to 3500 ml during forceful inspiration.

Expired air

Expired air Definition: The volume of air released by the respiratory system under resting conditions, during normal expiration or forceful expiration, is called expired air.

Volume: A person can normally expire 500 ml of air under resting conditions, but the volume increases to 1000 ml or 1500 ml during forceful expiration.

Alveolar air

Alveolar air Definition: The volume of air that enters the alveoli, located in both the lungs, during inspiration, is called alveolar air.

Volume: During normal inspiration, out of the 500 ml of inspired air, 150 ml remains as anatomical dead space air and the remaining 350 ml enters the alveoli.

Regulation Of Breathing

Unlike all other organ systems, the respiratory system demonstrates both involuntary as well as voluntary modulation. We breathe involuntarily, but we can willingly modify our breathing pattern.

We can hold our breath up to a certain point called “breaking point.” Beyond this point, we can no longer hold our breath.

Our breathing pattern is also modulated by the activities of speech and singing. Respiration is regulated by two mechanisms— The nervous or neural mechanism and the Chemical mechanism.

Nervous mechanism

The nervous mechanism involves respiratory centres and afferent and efferent nerves. The respiratory centres are stimulated by impulses received from various receptors.

Respiratory centres: The rhythmic pattern of breathing is carried out by four separate respiratory centres, located in the medulla oblongata and pons.

These centres in the medulla are divided into the Dorsal Respiratory Group (DRG) and the Ventral Respiratory Group (VRG). The respiratory centres in the pons are the pneumatic centre and the apneustic centre.

Dorsal respiratory groups (DRG): Dorsal group of respiratory neurons (DRG), also known as the inspiratory centre, is located in the dorsal part of the medulla oblongata. Apart from their voluntary control over respiration DRG centre, is located in the dorsal part of the medulla oblongata.

Apart from their voluntary control over respiration DRG also has a role in controlling speaking, crying, laughing, singing and other physical activities. However, the impulses originating from the DRG neurons are primarily responsible for inspiration.

Inspiration is initiated by a rhythmical on-off pattern. During the “on” part of the cycle, neurons in the DRG cause the inspiratory muscles to contract. This, in turn, initiates inspiration.

During the I “off” part of respiratory cycles, the neurons from the DRG stop sending impulses.

The inspiratory muscles relax and passive expiration occurs.

Ventral respiratory groups (VRG): The VRG, also known as the expiratory centre, is located in the central part of the medulla. It consists of both inspiratory and expiratory neurons. Although it receives nerve impulses from DRG, it is mainly responsible for expiration.

Apneustic centre: It is located in the posterior part of the pons. It has a stimulatory effect on the inspiratory centre. Thus, it lengthens inspiration by preventing inspiratory neurons from being “switched off”. This is accomplished by providing a constant stimulus to continue inspiration.

Pneumotaxic centre: It is located in the anterior part of the pons. It is a collection of neurons called the pontine respiratory group. Its primary role is to decrease inspiration or “switch off” the inspiratory neurons and initiate expiration.

It inhibits the impulses from the apneustic centre, which helps to slow inspiration and control the rate of respiration. Chemical control mechanism Chemical control of breathing is carried out by chemoreceptors.

The chemoreceptors are sensory nerve endings that are stimulated by changes in the chemical composition of blood. These changes in the composition of blood may be due to changes in the partial pressure of oxygen and carbon dioxide.

There are two groups of chemoreceptors—central and peripheral chemoreceptors. The central chemoreceptor is in the central nervous system. The other group, the peripheral chemoreceptor, is located in the periphery of the body and is exposed to the arterial blood.

Central chemoreceptor: These are specialised cells, present on the ventral surface of the medulla oblongata. At rest, most respiratory responses are mediated by these central chemoreceptors.

These cells are sensitive to CO₂-induced changes in hydrogen ion concentration in the brain interstitial fluid.

Hydrogen ion is not able to cross the blood-brain barrier but CO₂ can. This CO₂ enter into the CSF and reacts with a water molecule to form carbonic acid. Carbonic acid is broken down into H+ and HC03″.

So, increasing CO₂ concentration in blood also increases H+ concentration in CSF. The CSF has a very small amount of buffers; the excess of hydrogen ions (or low pH) in the CSF stimulates the central chemoreceptor in the medulla.

As a result, respiratory rate and tidal volume increase to decrease acidity (i.e., increased pH). The remaining carbon dioxide is exhaled into the atmosphere.

The arterial partial pressure of CO₂ ( Pco ) and hydrogen ion concentration of the CSF return to normal to maintain homeostasis of the gases.

In contrast, if the level of CO₂ and hydrogen ion concentration is low, then the respiratory rate decreases. This allows the concentration of CO₂ and H+ in the blood to increase.

Peripheral chemoreceptors: These are located in the arch of the aorta (aortic bodies) and in the bifurcation of carotid arteries (carotid bodies) in the neck.

They sense changes in pcO₂ and H+ concentration in the arterial blood. If pCO₂ and H+ concentrations increase (with a decrease in pH), the peripheral chemoreceptors sense hypoxia in the arterial blood.

The glossopharyngeal nerve carries the impulses from the carotid bodies, while the vagus nerve carries those from the aortic bodies.

This network of neurons in the medullary respiratory centre sends signals through motor neurons to the respiratory muscles.

These signals help to restore Pco to a relatively constant level. This is done by increasing the rate of respiration to increase carbon dioxide elimination from the body and increase blood pH.

Exchange Of Gases

The exchange of gases takes place inside the lungs and in the tissues. Transportation of sufficient O₂ from the lungs to the tissues is carried out by both the respiratory and circulatory systems simultaneously.

O₂ is transported to the tissues, CO₂ is transported from the tissues to the blood and eventually exhaled by the lungs.

Unique transport mechanisms not only enable these two processes (O₂ uptake and CO₂ release) to occur simultaneously, but they also facilitate each other. This exchange of gases in the tissues occurs by simple diffusion.

Exchange Of Gases In Lungs

The exchange of gases in the lungs occurs through two steps, which are discussed below. Entry of oxygen from alveolar air to pulmonary capillaries Every 100 ml of oxygenated blood (arterial blood) contains 19-20 ml of oxygen and 100 ml of deoxygenated blood (venous blood) contains 14-15 ml of oxygen.

Therefore, the average difference in oxygen content between the arterial and venous blood is approximately 4.5 ml.

Hence, it is to be noted that during the circulation of blood in pulmonary capillaries, approximately 4.5 ml of oxygen (per 100 ml) from the alveolar air diffuses to pulmonary capillaries.

The partial pressure of oxygen (PO₂) in alveolar air is 104 mm Hg. But PO₂ in deoxygenated blood of pulmonary capillaries is 40 mm Hg. Due to this difference, oxygen diffuses through the squamous epithelium layer of the alveolus and enters into the blood of pulmonary capillaries.

This process of diffusion continues until PO₂ in pulmonary capillaries increases to 100-104 mm Hg.

But, generally, this does not happen because some amount of oxygen gets absorbed by the alveolar tissue layer. So, the PO₂ in pulmonary capillaries is approximately 90-95 mm Hg.

Pulmonary shunt

Pulmonary shunt occurs when the alveoli and lungs are perfused with blood but due to some reason gaseous exchange (ventilation) cannot take place.So the pulmonary shunt is a route through which blood perfuses unventilated shunt.

Carbon Monoxide Poisoning

Carbon monoxide has 250 times more affinity towards haemoglobin than oxygen. In the presence of carbon monoxide, haemoglobin reacts fast and forms a carboxyhaemoglobin compound. [CO + Hb = CO.Hb].

As a result, the oxygen-carrying capacity of blood reduces and oxygen content decreases in tissues (hypoxia).

This results in increased CO₂ content (asphyxia) and may prove fatal. Generally, igniting a fire or stove in a closed room causes incomplete combustion, leading to the formation of carbon monoxide. This CO₂ combines with the blood, causing carbon monoxide poisoning.

A person affected by carbon monoxide poisoning is treated by supplying a mixture of oxygen and carbon dioxide. This mixture helps to bind haemoglobin, thereby releasing carbon monoxide from haemoglobin.

Release of carbon dioxide from pulmonary capillaries to alveolar air

Every 100 ml deoxygenated blood contains 52 ml carbon dioxide in different forms, such as bicarbonate (in red blood cells and blood plasma), carbamino compound and a smaller amount in dissolved state.

During the circulation of blood in pulmonary capillaries, 5 ml of carbon dioxide per 100 ml of deoxygenated blood is released in the alveoli.

The PcO₂ of deoxygenated blood is approximately 45-46 mm Hg. However, the Pco of alveolar air is approximately 40 mm Hg. Due to this difference, carbon dioxide from deoxygenated blood diffuses into alveoli.

Exchange Of Gases In Other Parts Of The Body

The exchange of gases in other parts of the body occurs through two steps, which are discussed below.

Entry of oxygen from blood capillaries to its associated tissues

Oxygen enters the associated tissues from the blood capillaries in the following steps—

1. At first, oxygen from oxygenated blood in capillaries of different body parts diffuses in plasma and then to associated viable cells.
2. The PO2 in blood capillaries is approximately 90-95 mm Hg, but pO2 in tissues is 40 mm Hg.
3. Due to the pressure gradient, O₂ diffuses from the blood to the tissues.
4. 4.5 ml O₂ per 100 ml of oxygenated blood diffuses in tissues and helps in oxidation. This 4.5 ml O₂ is formed from 0.2 ml dissolved O₂ in blood plasma, and approximately 4.3 ml from the dissociation of oxyhaemoglobin.

Conditions needed for the dissociation of oxyhaemoglobin are—a decrease in an increase in Pco in tissues, an increase in temperature in tissues and low pH.

Release of carbon dioxide from the tissues to blood capillaries Carbon dioxide is mainly generated during the oxidation of glucose in living cells.

Carbon dioxide diffuses from tissues to blood plasma and then to its associated capillaries. Carbon dioxide content is 48 ml per 100 ml of oxygenated blood and 52 ml per 100 ml of deoxygenated blood.

During the circulation of 100 ml of blood through various capillaries throughout the body, it collects 4-5 ml of carbon dioxide from tissues.

Pcc,2 in tissues is 46 mm Hg, but Pcc,2 in deoxygenated blood is 40 mm Hg. Due to this pressure gradient, carbon dioxide diffuses from tissues to blood capillaries.

This balance is maintained in order to transport carbon dioxide from tissues to capillaries.

Transport Of Gases

• The transportation of oxygen, from the atmosphere to the individual cells, occurs through a series of steps.
• The heart, lungs and blood circulation generate a flow of oxygenated blood to the tissues to maintain aerobic metabolism.
• However, oxygen transportation occurs with the concomitant expulsion of carbon dioxide.
• For this transportation, some respiratory pigments play an important role. In this section, the respiratory pigments and the process of transportation have been discussed.

Respiratory Pigments

The pigments present in animal bodies that carry O₂, and CO₂ and transport them for cellular respiration are called respiratory pigments.

The different types of respiratory pigments, their chemical nature and features are discussed below in the table.

Transport Of Oxygen

O₂ transport system in the body consists of the lungs and the cardiovascular system.

Deoxygenated blood releases carbon dioxide and gets saturated by oxygen. This oxygenated blood enters the left ventricle of the heart by the pulmonary vein.

From the heart, the blood passes through the aorta, arteries and arterioles and reaches capillaries surrounding different body organiser

Finally, it reaches the associated tissues. At normal atmospheric pressure, the number of oxygen molecules present in each litre of blood is normally equivalent to 200 ml of pure gaseous oxygen.

Oxygen is present in two forms—

1. As dissolved form and
2. A combined form (oxyhaemoglobin). Both these forms have been discussed below

Transport As Dissolved Gas

1. Oxygen diffuses passively from the alveoli and dissolves in the plasma. In its dissolved form, O₂ maintains its molecular structure and gaseous state.
2. The quantity of oxygen that dissolves in a fluid, such as blood, is predicted by gas laws (Henry’s law and Graham’s law) and it depends on the partial pressure of gas and temperature.
3. In a healthy normal adult, approximately 0.3 ml of O₂ is dissolved in 100 ml of blood.
4. This is commonly expressed as 0.3 volume per cent (vol%), where the vol% is equal to the millilitre of O₂ per 100 ml of blood.
5. Only a small percentage of O₂ is carried in this dissolved form, and its contribution to total O₂ transport is small.

Transport As Oxyhaemoglobin.

Amount to be transported: As oxygen is relatively insoluble in water, only 3 ml can be dissolved in every 1 L of blood at the normal arterial PO₂ of 100 mm Hg.

Among the rest 197 ml (out of 200 ml total oxygen) of oxygen in a litre of arterial blood is transported through the erythrocytes as oxyhaemoglobin.

So, more than 98% of oxygen content is transported through erythrocytes as oxyhaemoglobin.

Formation of oxyhaemoglobin: The dynamics of the reaction of haemoglobin with O₂ makes it a suitable O₂ carrier.

Each haemoglobin (Hb) molecule is a protein made up of four subunits joined together. Each of these subunits consists of a molecular group known as haem and a polypeptide attached to it.

Haem is a porphyrin ring complex that includes one ferrous ion (Fez+). Each of the four ferrous ions in Hb can reversibly bind one O₂ molecule each.

In normal adults, most of the haemoglobin molecules contain two or two chains.

The binding of O₂ to haemoglobin alters the absorption of Hb, which is responsible for the change in colour between oxygenated arterial blood (HbOz) and deoxygenated venous blood (Hb).

The binding of four O₂ molecules to each Hb molecule forms a complex called oxyhaemoglobin. This happens in four stages which are as follows

⇒ \(\left.\begin{array}{l}\text { (1) } \mathrm{Hb}_4+\mathrm{O}_2 \longrightarrow \mathrm{Hb}_4 \mathrm{O}_2 \\ \text { (2) } \mathrm{Hb}_4 \mathrm{O}_2+\mathrm{O}_2 \longrightarrow \mathrm{Hb}_4 \mathrm{O}_4 \\ \text { (3) } \mathrm{Hb}_4 \mathrm{O}_4+\mathrm{O}_2 \longrightarrow \mathrm{Hb}_4 \mathrm{O}_6\end{array}\right\} \begin{aligned} & \text { Unsaturated } \\ & \text { oxyhaemoglobin }\end{aligned}\)

⇒ \((4) \mathrm{Hb}_4 \mathrm{O}_6+\mathrm{O}_2 \longrightarrow \mathrm{Hb}_4 \mathrm{O}_8 \quad Saturated oxyhaemoglobin\)

The readily reversible binding of O₂ to haemoglobin facilitates the delivery of O₂ to the tissues from the blood. In saturated conditi ons, 1g of haemoglobin can transport 1.34 ml of oxygen.

Conditions required for oxyhaemoglobin Formation:

1. Increase in PO₂ in alveolus;
2. Decrease in PcO₂ in alveolus;
3. High pH in blood; Maintenance of warm condition in lungs.

The oxygen dissociation curve or ODC

The graph in which the partial pressure of O₂ (PO₂ is plotted against the X-axis and percentage saturation of haemoglobin (Hb) by O₂, along the Y-axis to determine the rate of O₂ binding to haemoglobin is called the oxyhaemoglobin dissociation curve or oxygen dissociation curve or ODC.

The O₂ molecules present in the pulmonary capillaries of human lungs mostly bind with haemoglobin to form oxyhaemoglobin.

The formation of oxyhaemoglobin is dependent on various factors, such as PO₂ Pcc,2, H+ ion concentration in blood, temperature, etc.

The curve, obtained by plotting oxyhaemoglobin formation against PO₂ is S-shaped or sigmoid. The effect of the factors stated above can be studied by this plot.

Reasons behind the sigmoidal shape of the oxygen dissociation curve: Each haemoglobin molecule contains four haem groups that can bind with four O₂ molecules, at the most.

The binding occurs sequentially in the following manner—

The binding of the first haem in the Hb molecule with O₂ occurs slowly so that no change in ODC occurs. However, this changes the structure of haemoglobin and increases the affinity of the second haem for O₂.

Oxygenation of the second haem increases the affinity of the third and these reactions occur easily. As a result, ODC shifts to the right.

When PO₂ is 25 mm Hg, Hb becomes 50% saturated by O₂ At this portion of the curve, a large amount of O₂ is released from haemoglobin with only a small change in PO₂. This response facilitates the diffusion of O₂ to the tissues.

The point on the curve at which 50% of a haemoglobin molecule is saturated with O₂ (two oxygen molecules on one Hb molecule) is called the P50.

After binding of the three haem groups, however, binding of the fourth O₂ molecule is a bit difficult, this type of binding is known as co-operative binding.

After this binding, ODC becomes almost plateau. From this point, a further increase in PO₂ produces only a smaller increase in oxygen binding.

The significance of the flat portion of the ODC is that a drop in PO₂ from about 100 mm Hg to about 60 mm Hg still results in a haemoglobin saturation of more than 90%. This virtually ensures adequate O₂ transport.

Factors controlling the oxygen dissociation curve

Three important factors that affect ODC are pH, CO₂, temperature and concentration of 2,3-diphosphoglycerate (2,3-DPGA).

1. pH: A decrease in pH shifts ODC to the right (enhancing O₂ dissociation) whereas an increase in pH shifts the curve to the left (increasing O₂ affinity). During cellular metabolism, CO₂ is released into the blood and therefore hydrogen ion concentration increases and thus, a decrease in pH occurs.
2. CO2: An increase in CO₂ concentration leads to an increase in H+ ion concentration. This causes ODC to shift to the right. CO₂ has a direct effect on haemoglobin. This effect is known as the Bohr effect.
3. Temperature: The body temperature increases during muscular exercise. This effect shifts the dissociation curve to the right and it enables more O₂ to be released in the tissues. When the weather is cold, a decrease in body temperature shifts the O₂ dissociation curve to the left (higher Hb affinity).
4. 2,3-Diphosphoglycerate (2,3-DPGA): 2, 3-Diphosphoglycerate is produced during glycolysis. It is found abundant in red blood cells. The affinity of 2, 3-DPGA for Hb is greater than that of O₂ As a result, 2, 3-DPGA directly competes with O₂ and binds to the p chain of Hb binding sites. This reduces the transport of oxygen by Hb.

⇒ \(\mathrm{HbO}_2+2,3-\mathrm{DPGA} \rightleftharpoons \mathrm{Hb}-2,3-\mathrm{DPGA}+\mathrm{O}_2\)

Bohr effect

It is the phenomenon in which an increase of carbon dioxide in the blood and a decrease in pH results in a reduction of the affinity of haemoglobin for oxygen.

It results in a right shift of the ODC reflecting the release of oxygen from haemoglobin. The Bohr effect is closely related to the fact that deoxygenated haemoglobin binds H+ more actively than oxygenated haemoglobin.

This phenomenon was first described by the Danish scientist Christian Bohr(1904).

Thus, the oxyhaemoglobin dissociation curve is shifted to the right. Conditions that increase 2, 3-DPGA include hypoxia, decreased Hb concentration, and increased pH.

Haldane effect

Haldane effect states the effect of oxygen on carbon dioxide transport. This phenomenon describes the binding of oxygen to haemoglobin and promotes the release of carbon dioxide from carbaminohaemoglobin. This effect was first observed by J.S.Haldane(1914)

Transport of carbon dioxide

During cellular respiration, CO₂ is produced and reaches the heart through blood vessels. From the heart, the deoxygenated blood comes to the lungs by pulmonary arteries.

The CO₂ content is approximately 52.1 ml per 100 ml of deoxygenated blood and 48.3 ml per 100 ml of oxygenated blood.

CO₂ is carried and transported through the blood in both plasma and red blood cells as three distinct chemical forms.

These are namely, bicarbonate (HC03 ), dissolved CO₂ and carbamino protein complexes. In plasma, CO₂ binds to various plasma proteins, and in red blood cells, CO₂ binds to haemoglobin. By far, the predominant transport mechanism of CO₂ is HC03 within RBC.

Transport as bicarbonate (HC03)

Every 100 ml of deoxygenated blood containing 45.7 ml of CO₂ and 100 ml of oxygenated blood containing 42.9 ml CO₂ is transported as different bicarbonate compounds by blood and plasma.

Formation of bicarbonates in blood plasma: The plasma of 100 ml each of deoxygenated and oxygenated blood contains 35.2 ml and 33.1 ml CO₂ respectively, as bicarbonate compounds. The reactions that are involved are as follows.

The plasma protein (Pr) of blood plasma remains bound to sodium (Na) which is designated as NaPr. This reacts with H2C03 to form sodium bicarbonate (NaHC03)

⇒ \(\begin{gathered}
\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{CO}_3 \\
\mathrm{NaPr}+\mathrm{H}_2 \mathrm{CO}_3 \rightarrow \mathrm{NaHCO}_3+\mathrm{HPr}
\end{gathered}\)

Blood plasma contains alkaline phosphates such as disodium hydrogen phosphate (Na₂HPO₄). This Na₂HPO₄ reacts with carbonic acid to form sodium bicarbonate (NaHCO₃) and sodium dihydrogen phosphate (Na₂PO₄).

⇒ \(\mathrm{Na}_2 \mathrm{HPO}_4+\mathrm{H}_2 \mathrm{CO}_3 \rightarrow \mathrm{NaHCO}_3+\mathrm{NaH}_2 \mathrm{PO}_4\)

Formation of bicarbonate compound in red blood cells: RBC of 100 ml deoxygenated and oxygenated blood contain 10.5 ml and 9.8 ml of CO₂ respectively, as bicarbonate compounds.

CO₂ in RBCs is transported as potassium bicarbonate (KHCO₃). Due to the presence of the enzyme carbonic anhydrase, CO₂ and water react to form carbonic acid. Carbonic acid dissociates to form bicarbonate and H+ ions.

Hb forms the compound KHb with potassium in RBC which then reacts with carbonic acid. Bicarbonate ions react with potassium to form potassium bicarbonate and the H+ ions bind to haemoglobin to form a HHb complex.

Transport as carbamino protein complexes Each 100 ml of deoxygenated and oxygenated blood contains 3.7 ml and 3.0 ml CO₂ respectively, as carbamino compounds.

Out of these, some amount of CO₂ remains in plasma as carbamino-plasma and some of the amount remains in RBCs as carbaminohaemoglobin.

In blood plasma: 100 ml of deoxygenated blood contains 1.1 ml and 100 ml of oxygenated blood contains 1.0 ml of carbon dioxide which is transported as carbamino compounds. The amino acids of plasma proteins react with CO₂ to form carbamino proteins.

⇒ \(
\mathrm{Pr} \cdot \mathrm{NH}_2+\mathrm{CO}_2 \longrightarrow \mathrm{Pr} \cdot \mathrm{NH} \cdot \mathrm{COOH}
(Carbamino protein)\)

In red blood cells: 100 ml of deoxygenated blood j contains 2.6 ml of CO₂. 100 ml oxygenated blood contains 2.0 ml of carbon dioxide which reacts with Hb of RBCs to form carbaminohaemoglobin. In this case, carbonic anhydrase plays no role.

Transport as dissolved CO₂

The remaining 5% of the CO₂ is carried in the blood as dissolved gas. Although CO₂ is 20 times more soluble in water than O₂, the dissolved form of CO₂ still constitutes a small fraction of the total transported CO₂.

Hence, the dissolved form of CO₂ is much more important in CO₂ transport than the dissolved form of O₂. Cv dissolves in red blood cells and blood plasma to form carbonic acid.

RBCs and plasma of 100 ml of deoxygenated blood contain 0.9 ml and 1.8 ml of CO₂ respectively, i.e., a total of 2.7 ml in dissolved state. RBCs and plasma of 100 m oxygenated blood contain 0.8 ml and 1.6 ml CO₂ respectively, i.e., a total of 2.4 ml in the dissolved state.

Carbon Dioxide Dissociation Curve

In contrast to the behaviour of O2, the dissociation of CO₂ from blood is almost directly proportional to the Pco.

Hence, the dissociation curve for CO₂ is linear, the saturation of haemoglobin with O₂ has a major effect on the CO₂ dissociation curve.

Although O₂ and CO₂ both bind to haemoglobin at different sites, deoxygenated haemoglobin has a greater affinity for CO₂ than oxygenated haemoglobin.

The deoxygenated Hb more readily forms carbamino compounds and also binds faster with free H+ ions that are released during the formation of HCO₂. Thus, deoxygenated blood (venous blood) freely takes up and transports more CO₂ than oxygenated arterial blood.

Chloride Shift Or Hamburger Phenomenon

When CO₂ enters the cellular capillaries, NaCI in blood plasma breaks down to release Cl” ions which enter into the RBC.

The reverse phenomenon occurs in the lungs. As CO₂ enters pulmonary capillaries, RBCs release Cl- to regenerate NaCI in plasma.

This phenomenon regarding the shifting of chloride ions is known as the chloride shift or Hamburger phenomenon. It was discovered by Hamburger in 1927.

Chloride Shift In Cellular Capillaries

Cations like K+, and Na+ cannot move across the membrane of RBCs but anions like Cl-, and HCO₃ can. Carbon dioxide produced in tissues of body parts, other than the lungs, enters into the blood.

The majority of this carbon dioxide enters into the RBC. Due to the presence of the enzyme, carbonic anhydrase (CA) in RBCs, carbon dioxide reacts with water to form carbonic acid (H2C03).

H2C03 produced in RBC reacts with haemoglobin bound to potassium (KHb) to form potassium bicarbonate (KHCO₃) and haemoglobinic acid (HHb). Due to the production of KHC03, the content of bicarbonate in RBCs increases and it becomes slightly alkaline.

To maintain the pH of blood, either K+ ions should be released from the blood or some anions like Cl” should enter the blood.

Since K+ ions cannot move across the membrane of RBC, NaCI dissociates and releases Cl” ions. These ions penetrate through the membrane of RBC and react with KHCO₃ to form KCI and HCO₃.

Due to the formation of HCO₃ in RBCs, it becomes slightly acidic. To neutralise this, HCOJ diffuses into blood plasma and reacts with Na+ to form sodium bicarbonate (NaHCO₃)

Chloride shift in lung capillaries

Cl- from RBCs of pulmonary capillaries gets released into blood plasma. This Cl- reacts with NaHCO₃ to form NaCl and H2CO₃.

This H2CO₃ dissociates to produce carbon dioxide that diffuses into alveoli. Chloride shift occurs reversibly in the capillaries of other body parts except the lungs.

Regulation Of Respiration

Respiration is regulated by a number of factors which have been discussed separately.

Central chemoreceptors: The imbalance of gaseous components (O₂, CO₂) in blood and CSF causes stimulation of these receptors in the spinal cord and hypothalamus. Details have been discussed in previous sections.

Peripheral chemoreceptors: Receptors in blood vessels sense an imbalance in gaseous components (O₂, CO₂) in blood and body fluid. These receptors send signals to the respiratory centres in the brain and thus control respiration. Details have been discussed in previous sections.

Baroreceptors: Baroreceptors in blood vessels, tubular organs, muscles, joints of bones etc., can distinguish between expansion and contraction, thus controlling respiration.

Reflex: Reflex in alveoli, visceral pleura, respiratory tract etc., generates sensory impulse that triggers the respiratory centres in the brain, thus controlling respiration.

Temperature: The rate of respiration is proportional to body temperature. On increasing temperature, the rate of respiration also increases. Again, on decreasing the temperature, the rate of respiration decreases too.

Blood pressure: The rate of respiration is also proportional to blood pressure. On increasing the blood pressure, the rate of respiration increases and vice-versa.

Pain: Due to excess pain, respiration may stop. Pain in visceral organs reduces the rate of respiration.

Tracheal stimulation: Due to tracheal stimulation, respiration may cease for a moment and this, in turn, causes a cough or sneeze.

Stimulation in the limbic system: The limbic system in the human brain gets excited due to emotional stimuli which influences respiration accordingly.

Respiratory And Pulmonary Volumes

The total amount of air that the lungs can accommodate is divided into a number of volumes.

Combinations of these lung volumes can be used to determine lung can accommodate varies primarily with the age, height and sex of the individual.

The various lung volumes and capacities and their definition.

Importance of RV

The residual volume maintains gaseous exchange between O₂ and CO₂ in the alveoli and associated pulmonary capillaries during breathing. Otherwise, deficiency of oxygen may be observed due to oxidation of glucose and cellular respiration would be hampered.

Release of RV

RV cannot be released in living conditions. However, if holes are made in the wall of the thoracic region or in the pneumothorax region of a person, atmospheric air enters the thoracic cavity through these holes.

As a result, air pressure in the thoracic cavity increases. This causes the collapse of the lungs and RV releases from the body. During open heart surgery, the lungs collapse totally. RV is also released during death.

Instrument for measurement of pulmonary volume—spirometer

A spirometer is an instrument used to measure different volumes of air involved in breathing. spirometry is a basic test for the study of lung functioning. Basically, there are two vessels in a spirometer—

A larger vessel containing water and having a breathing hose attached to it,

A smaller vessel inverted and suspended in the water. A counterweight and indicator are attached to the inverted chamber.

The indicator is associated with a machine named Kymograph. The kymograph cylinder is provided with graph paper which records data.

This is known as spirogram. Air blown into the inverted chamber will cause it to rise, thus moving an indicator arrow along the horizontal scale.

It is calibrated in litres to give lung volume measurements. If the spirometer index moves by 1 mm up-down, it indicates 30 ml of air. It is essential for the evaluation and monitoring of respiratory diseases.

Dead Space

Dead Space Definition: The part of the respiratory system which does not take part in the gas exchange process is called dead space.

Types: Based on the volume of air contained in these spaces, dead spaces are classified into two types—

Anatomical dead space and physiological dead space

Anatomical dead space and anatomical dead space air: The airways from the human nasal passage to the terminal bronchiole where no gaseous exchange with pulmonary capillaries takes place, are known as anatomical dead space.

The air present in anatomical dead space is known as anatomical dead space air. In human beings, it is approximately 150 ml in volume and constitutes 20% – 30% of TV (tidal volume).

Physiological dead space and physiological dead space air: The frontal part of the lungs where alveoli cannot take part in gas exchange due to uneven distribution of several respiratory disorders seen in the human body capillaries is known as physiological dead space.

The air present in physiological dead space is known as physiological dead space air.

For a normal healthy individual anatomical dead space air volume is almost the same as physiological dead space air volume.

Its normal volume is approximately 150-200 ml.

Importance: Inspired air becomes saturated with water vapour, in the anatomical dead space and enters the alveoli.

Some particles of size more than 0.2 mm are trapped in that space before entering the alveoli.

An increase in physiological dead space air and air volume indicates diseased or ill-condition alveoli.

It helps to determine lung diseases. For example, in a respiratory disorder called emphysema, blood circulation decreases due to the presence of coagulated blood. This means during any lung disease physiological dead space air volume increases.

Apneustic breathing: It is another abnormal breathing pattern that is characterised by sustained periods of inspiration, separated by brief periods of exhalation.

Loss of inspiratory-inhibitory activities is the mechanism responsible for this type of ventilatory pattern. Individuals with central nervous system injury.

Disorders Related To Respiratory System

Several respiratory disorders are seen in the human body and some of these disorders.

Asthma

Asthma is a disease of the bronchi and bronchioles that is marked by wheezing, breathlessness, and sometimes cough and expectoration of mucus.

Asthma Types and causes: There are two types of asthma that are discussed below.

Allergic or atopic asthma: This type of asthma generally occurs due to air pollution. Different foreign substances and allergens, such as dust particles, pollen grains etc., on entering the body cause allergy.

In this condition, the number of eosinophils and mast cells increases. They secrete leucotriene, histamine etc. These substances constrict the bronchioles and cause allergic asthma. Generally, this type of asthma is observed during childhood.

Non-atopic or idiosyncratic asthma: This type of asthma affects elderly people. It is caused due to viral infection, polyp formation or excessive smoking. During viral infection, inflammation of the wall of bronchioles occurs.

As a result, the diameter of the inner cavity of bronchioles reduces and non-atopic asthma is observed.

The asthmatic trauma becomes frequent during dawn or late night. Cold air enhances the chances of such asthma. Anxiety, mental pressure etc., also increase the chances of this disease.

Asthma Symptoms:

1. Contraction of smooth muscles of the wall of bronchioles occurs which reduces the diameter of the lumen of bronchioles. So, the circulation of air through it gets obstructed and this creates difficulty in breathing.
2. Inflammation of the wall of bronchioles occurs. From that layer, a thick mucus is secreted that sticks to the smaller cavities of bronchioles. This mucus obstructs the airways and causes dyspnoea (painful respiration).
3. Asthmatic patients face difficulty in expiration than inspiration and in some cases, whistling sound is observed during expiration.
4. Tachycardia or an increase in the rate of heartbeat is observed.
5. During chronic asthma, the skin may turn blue (due to deficiency of O₂), and loss of consciousness, and respiratory arrest leading to death may also occur.

Asthma Prevention and Treatment:

1. Air pollution and smoking are the two main causes of asthma. So, prevention of smoking and avoiding inhalation of polluted air are the two best possible ways to prevent asthma.
2. Cold waves of air increase the disease. So, care should be taken that the patient is not exposed to cold air during dawn.
3. At present, corticosteroids or other medicines are used to treat asthma. They act as bronchodilators.
4. Homes should be cleaned and made germ-free or dust-free.
5. The patient should not suffer from any anxiety or tension.

Emphysema

Emphysema is a chronic disease or disorder of the lungs due to which the elasticity of the walls of the lungs reduces, the lungs expand and the surface area for absorption of oxygen reduces.

Emphysema Causes:

1. Cigarette smoking, air pollution, lung infections and some occupational hazards are the main causes of emphysema. It occurs due to the release of proteolytic enzymes as part of the inflammatory process that follows irritation of the lungs.
2. The lungs actually undergo self-destruction by these proteolytic enzymes secreted by leukocytes in the lungs.
3. In about 2% of cases, it’s a rare hereditary disease.

Emphysema Symptoms: The symptoms of emphysema.

• Reduction or loss of elasticity of the walls of the lungs: The elasticity of the walls of the lungs is reduced thereby causing narrow airways.

Excessive smoking leads to an increase in the number of macrophages. These macrophages secrete chemicals which attract WBCs. These WBCs then secrete elastase and other proteolytic enzymes. These enzymes cause damage to the lungs and reduce their elasticity.

Hypoxia: This occurs due to the malfunctioning of the lung tissue during emphysema and uneven blood flow due to the destruction of capillaries.

Enlargement of chest: Emphysema may lead to an enlarged or barrel-shaped chest.

Enlargement of heart: The right side of the heart is observed to be enlarged for people suffering from emphysema.

Types: The different types of emphysema are discussed below.

1. Panacinar emphysema: In this case, walls of adjacent alveoli break down, alveolar tubules become swollen and intra-alveolar elastic tissue gets damaged.
2. Centrilobular emphysema: In this case, bronchioles become thick and so when air enters these lobules (i.e., bronchioles and alveolar region) oxygen pressure reduces and hypoxia occurs.
3. Interstitial emphysema: When air occupies intrathoracic tissue, the condition is known as interstitial emphysema. External wounds by knife or breaking of the rib cage, and asthma may create such conditions.

Emphysema Prevention and treatment: Currently no form of treatment can cure emphysema reverse the damage to the lungs, or even control the disease and its symptoms. However, emphysema can be prevented by avoiding cigarette smoking.

Antibiotics may be used to treat and prevent infections within the respiratory system. Special breathing exercises are often helpful and breathing equipment that delivers extra oxygen and medications may be provided for home use.

Corticosteroids or steroids are used to decrease the inflammation of the airway walls.

Occupational Respiratory Disorders

Occupational respiratory disorders are a group of disorders that are caused by long or brief exposure to toxic substances while working in coal mines, factories, etc.

Common occupational lung diseases include anthracosis, mesothelioma, occupational asthma, silicosis, and asbestosis.

Occupational respiratory disorders Symptoms: In the case of occupational diseases, different particulate matter enter, the lungs along with the inhaled air and get deposited on the walls of the lungs as well as those of the alveoli. As a result, the wall of the alveoli thickens. Due to this, gaseous exchange slows down, and difficulty in breathing and coughing are experienced.

Occupational respiratory disorders Types: Some occupational diseases are discussed below. Black lung disease or Anthracosis: The people who work in coal mines inhale minute particulate matter like small fine particles of coal during inspiration.

These particles deposit on the inner walls of the lungs and create a black layer over them. This disease is known as black lung disease or anthracosis. In this case, symptoms of bronchitis and emphysema can also be observed.

Asbestosis: Asbestosis is a respiratory disease that results from the presence of microscopic asbestos fibres in the air. These small asbestos fibres deposit on the lungs over time and can cause scarring or fibrosis of the lungs.

This scarring causes the lungs to stiffen and makes it hard to breathe. Earlier, asbestos was used widely as a fireproofing and insulating agent, whereby unwanted exposure has occurred, causing asbestosis.

Mesothelioma: Mesothelioma is a fatal type of cancer also caused by exposure to asbestos. Millions of construction and general industry workers have been exposed to asbestos fibres over a very long period of time, causing mesothelioma.

Usually, mesothelioma does not show up until 20 to 40 years after exposure. Most of the deaths resulting from the disease are the result of exposures that occurred decades ago.

Silicosis: People who work in gold, copper mines or factories where glass, slate etc., are manufactured, suffer from this disease.

It includes the deposition of silicon particles within the alveoli. WBCs and macrophage phagocytose silicon particles.

However, sometimes these particles may not get removed from the body. The X-ray plates of the lungs of patients suffering from this disease show an orange line, showing the presence of silica.

Byssinosis: The people who work in textile manufacturing industries for a long time, inhale thin fibres of cotton, jute, silk etc. These fibres enter the lungs and cause the disease byssinosis. The symptoms of this disease are breathing trouble, asthma, chest pain etc.

Prevention and treatment: To prevent these diseases, workers must be aware of the consequences. They must be provided with proper aprons, masks, gloves etc., that prevent them from coming in contact with this harmful substance.

Hypoxia

Hypoxia is a condition where the supply of oxygen to the tissues is less than the volume of oxygen required by them.

Hypoxia Types: It is mainly of four types. Each of them is discussed below

Hypoxic hypoxia: The reduction in oxygen content in the tissues due to a reduction in partial pressure of oxygen in the artery, is known as hypoxic hypoxia.

In this form of hypoxia, the arterial PO₂ is below normal because either the alveolar Pÿ is reduced for environmental reasons such as altitude or the blood is unable to equilibrate fully with the alveolar air as in lung diseases.

Anaemic hypoxia: The reduction of oxygen content in the tissues due to anaemia and less haemoglobin in blood, is known as anaemic hypoxia.

Ischemic or hypokinetic hypoxia: Reduced oxygen content in tissues due to a reduction in blood flow because of a reduction in the activity of the heart, is known as ischemic or hypokinetic hypoxia.

Low cardiac output or perfusion (shock or ischemia), severe hypotension or low pulmonary venous outflow are examples of conditions that reduce circulation, seriously decreasing oxygen transport to the tissues.

Histotoxic hypoxia: The reduction in oxygen content due to the toxicity of tissues is known as histotoxic hypoxia, This is the most severe form of hypoxia.

Lung cancer

Cancerous growth may develop in epithelial cells lining the bronchi and bronchioles. It is most common among smokers.

The smoke released by burning tobacco in a cigarette, etc., contains harmful compounds that cause cancer.

Pulmonary Tuberculosis

Tuberculosis is an infectious disease which usually affects the lungs. It is also known as pulmonary tuberculosis. In humans, tuberculosis is caused by the bacterium Mycobacterium tuberculosis.

Mountain sickness and acclimatisation When a person reaches 12,000-18,000 ft altitude, he/she suffers from mountain sickness due to a lower level of oxygen. Its symptoms are headache, nausea and dyspnoea.

But after 4-8 days, several physiological changes are observed in his body. Due to these changes, the person can adjust to the changing environment. This phenomenon is called acclimatisation.

These physiological changes enable proper oxygen supply to the body. For example, the spleen contracts, the amount of blood in blood capillaries and the number of RBCs increase, and heart rate and rate of blood flow also increase.

As a result, sufficient oxygen reaches the tissues. Under the effect of erythropoietin, RBC production increases in the bone marrow.

Breathing And Exchange Of Gase Notes

• Allergy: It is a symptom that occurs due to hypersensitivity of the immune system which is caused by some environmental factors.
• Antibiotic: It is a drug or medicine which inhibits the growth of a micro-organism or destroys it.
• Bradycardia: Decrease in heart rate.
• Bronchodilator: A drug which helps in the widening of bronchi.
• Chemotherapy: Use of drug to treat any disease (mainly cancer)
• Corticosteroid: A class of steroid hormones which is produced in the adrenal cortex.
• Dyspnea: Painful respiration due to some reason.
• Pigment: It is a natural colouring matter of animal or plant tissue.
• Phonation: Production of speech sound.
• Radiation therapy: It is a process where high-energy particles or waves are used to treat cancer.
• Tachycardia: Increase in heartbeat

Points To Remember

1. Breathing is a physical process by which animals exchange gases between their body and the atmosphere.
2. Human breathing takes place in two stages— inspiration and expiration.
3. Each lung is covered by a double-layered membrane, known as pleura. The outer layer of the pleura is called the parietal pleura and the inner layer is called the visceral pleura.
4. The nerve present in the diaphragm is the phrenic nerve
5. Inflammation of the mucous membrane of the nasal passage is called rhinitis.
6. Arrest of the functioning of the central nervous system along with its respiratory control systems is known as narcosis.
7. Inflammation of the larynx is called laryngitis.
8. Tuberculosis or TB occurs due to the infection of the bacteria Mycobacterium tuberculosis. The infection mostly occurs in the lungs and pleural membrane.
9. Inflammation of bronchi is known as bronchitis.
10. Infection and inflammation of the pleura are known as pleurisy.
11. In dry pleura or pleuritis, the pleura becomes rough and the sound of rubbing against the rough pleura can be heard by a stethoscope.
12. Fluid may deposit in interpleural space due to infection. This is known as pleural effusion. Haemothorax, pyothorax, chylothorax etc., are examples of pleural effusion.
13. The abnormal condition in which the intrapleural region is filled with air is known as pneumothorax.
14. Deposition of blood in the intrapleural region is known as haemothorax
15. The deposition of pus between the two layers of the pleura is known as pyothorax.
16. In the interpleural region, deposition of chyle i.e., the lymph appearing as a milky fluid, containing undissolved fat, is known as chylothorax.
17. Although the volume of fluid deposition is not too high, yet occurrence of this condition is an indication of cancer (e.g., lymphoma) and choking of important lymphatic vessels.
18. The presence of coal particles in the lungs causes bronchitis, emphysema etc. This condition is known as anthracosis.
19. The disease that occurs due to inhalation and deposition of silicon compounds in the lungs, is known as silicosis.
20. Inhalation of thin fibres of cotton, jute etc., for a long time causes byssinosis.
21. A normal respiration rate is known as eupnea.
22. The arterial oxygen that is utilised by the tissues is expressed by the oxygen utilisation coefficient. Its value is expressed in terms of the following ratio.
23. ⇒ \(\frac{\text { Amount of oxygen utilised in tissues }}{\text { Amount of oxygen present in artery }} \times 100\)
24. The normal value of the oxygen utilisation coefficient is 25%. But during exercise, its value increases to 75-80%.
25. Dissociation of oxyhaemoglobin increases in the presence of 2, 3-BPG or 2, 3-bisphosphoglycerate. So, this compound is important for acclimatisation.
26. The vital capacity of athletes and people living in mountains is more than the vital capacity of people living in plains. Due to their, necessity for more energy, they require more oxygen.
27. Red blood cells contain zinc-containing enzymes known as carbonic anhydrase. It increases the rate of binding of RBC to carbon dioxide by 5000 times.
28. Due to this reason, 70% of the amount of carbon dioxide that enters the blood from tissues, remains bound to red blood cells.
29. Animals belonging to the class Insecta have a complex respiratory system called the trachea. By the trachea (10 pairs), they transport air to the tissues directly.
30. Fishes breathe by gills. The blood vessels in the gills are so arranged that the flow of blood in the vessels is in opposite directions to the water flow.
31. Inflammation of the alveoli of the lungs due to the infection of the bacteria Streptococcus pneumoniae is known as pneumonia.
32. Lung cancer occurs due to excessive smoking. In this disease, cells of the lungs divide uncontrollably. As a result, blood vessels rupture and death occurs due to loss of blood (haemorrhage).
33. A decrease in the normal rate of respiration is known as hypopnoea.
34. An increase in the normal rate of respiration is known as hyperopnoea.
35. A deficiency of carbon dioxide in the blood is known as hypocapnia.
36. The absence of CO₂ in the blood is known as apnoea.
37. Excess carbon dioxide in the blood is known as hypercapnia.
38. Painful respiration due to some reason is called dyspnea.
39. Obstruction of respiration due to some reason is known as apnea.

 

Locomotion And Movement Introduction

Definition Of Movement: Movement is the Process by Which An Organism Is Able To move its organs Or body parts in response to stimuli without changing the location of the whole body.

Definition of locomotion: Locomotion is the process by which an organism can move from one place to another in response to stimuli.

While watching the Olympic games, don’t you become amazed to see the performance of the athletes? Some of them can jump very high, some can run very fast, some can lift heavy weights and so on. All these abilities depend on the functions of various muscles and bones present in the body.

Movement is a change of position of any part or organ of the body of a living organism.

• Movement can be done by both plants and animals. An animal can also change its location by simply moving different parts of its body. It is termed locomotion.
• A large number of animals move around using their limbs. The limbs are moved by muscles and the muscles are attached either to the endoskeleton or the exoskeleton.
• In almost all vertebrates, movement and locomotion are controlled by muscles and bones. Some organisms such as worms, snails, protozoans, etc., do not have limbs.
• They move around either by changing their body shape or by using appendages or by floating in water.
• Among all the multicellular organisms, only animals explore their environment in an active way, through locomotion.

Significance Of Movement And Locomotion

The significance of movement and locomotion is discussed below.

Search for food: All living organisms have to move to places in search of food. Plants synthesise their own food through photosynthesis.

Thus, locomotion is not that necessary for them. Animals cannot produce their food through photosynthesis. Hence, they need to move from one place to another in search of food.

Reproduction: Locomotion is also important for reproduction in animals and in some lower groups of plants (algae and bryophytes).

Certain fishes (Example salmon) live in sea water but they require fresh water for reproduction. So, they move from sea to fresh water at the time of reproduction.

Shelter: All living beings require a safe and comfortable shelter for living and reproducing.

Animals move from one place to another in search of a suitable shelter where they will have enough food and water as well as less threat from other predatory animals.

Search for a favourable environment: All plants and animals require various environmental components such as light, air, water, etc., for their survival. So, they move from one place to another in search of favourable environmental conditions required for their survival.

Different types of movement are observed in living organisms.

Ciliary Movements

Ciliary Movements Definition: The type of movement which includes rhythmic movements of cilia, present on the cell surface, is known as a ciliary movement.

Cilia are microscopic hair-like organelles found in eukaryotic cells. They project from the surface of the cells. They show movement which helps cells in locomotion. They also help the cells to remain anchored in the tissues.

Ciliary Movements Example:

1. In multicellular organisms such as human beings, cilia are found in some internal organiser
2. Cilia present in the respiratory tracts help to remove pathogens and dust particles. Cilia are also found in reproductive tracts. They help in the transportation of egg and sperm cells.
3. In unicellular organisms such as Paramoecium, cilia are used for locomotion.
4. Cilia is also found in the flame cells present in tapeworm. These cells are the excretory cells of tapeworms. The cilia of flame cells help to filter the body fluid by removing waste products from it.

Ciliary Movements Process:

1. The organisms use their cilia similar to an oar used in a boat.
2. Cilia can move both in anterior and posterior directions.
3. Cilia are arranged in bands or clumps. The movement of each cilium must be closely coordinated with the movements of other cilia.
4. Paramoecium uses its holotrichous cilia as a locomotory organiser During swimming, cilia move in a whip-like motion by producing two types of strokes water-power stroke and recovery stroke.
5. During a power stroke, cilia push the water backwards and give a forward push.
6. The power stroke is followed by the recovery stroke. In this movement, the cilia relax and return to their original position.
7. In this way, the rows of cilia move one by one which helps to move the animal in water.

Flagellar Movement

Flagellar Movement Definition: Some organisms’ locomotion by movement of a type of cytoplasmic filament called flagella, is known as flagellary movement.

A flagellum (Latin: flagellum means ‘whip’) is a lash-like appendage that protrudes from the cell body of certain prokaryotic and eukaryotic cells.

The primary role of the flagellum is the locomotion of the organism in a liquid medium, but it also often functions as a sensory organ.

Flagellar Movement Example:

1. Some unicellular organisms such as Euglena, Trypanosoma gambiense, etc., use their flagella for locomotion.
2. In some multicellular organisms such as sponges, a water canal system is found. The large, central cavity included in this system is known as spongocoel.
3. The inner wall of this spongocoel contains special flagellated cells known as choanocytes or collar cells.
4. Each cell contains a central flagellum. This flagellum helps in the unidirectional flow of water into the spongocoel.

Flagellar Movement Process:

1. Euglena and other flagellated protozoans move by the spiral undulation of their flagellum.
2. Euglena swims by lashing the flagellae sideways and the body is directed obliquely backward. This movement helps Euglena to move forward by pushing the water backwards.
3. The energy required for the movement is obtained by the breakdown of ATP into ADP and inorganic phosphate.

Amoeboid Movement

Amoeboid Movement Definition: The movement or locomotion that depends on the movement by extensions of the cell body called pseudopodia, is known as amoeboid movement.

Amoeboid movement is the most common mode of locomotion in eukaryotic cells. It is a crawling-like movement accomplished by cytoplasmic protrusion of the cells called pseudopodia.

Example:

1. This kind of movement can be found in different cells such as leucocytes and macrophages, etc., present in our body.
2. Unicellular organisms such as Amoeba, move from one place to another by this type of movement.

Amoeboid Movement Process:

1. By changing the viscosity of its protoplasm and with the help of microfibrils inside the cells, Amoeba gives rise to pseudopodia.
2. The pseudopodium attaches to the substratum (on which movement of Amoeba occurs) and microfibrils inside the cell contract to bring the rest of the cell towards the pseudopodium.
3. By repeating this movement, Amoeba moves from one place to another.

Muscular Movement

Muscular Movement Definition: The movement of the body which occurs with the help of muscles is known as muscular movement.

Example: Muscular movement is found in vertebrates and some invertebrate animals (annelids, arthropods, molluscs, etc.).

Muscular Movement Process:

1. Typical nature of muscles are excitability, contractility and elasticity.
2. Due to nervous stimulation, the muscle fibres contract and then return to their original state.
3. Some physiological processes occur due to the involuntary action of muscles. Some of such actions are peristalsis, movement of the heart muscles, etc.
4. The voluntary action of muscles also helps the organisms in the movement of their body parts as well as locomotion.

Skeletal Muscle Contractile Proteins And Muscle Contraction

Skeletal Muscle Contractile Proteins And Muscle Contraction Definition: The muscles which are attached to the bones, can contract voluntarily and are mainly involved in the movement of the bones, are known as skeletal muscles.

Skeletal Muscle Contractile Proteins And Muscle Contraction Location: In humans and other vertebrates, these muscles are attached to the endoskeleton by means of a thick band of collagenous connective tissues called tendons.

As these muscles are attached to the skeletal system, they are named as skeletal muscles.

Skeletal Muscle Contractile Proteins And Muscle Contraction Characteristics:

1. Skeletal muscle is responsible for the movement of various parts of the body and locomotion. These muscles are attached to the bones with the help of tendons.
2. These muscles are able to contract voluntarily.
3. These are striated.
4. Muscle fibres are the individual contractile units of muscle.
5. A number of undifferentiated, mononucleated muscle cells (myoblasts), unite to form a single, cylindrical, multinucleated cell (syncytium). These syncytia play a fundamental role in muscle function as they form the muscle fibres.
6. Every muscle cell is covered with a thin transparent sheath, known as sarcolemma. The cytoplasm of muscle cells is called sarcoplasm. The sarcolemma contains some deep invagination called T-tubules (or transverse tubules).
7. The bulk of fibre is arranged in the form of myofibrils that have characteristic striations due to the precise organisation of the contractile proteins actin and myosin.
8. The myofibrils are arranged in a section, called a sarcomere joined end to end all along the length of a muscle fibre.
9. Sarcomeres are separated by thin, comparatively dense, zigzag lines or discs called Z-line or Z-disc.
10. A myofibril has dark and light bands. The dark bands are called the A-band or anisotropic bands. The light bands are called l-band or isotropic bands.
11. The two bands are separated by the H-zone (Hensen zone).
12. The centre of the H-zone has a line called the M-line.
13. The myofibrils have characteristic striations on them due to the precise organisation of two contractile protein filaments—
    • Actin and
    • Myosin.
14. Adult skeletal muscle fibres have diameters between 10 and 100 /μm and lengths that may extend up to 20 cm.

Connective Tissue Covering Of Muscle Fibres

Skeletal muscle fibres are covered with layers of connective tissues.

Blood vessels, nerves, lymphatic vessels, etc., are present in this covering. This covering is divided into three layers—

1. Endomysium: The thin layer of areolar connective tissue around each muscle fibre is known as endomysium.
2. Perimysium: The thin sheath of connective tissue that covers a bundle of muscle fibres which are already surrounded with endomysium, is known as perimysium. Each bundle of muscle fibres covered with perimysium is known as fasciculus (plural: fasciculi).
3. Epimysium: The layer of connective tissue that surrounds some bundles of muscle fibres together is known as epimysium.

Skeletal Muscle Contractile Proteins Physiological properties: The physiological properties of skeletal muscles are as follows—

• Excitability: If a suitable stimulus is applied to a voluntary muscle fibre, then it will get excited. This phenomenon is known as excitability. The stimulus can be chemical, electrical, mechanical, thermal, etc.
• Contractility: Muscle fibres contract due to the excitation caused by the appropriate stimulus. This is known as contractility. It is an important property of muscle fibres. Mainly contractile proteins are responsible for the contraction of the muscle fibres.
• Refractory period: A voluntary muscle fibre contracts when a threshold stimulus (an optimum stimulus that could generate a contraction) is applied to it. After this, for a small interval of time, the muscle fibre will not contract further, if another stimulus is applied at this stage.
• The small interval of time is known as the refractory period. In mammals, the absolute refractory period is 0.002-0.005 seconds.

This period can be categorised into two types—

• The absolute refractory period is the initial period of the refractory period during which the muscle fibres will not contract even if a very strong stimulus is applied.
• The relative refractory period is the end period of the refractory period when the muscle starts to contract by the effect of a strong stimulus.

All-or-none law:

• According to this law or principle, the response of a muscle fibre to a stimulus is independent of the stimulus.
• If the muscle fibre is given a stimulus that exceeds the threshold potential, the nerve of the muscle fibre will give a complete response; otherwise, there is no response.
• The contraction will not increase further even if the force of the stimulus is increased. This is the all-or-none law.

Latent period: It is the time gap between the application of a stimulus and the generation of the response.

Simple Muscle Curve

Contraction and relaxation of muscle fibres occur due to excitation. The recording of relaxation and contraction of muscle fibres by the kymograph is known as a simple muscle curve.

Summation of stimuli: In case of a stimulus lower than the threshold level, no contraction occurs within the voluntary muscles.

But if the same stimulus is applied repeatedly, between the latent period and refractory period of the first stimulus, then contraction of muscles can be observed.

Individually the stimulus could not evoke a contraction. However, the summation of those separate stimuli was able to generate a contraction as their cumulative value exceeded the threshold level.

Muscle twitch: A muscle fibre contracts only once if it is stimulated by a single stimulus of adequate strength. It relaxes immediately after the contraction. Together, this individual event of contraction and relaxation of muscle is known as muscle twitch.

Tetanus: If a muscle fibre is introduced to a rapid succession of stimuli for a certain period of time, then the rate of stimulation will be so high that the muscle fibres will be unable to relax between the two stimuli.

This will lead the muscle to a state when muscle twitches are converted to sustained contraction as long as the stimuli continue. This is known as tetanus.

Rigour mortis: After death, the muscle cells can no longer produce ATP and therefore the cross-bridges between muscle fibres cannot be broken. This causes muscle stiffness called rigour mortis.

Fatigue: Muscle fatigue refers to the use-dependent decrease in the ability of a muscle to generate force.

The causes of fatigue are not entirely understood. In most cases, however, muscle fatigue is correlated with the production of lactic acid due to excessive working of the muscles.

Lactic acid is produced by the anaerobic respiration of glucose, which is obtained from glycogen present in the muscles and blood. Lactate production and muscle fatigue are therefore also related to the depletion of muscle glycogen.

Muscle Fatigue

During excess muscular activity or if a muscle fibre is stimulated continuously, the contracting ability of the muscle gradually decreases and ultimately fails to contract for some time. This condition is known as muscle fatigue.

Muscle Fatigue Causes:

• The large amount of energy required for contraction and relaxation of muscles is produced by the aerobic oxidation of glycogen.
• However, due to excess oxidation, the muscle cells receive insufficient oxygen.
• As a result of which, partial oxidation of the glycogen takes place. This oxidation produces lactic acid.
• Thus, the concentration of lactic acid increases in the muscles. The stored lactic acid stops the muscle contraction for some time and causes muscle fatigue.

Relief from muscle fatigue: If the fatigued muscles remain in the resting stage for some time, then they will get a sufficient amount of oxygen. As a result, complete oxidation occurs and the muscles can overcome the fatigued condition slowly.

Structure of skeletal muscle: A detailed description of this topic has been provided.

Muscle Fatigue  Types: Depending on structure and function, skeletal muscles are of different types.

On the basis of function: Details are discussed in the table given below.

On the basis of structure: On the basis of structure, skeletal muscles are of two types—white and red muscles.

White muscles: Some of the muscles possess a very small quantity of myoglobin and therefore, appear pale or whitish. These are categorised as white muscles.

The number of mitochondria is also few in them, but the amount of sarcoplasmic reticulum is high. They depend on anaerobic processes for energy.

Example: White muscles are mostly found in amphibians and birds.

Red muscles: Some muscles contain a high concentration of myoglobin and hence appear red in colour.

These muscles are called the red muscles. They contain a large number of mitochondria which produce ATP by aerobic metabolism.

These muscles, therefore, can also be called aerobic muscles. Example: Red muscles are found in mammals.

Structure Of Contractile Proteins

• Every skeletal muscle is composed of myofibrils. These myofibrils are composed of myofilaments.
• The proteins present in the myofilaments are responsible for the contraction and relaxation of myofibrils.
• These are known as contractile proteins. Mainly, two types of contractile proteins are found in human beings.

Actin Filament

The thin filamentous protein among the contractile proteins of myofibrils is known as actin filament. It is also known as secondary myofilament.

Actin Filament Number: Each myofibril is composed of about 3,000 actin filaments.

Actin Filament Characteristics:

1. Actin filaments are present in between two Z-lines (except the H-zone), generally all over the sarcomere.
2. The length and diameter of each fibre are lum and SOA respectively.
3. Actin filament is composed of mainly three types of proteins—actin, tropomyosin and troponin.
4. One end of these fibres remains attached to the Z-line (a zigzag line that separates two adjacent units of muscle or sarcomere) and another end is attached between the two myosin filaments.

Actin Filament Structure: The actin filament is composed of three types of proteins. They are discussed below.

1. Actin: An actin filament consists of actin monomers polymerised into a large molecule that looks like two strands of pearls twisted together.
2. Actin is found in two forms —G-actin and F-actin.
3. When actin is present in the form of small monomers, then it is known as G-actin. Its molecular weight is about 42,000 Da.
4. G-actin proteins form fibrous polymers in the presence of Mgz+. The filamentous structure formed by these fibrous polymers is known as F-actin.
5. Two F-actin fibres form a double helix which forms a single main actin filament (lpm in length).
6. During muscle contraction, myosin fibre attaches to the active site of the F-actin.

Tropomyosin:

• Tropomyosin is a type of long and thin protein molecule, present around the grooves of the actin chains.
• Its molecular weight is about 70,000 Da and its length is 40nm.
• During normal conditions, tropomyosin molecules remain attached to the active site of the F-actin.

Troponin: Troponins are small globular proteins present in tropomyosin at intervals. Each troponin molecule has three peptide subunits—

1. Troponin-T: Connects the troponin with tropomyosin,
2. Troponin-I: Prevents the attachment of myosin and actin.
3. Troponin-C: Helps in muscle contraction in the presence of Ca2+ ions. The troponin complex (troponin-C, troponin-l and troponin-T) together with tropomyosin, form the calcium-sensitive switch that activates muscle contraction.

Myosin Filament

The thick filamentous protein among the contractile proteins of myofibrils is known as myosin filament. It is also known as primary myofilament.

Myosin Filament Number: Each myofibril is composed of 1,500 myosin fibres.

Myosin Filament Characteristics:

1. These fibres are present in the A-band of the sarcomere.
2. The length and diameter of each myosin fibre are about 1.6 fim and 100A respectively.
3. Different types of myosin proteins are present in myosin fibres. Among them, myosin-II protein (also known as conventional myosin) helps in the contraction of skeletal muscles.
4. Both ends of the myosin fibres are free.

Myosin Filament Structure:

1. Each filament contains about 200 myosin molecules.
2. Each myosin molecule is composed of six polypeptide chains. Among these, two are heavy chains and four are light chains.
3. Each myosin fibre has a head region and a long tail part.
4. The long tails (heavy chains) of two myosin fibres wrap around each other to form a dimer, which in turn aggregates to form the myosin filament.
5. Their head regions (light chains) are lined precisely opposite to the actin filaments.
6. When viewed in transverse sections, the actin filaments form a hexagonal pattern around each myosin filament.
7. The bundles of myosin filaments are held in a centred position within the sarcomere by a protein called titin.
8. The myosin head contains a specific binding site for actin fibre.
9. The head can act as an ATPase enzyme. Due to this property, it gathers energy for contraction by hydrolysis of the ATP molecules.

Contraction Of Skeletal Muscle

• The region of the sarcolemma that lies just below the terminal end of the axon, is known as the motor endplate.
• The synapse or junction formed between the axon terminal and the motor endplate is known as a neuromuscular junction.
• Each branch of the axon ends in an axon terminal that lies in close proximity to the sarcolemma of a muscle fibre.
• These neurons release acetylcholine with the help of Ca+2 ions in response to action potentials (i.e., change in j electrical potential in cells during a nerve impulse).
• Acetylcholine is a chemical substance that transmits the impulse between the nerve and muscle cell, hence, called a neurotransmitter. Acetylcholine is present at the neuromuscular junction.
• The contractile proteins (actin and myosin) cause contraction of muscle fibres when the impulse is transmitted.
• Muscle fibres are innervated and are stimulated to contract by motor neurons.
• These neurons have their cell body in the spinal cord and their axons are projected outside to enter the effector organiser
• The axon of one motor neuron has several branches and can stimulate a few to several muscle fibres of a particular muscle.

Neuromuscular Junction

• The junction between the axon terminal of a motor neuron and the skeletal muscle fibre is known as a neuromuscular junction or motor end plate.
• The nerve impulses from the motor neurons cross the neuromuscular junction and are transported to skeletal muscles by the neurotransmitter molecules.
• Here the end of the motor neurons become flat, broad and extended which is known as end feet.
• The membrane present at the nerve end is known as the presynaptic membrane. This portion is rich in mitochondria. The spaces between the motor neurons and the sarcolemma of the skeletal muscles are known as synaptic clefts.
• Synaptic cleft contains numerous vesicles containing acetylcholine, a neurotransmitter. These vesicles are called synaptic vesicles. The sarcolemma is folded at the neuromuscular junctions of the skeletal muscles.
• These are known as postsynaptic membranes. The folds that occur in the membranes are known as subneural clefts.
• These clefts contain many receptor proteins. The acetylcholine molecules are released by the synaptic vesicles in the synaptic cleft.
• These neurotransmitters are transported to skeletal muscle fibres by binding with acceptor proteins present in the subneural clefts.

Mechanism of contraction of skeletal muscle was The mechanism of contraction of skeletal muscle best explained through—‘Sliding filament theory’. This was proposed by two groups of scientists—A. F.

Huxley and R. Nierdergerke, 1954; H. E. Huxley and J. Hansen, 1954.

According to this theory, events of muscle contraction are divided into two parts—

Physical events of muscle contraction, and biochemical and electrical events of muscle contraction.

Physical Events of Muscle Contraction:

• During muscle contraction, thick myosin fibres and thin actin fibres combine with each other by forming a cross-bridge. After this, myosin fibres pull the actin fibres towards the centre of the sarcomere.
• At this time Z-lines of the muscle fibres come closer to each other. As a result, the length of the sarcomere decreases.
• Contraction of l-band and H-zone (Heller zone from ‘heller’ in German, meaning brighter) takes place. The M-line crossing the H-zone centrally is also called the Micellar line or programme line.
• Only the length of the A-band remains unchanged.
• Due to contraction, the length of the muscle decreases but the volume remains the same.

Biochemical and electrical events during muscle contraction: The steps of biochemical and electrical events during muscle contraction are as follows—

Discharge of neurotransmitter by motor nerve: The motor neurons arising from the spinal cord or brain stem, release the neurotransmitter acetylcholine at the neuromuscular junction. In response to this, action potentials travel down the axon towards the skeletal muscle cell.

Binding of neurotransmitter at skeletal muscle fibre: The amount of acetylcholine released is proportional to the frequency of action potentials travelling down the axon. Acetylcholine diffuses across the synaptic cleft and binds to a receptor protein, the nicotinic receptor, on the muscle cell membrane.

Generation of Action Potential at Skeletal Muscle Fibre:

• The nicotinic receptors form channels that allow sodium and potassium ion influx to the muscle cells.
• This causes partial depolarisation of the cell membrane in the synaptic cleft.
• This depolarization generates an endplate action potential or action potential. This action potential acts as an indicator of muscle contraction.

Release of Ca2+ ions:

• Muscle fibres store Ca2+ in a modified endoplasmic reticulum known as sarcoplasmic reticulum (SR).
• When a muscle fibre is stimulated to contract, an electrical impulse travels into T tubules. This triggers the release of Ca2+ from the sarcoplasmic reticulum.

Release of the Active site of Actin Filament by Ca2+ ion:

• When a muscle is relaxed, myosin heads are unable to bind to actin.
• This is because the active sites (attachment sites) for the myosin heads on the actin are blocked by tropomyosin.
• Thus, myosins are unable to bind with actin. Tropomyosin is held in place by troponin.
• When the concentration of Ca2+ is raised in the sarcoplasm, Ca2+ binds to troponin. This causes the conformational change in troponin. This causes the troponin-tropomyosin complex to be shifted away from the active sites.

Formation of Energised Myosin Heads:

• An ATP molecule attaches to the active site of the myosin head. The head acts as an ATPase enzyme.
• This causes hydrolysis of the ATP in the presence of Ca2+ and Mg2+ to produce ADP and Pi. The myosin head takes up the energy released and becomes highly energised.

Formation of Cross-Bridges:

• The energised myosin heads are attached to actin by forming cross-bridges.
• After binding, the myosin heads release the inorganic phosphate and initiate an inward pull called power stroke.
• This causes inward bending of myosin filament, thereby shortening the sarcomere.
• The myosin heads then release the ADP molecule but still remain tightly bound to the active, until a new ATP molecule is bound to it. Then the heads separate from the actin filament.
• This process occurs in a cyclic manner as long as ATP and Ca2+ are present in the sarcoplasm. This is how actin slides and contraction of muscles takes place.

Types of Muscle Contraction are—

1. Isometric muscle contraction, and
2. Isotonic muscle contraction.

Isometric Muscle Contraction: The muscle contractions in which the length of the muscle remains the same but tension or force increases, are called isometric muscle contractions.

The term ‘isometric1 has been derived from the Greek words isos meaning equal and metric meaning measuring

The characteristics of Isometric Muscle Contractions are—

Isometric contractions occur as a phase of normal muscle contraction but also provide tautness and stability to the body.

• The weight of the muscles is more than the force of this contraction.
• No mechanical work is carried out by this type of contraction in muscles.
• Most of the free energy produced during this type of contraction of the muscles is released as heat energy.

Example: The contraction of muscles during chewing, writing, etc.

Isotonic Muscle Contraction: The muscle contractions in which the tension within the muscle during contraction remains the same but the length of the muscle changes are called isotonic muscle contractions.

The term ‘isotonic’ has been derived from two Greek words—isos meaning equal and tonic meaning strength.

The characteristics of Isometric Muscle Contractions are—

Isometric contractions occur as a phase of normal muscle contraction but also provide tautness and stability to the body.

• The weight of the muscles is more than the force of this contraction.
• No mechanical work is carried out by this type of contraction in muscles.
• Most of the free energy produced during this type of contraction of the muscles is released as heat energy.

Example: The contraction of muscles during chewing, writing, etc.

Isotonic Muscle Contraction: The muscle contractions in which the tension within the muscle during contraction remains the same but the length of the muscle changes are called isotonic muscle contractions.

The term ‘isotonic’ has been derived from two Greek words—isos meaning equal and tonic meaning strength.

Changes occurring during muscle contraction The changes occurring during muscle contraction can be divided into several types.

Physical Change: The length of the muscle fibres decreases and thickness increases during muscle contraction. The volume of the muscles remains unchanged.

Chemical Change: The energy required for muscle contraction is produced by hydrolysis of ATP.

Due to repeated hydrolysis of ATP, its amount gradually decreases in the cells. It is replenished by various chemical reactions. These chemical reactions are discussed under separate heads.

Glycolysis, Krebs cycle and terminal respiration: Synthesis of ATP, C02, water, lactic acid, etc., take place due to these chemical reactions.

Changes in creatine phosphate or phosphagen: Creatine phosphate is a highly energised compound. This compound is present in large amounts in muscle cells. It supplies inorganic phosphates during the conversion of ADP into ATP during muscle contraction.

Change in pH: During the initial stage of normal muscle contraction, the pH remains basic. But if the muscle contracts for a prolonged period of time, then due to the accumulation of lactic acid, the pH becomes acidic. ATP synthesis also occurs during this stage.

Thermal change: During muscle contraction, some of the energy is released in the form of heat energy. This energy is released in three forms—

The heat of activation: The heat energy released from the muscles just after receiving the stimulus, prior to contraction is called heat of activation. The amount of heat gradually decreases with muscle contraction.

Heat of shortening: The heat energy produced due to various structural changes during muscle contraction is called heat of shortening.

Recovery heat: The heat energy produced after muscle contraction, during re-synthesis of substances dissociated during muscle contraction, is called recovery heat.

Electric charge: The electric potential found in the muscles during their resting stage, is known as resting potential.

Its value is -70 mV. During muscle contraction, different ions are transported between the intracellular and extracellular fluids in the stimulated region of the sarcomere.

As a result, the resting potential is converted to the action potential. Its value is +35 mV.

Functions Of Skeletal Muscle

The functions of skeletal muscles are discussed below.

Movement of body parts: The skeleton muscles allow flexible body movement. These muscles remain attached to bones with the help of tendons. So, the muscle contraction leads to movement of different body parts.

Storage of glucose: Excess glucose present in the blood converts into glycogen by the process of glycogenesis. The glucose remains stored in the skeletal muscles as glycogen.

Maintainance of body structure: Skeletal muscles along with the bones maintain the structure of the body.

Maintainance of body balance: Skeletal muscles help to maintain the body balance.

Respiration: Skeletal muscles, especially the intercostal muscles, help in respiration.

Locomotion in animals is accomplished by the force of muscles acting on a rigid skeletal system.

The skeletal system consists of the bones and joints, along with cartilage and ligaments.

There are two types of skeletal systems found in the animal kingdom—exoskeleton, and endoskeleton. These are discussed under separate heads.

Exoskeleton: Exoskeletons can be of different forms, including horns, claws, nails, hooves, scales, etc.

• In general, they are hard outer coverings, that provide physical protection to soft internal body parts of animals. Exoskeletons also provide sites for muscle attachment. In most of the species, this structure surrounds the body as a rigid hard case.
• Arthropods, such as crustaceans and insects, have exoskeletons made of the polysaccharide, chitin. Having an exoskeleton also limits the size of the animal.
• Animals with exoskeletons cannot grow too large. This is because if the animal grows larger, its exoskeleton will become thicker and heavier and will cause hindrance in movement and locomotion.

Endoskeleton: An endoskeleton consists of hard, mineralised structures located within the soft tissues of organisms. The endoskeleton is present in vertebrates.

An example of a primitive endoskeletal structure is the spicule of sponges. The bones of vertebrates are composed of tissues, whereas sponges have no true tissues.

Endoskeletons provide support to the body, protect internal organs, and allow movement through the contraction of muscles, attached to the skeleton.

Description Of The Human Skeletal System

The skeletal system, in an adult body, is made up of 206 individual bones.

The endoskeleton is arranged into two major divisions—the axial skeleton and the appendicular skeleton.

The tissues of the axial and appendicular skeletons are bone (both compact and Fh spongy), cartilage (hyaline, fibrocartilage, and elastic cartilage), and dense connective tissue (a type of fibrous connective tissue).

Axial Skeleton

Axial Skeleton Definition: The bones of the skeleton, which form the main longitudinal axis of the body in humans and other vertebrates, are together known as the axial skeleton, It is formed of 80 bones.

The appendicular skeleton is attached to the axial skeleton. The main parts of the axial skeleton are—the skull, thoracic cage and vertebral column.

Skull

The skull is the bony, hollow, round-shaped structure of the axial skeletal system, that protects our brain.

Characteristics:

1. The skull is composed of 29 bones, that are fused together, except the mandible.
2. These skull bones are not fused in children, but rather loosely attached with connective tissues.
3. This allows the growth of the skull. In adults, the skull bones fuse to give added strength and protection to the soft tissues and brain.

Parts: The four main parts of the skull are—the cranium, facial bones, hyoid bone and auditory ossicles.

Cranium: The box-shaped structure made of 8 flat bones, present at the superior portion of the skull is known as the cranium.

The characteristics of the cranium are as follows—

1. It is formed of 8 flat bones.
2. These bones are separated from each other by immovable joints called sutures.
3. The cranium is composed of one frontal bone, two parietal bones, one occipital bone, two temporal bones, one sphenoid bone, and one ethmoid bone.
4. An orifice, known as the foramen magnum, is present behind the skull. The spinal cord remains attached to the brain through this orifice.
5. The cranium is also known as the neurocranium and rainbow, as it contains the brain. It protects the brain from damage.

Facial bones: The bones of the inferior and anterior portions of the skull are known as facial bones.

The characteristics of facial bone are as follows—

• There are 14 facial bones.
• These bones generally form both the jaws, some sense organs like eyes, nose, ears, mouth and a part of the nasal passage.
• The face is composed of one mandible (the lower jaw), the only movable bone in the skull.
• Other bones of the face are two maxillae (upper jaw), two palatine, two zygomatic, two lacrimal, two nasal, one vomer and two inferior nasal conchae.
• These bones support the eyes nose and mouth.

Hyoid bone: The hyoid bone is a small, U-shaped bone found just inferior to the mandible.

It is the only bone in the body that does not have any joint and it is a floating bone.

The functions of the hyoid bone are as follows—

• The hyoid bone helps in various movements of the tongue, larynx and pharynx.
• Its function is to hold the trachea open and to form a bony connection for the tongue muscles.

Auditory ossicles:

• The three bones present in the ear malleus, incus, and stapes, are collectively known as the auditory ossicles.
• They are found in a small cavity inside the temporal bone. These bones are among the smallest bones in the human body.
• Malleus, incus and stapes are commonly known as hammer, anvil and stirrup, respectively.

The functions of auditory ossicles are as follows—

The ossicles help to transmit vibrations of sound waves, from the tympanum through the middle ear to the oval window of the inner ear.

They also protect the ear from damage caused by loud sounds.

Functions of the skull:

• The skull protects the brain.
• It also protects our main sensory organs—eyes, ears, nose and tongue.
• Jaws and teeth help in the mastication of food.
• The skull also protects the pituitary gland located at a depression (sella turcica) on the sphenoid bone of the skull.
• The opening present at the end of the skull called the foramen magnum, connects the spinal cord to the brain.

Thoracic Cage (Rid Cage)

A frame-like structure formed by flat bones, that is present within the thoracic region is known as a thoracic cage.

Thoracic Cage (Rid Cage) Characteristics:

• The rib cage, sometimes called the thoracic cage is the arrangement of bones in the thorax region.
• It is formed by thoracic vertebrae 12 of the vertebral column, ribs and associated cartilages, and sternum.
• The rib cage encloses the heart and lungs.

Parts: The parts of the rib cage have been described under separate heads below.

Ribs: The 12 pairs of narrow and flat bones, originating from both sides of the vertebral column, are known as ribs.

Characteristics:

• All twelve pairs of ribs are attached to the thoracic vertebrae of the vertebral column,
• The first seven pairs of ribs (upper ribs) connect the thoracic vertebrae directly to the sternum and are known as “true ribs”,
• The next three pairs of ribs (Ribs 8, 9, and 10) originated from the 8th, 9th and 10th thoracic vertebrae and are connected to the sternum indirectly via the costal cartilage of the 7th rib, and due to this they are considered as “false ribs.”
• The last two pairs of ribs (11 and 12), are not attached to the sternum. They remain free in front, and are termed as “floating ribs”.

Function: The functions of the ribs are, the formation of the thoracic cage and also protection of internal organs such as the lungs, heart, etc. The elasticity of the false ribs allows movement of the rib cage during respiratory activity.

Sternum; The sternum, or breastbone, is a thin, knife-shaped bone located at the anterior side of the thoracic region along the midline of the body.

Characteristics:

• The sternum is connected to the ribs by thin bands of cartilage, called the costal cartilage,
• The sternum is composed of three bones—manubrium, body, and xiphoid process, that fuse during foetal development.

Function:

• The sternum participates in the formation of the thoracic cavity,
• It helps to join different muscles.
• It also protects the heart, aorta, vena cava, etc.

Functions of the thoracic cage:

• The thoracic cage protects the heart, lungs and the main blood vessels.
• The functions of ribs are dependent on the function of intercostal muscles.
• Due to the action of these muscles, ribs help the lungs to expand and relax during breathing. Thus, the ribs play a key role in respiration.
• In humans, ribs contain red bone marrow throughout their life. So, they produce RBCs, some WBCs and platelets in them.
• The two pairs of floating ribs protect the liver and other organs in the abdominal cavity.

Vertebral column

A long pillar-like structure, composed of bones called vertebrae, present as the longitudinal axis, at the dorsal side of the body, is called the vertebral column.

Characteristics:

1. The vertebral column, also known as the backbone or spine, extends from the skull to the pelvis.
2. It consists of 33 bones, (In an adult person it becomes 26 in total due to the fusion of some lower vertebrae) the vertebrae.
3. They are separated by pads of fibrocartilage, known as the intervertebral discs.
4. The vertebral column forms the vertical axis and is located in the middorsal region.
5. The skull rests on the superior end of the vertebral column.
6. The vertebral column supports the rib cage and serves as a point of attachment for the pelvic girdle.
7. The vertebral column also protects the spinal cord (part of the central nervous system), which passes through a hollow space called the vertebral canal, created by vertebrae.
8. They are named according to their location, such as—cervical vertebrae
9. Present at the neck region, thoracic vertebrae
10. (12) Present at the thorax region, lumbar vertebrae (5) present at the lower back region, sacrum or sacral vertebra (1) present at the end of the spine and coccyx or caudal vertebra (1) present as the tailbone.
11. With the exception of the sacrum and coccyx, each vertebra is named as the first letter of its region and its position along the superior-inferior axis.
12. For example, the most superior thoracic vertebra is called Tx and the most inferior is called T12.
13. The cervical, thoracic and lumbar vertebrae remain separate and are known as movable vertebrae.
14. In adults, 5 vertebrae fuse together to form the sacrum and 4 vertebrae fuse to form the coccyx.
15. Every vertebra has a large, anterior middle portion called the centrum or body. Each vertebra also contains a central hole, called vertebral foramen (plural—foramina). The foramina of the vertebrae together form a hollow channel, called a neural canal. The spinal cord passes through this channel.
16. The vertebral column is not a straight structure, rather it is curved.

• Curvatures of the vertebral column: There are four curves present in the vertebral column known as curvatures.
• Cervical curvature: The posterior convex curve, present in the region of neck vertebrae, is known as the cervical curve or curvature.
• Thoraciccmvatme:‘The anterior concave curve, present at the chest region, is known as thoracic curvature.
• Lumbar curvature: The posterior convex curve, present in the abdominal region, is known as lumbar curvature.
• Sacral curvature: The anterior concave curve, present Sacml curvature.

Functions of the vertebral column:

1. It acts as the longitudinal axis of the axial skeleton, and the skull is situated on it.
2. The vertebral column protects the spinal cord running through it.
3. It also maintains body balance.
4. The intervertebral discs provide the flexible movement of the vertebral column.

Appendicular Skeleton

Appendicular Skeleton Definition: The skeleton, attached to the axial skeleton that includes the bones of girdles and limbs is known as the appendicular skeleton.

The appendicular skeleton is made up of 126 bones. The appendicular skeleton has four parts. These pectoral girdle, arms, pelvic girdle, and legs.

Pectoral Girdle

The part of the skeleton, which connects the arm bones to the axial skeleton, is known as the pectoral girdle.

Pectoral girdle Characteristics:

• The pectoral girdle connects the upper limb (arm) bones to the axial skeleton.
• The pectoral girdle (shoulder girdle) contains four bones— two clavicles and two scapulae.
• The bones of this girdle are weakly attached and held in place by ligaments and muscles.

Pectoral Girdle Functions: It supports the arms and serves as a point of attachment for muscles, responsible for the movement of the arms.

Arms

The arms are part of the skeleton, which includes the bones of the upper arm, forearm, wrist joint, palm and five fingers

Arms Characteristics: Arms are attached to the pectoral girdle. They are composed of 60 (30 in each upper limb) bones.

The characteristics of the bones of the arms are discussed in the table below.

Arms Functions: Arms Help The Animal to perform different works such as grasping climbing etc.

Pelvic Girdle

The part of the skeleton, connecting the bones of the legs to the axial skeleton, is known as the pelvic girdle.

Pelvic Girdle Characteristics:

1. The pelvic girdle is formed by the left and right of the coxae or hip bones.
2. The pelvic girdle also contains the sacrum and coccyx, wedged in between two hip bones.
3. The true pelvis is the portion of the trunk bounded by the sacrum, lower ilium, ischium, and pubic bones. It is inferior to the false pelvis.
4. The upper part of the ilium is broad and flat, which is connected to the sacral segment of the vertebral column and forms the sacroiliac joint.
5. Pubis and ischium remain attached at the lower part of the ilium.
6. The obturator foramen (a hole) is present between the pubis and ischium.
7. The cup-shaped cavity of the pelvic girdle is known as the acetabulum. The femur remains attached to the pelvic girdle by forming a ball and socket joint with this acetabulum.
8. The ends of the pubis are connected with slightly movable joints and are known as symphysis pubis.

Pelvic Girdle Functions:

1. The strong bones of the pelvic girdle bear the weight of the body.
2. The pelvis also serves as the point of attachment for the lower limbs. It provides protection to the urinary bladder, reproductive organs, and a portion of the large intestine.

Legs

The legs are the lower parts of the skeleton which include the bones of the thigh (femur), kneecap (patella), leg (tibia and fibula) and foot (tarsals, metatarsals and phalanges)

Legs Characteristics: Legs are attached to the pelvic girdle. They are composed of 60 (30 in each leg) bones. The characteristics of the bones of the legs are discussed in the table below

Legs Functions:

1. In infants, the patella gives support to the knee during walking and crawling.
2. The tibia bears almost the whole weight of the body.
3. The fibula is mainly a point of muscle attachment and it maintains the body balance.

Functions Of The Human Skeletal System

The endoskeleton of a human being is composed of bones, cartilage, ligaments, etc. It plays various roles in our body.

Human Skeletal System Structure formation: It forms a rigid structure of the body and provides mechanical strength to it.

The endoskeleton is covered with muscles, skin, etc., which provide a definite shape to the body.

Human Skeletal System Role in increasing length: During adolescence, the cartilage present at the end of the long bones divides frequently causing the length of the bones to increase.

As a result, height increases during adolescence. Protects various internal organs: It protects various important parts of our body, such as the brain, neural canal, lungs, etc., from external damage.

Human Skeletal System Helps in movement and locomotion: Skeletal muscles move due to the activity of voluntary muscles. This causes movement of different parts of the body and bipedal locomotion in human beings.

Human Skeletal System Acts as storage: Stores minerals, such as calcium and phosphate. It also maintains the balance of minerals in the blood.

Human Skeletal System Production of blood cells:

• During birth red bone marrow is present in all the bones. These are the main sites for RBC formation.
• Besides RBCs, some WBCs, such as neutrophils, basophils, eosinophils, etc., are also formed from red bone marrow.
• In human beings, platelets are formed from human beings, platelets are formed from megakaryocyte cells.

Human Skeletal System Role in hearing: Helps in hearing by transmitting sound waves to the inner ear through the middle ear.

The auditory ossicles act as an amplifying device for sound waves and also protect the inner ear from loud sound.

Human Skeletal System Role in breathing: The sternum and ribs of the thoracic cage help in breathing. They enable the expansion and relaxation of the lungs.

Bone Joints Or Articulations

Bone Joints Or Articulations Definition: A joint or articulation is a point of contact between bones, between a bone and a cartilage, or between a bone and a tooth.

The study of joints is termed as arthrology.

Bone Joints Or Articulations Types: Depending on structure and function, there are various types of joints in our body.

Immovable Bone Joints Or Synarthroses

Immovable Bone Joints Or Synarthroses Definition: The synarthroses (singular: synarthrosis) or immovable bone joints are the type of joints, where two or more bones are joined together by fibrous tissue and are unable to move.

These are also known as fibrous joints. Immovable joints are mainly of three types—

1. Suture, bones with irregular surfaces, are tightly joined with each other by fibrous connective tissues. These types of fibrous joints are known as sutures. Example joints present between bones of the cranium.
2. Gomphosis, which are immovable fibrous joints, where a pointed or peg-like portion of a bone is inserted into the socket or cavity of another bone, are known as gomphosis (singular: gomphosis). Example joints that bind the teeth in their sockets in the maxillary bone and mandible.
3. Syndesmosis, are immovable joints, where the bones are connected by connective tissue or collagen fibres, are known as immovable fibrous joints or syndesmoses (singular: syndesmosis). For example the joint between the tibia and the fibula.

Slightly Movable Bone Joints Or Amphiarthroses

Slightly Movable Bone Joints Or Amphiarthroses Definition: The slightly movable bone joints or amphiarthroses (singular: amphiarthrosis) joints are the type of joints, where two or more bones are joined together by collagen fibres or cartilage and are able to move slightly.

These are also known as cartilaginous joints. Slightly movable joints are mainly of two types—

• Synchondrosis, these are slightly movable joints, where the bones are joined with each other by cartilage, known as synchondroses (singular: synchondrosis). For example the first pair of ribs are joined with the sternum by synchondrosis.
• Symphysis, these are slightly movable joints, where the bones are joined with one another by fibrous cartilage, are known as slightly movable cartilaginous joints or symphysis (singular: symphysis).
• For example joints between two adjacent vertebrae and pubic symphysis between two hip bones.

Freely Movable Bone Joints Bone Or Synovial Bone Joints Or Diarthroses

Diarthroses Definition: The freely movable joints or synovial bone joints or diarthroses (singular: diarthrosis) are the type of joints, where the bones are connected to each other through a joint cavity.

On the basis of structure and function, synovial joints are of six types.

Ball and socket joint: In a ball-and-socket joint, a rounded, ball-like end of one bone fits into a cup-like socket of another bone. This organisation allows movements of the bones in all directions. Example the shoulder and hip joints.

Hinge joint: In the hinge joint, the slightly rounded end of one bone fits into the slightly hollow end of the other bone.

These bones are attached in such a manner so that movement will be possible only in one plane. The movement of these joints is similar to the hinge of a door. Example the elbow and knee joints.

Plane or planar joint or gliding joint: In the plane or planar joints, bodies with flat or slightly curved articulating surfaces are packed closely together or held in place by ligaments.

These joints allow gliding movements and hence, these joints are also referred to as gliding joints. Example the articulation between carpal bones in the hand and the tarsal bones of the foot.

Structure Of Synovial Joint

Articular cartilage: The cartilage, that lines the epiphyses of the bones, present at the synovial joint, is known as articular cartilage.

The cartilaginous covering makes the end of the bones smoother reducing friction during movement of the joint. It contains a layer of connective tissue known as perichondrium.

Capsular ligament: These ligaments form capsules and are present at the outer surface of the synovial joints. These ligaments are known as extra-capsular ligaments.

Sometimes inner parts of the bones present at these joints also remain connected with some ligaments. These are known as intra-capsular ligaments. The capsular ligaments keep the bones in their proper position.

Synovial membrane and synovial fluid: A soft layer of glandular epithelial tissue is found between the articular capsule and the synovial cavity.

The space covered with the synovial membrane between the bones of the synovial joint is known as a synovial cavity.

The glands of the synovial membrane secrete a fluid known as synovial fluid. The synovial cavity is filled with this synovial fluid. Synovial fluid provides nutrients to the surrounding cells and reduces friction during movement.

Pivot joint or trochoid joint: In the pivot joint, the rounded end of one bone fits into a shallow pit formed by the other bone.

This structure allows rotational movement, as the rounded bone moves around its own axis. For, the joint of the first and second vertebrae of the neck that allows the head to move back and forth is of this type.

Angular or ellipsoid or condyloid joint: In a condyloid joint, an oval-shaped end of one bone fits into a similar oval-shaped hollow of another bone.

This is also sometimes called an ellipsoidal joint or angular joint. This type of joint allows movement in two directions, side to side and back to forth. For example, the joints of the wrist and fingers can move both sideways. and also up and down.

Saddle joint: In a saddle joint, concave and convex portions of the two bones fit together, just like a saddle and a rider.

Saddle joints allow movements similar to condyloid joints. For example, the thumb joint, can move back and forth and up and down. It can move more freely than the wrist or fingers.

Disorders Of Muscular And Skeletal System

Improper functioning of the muscular and skeletal system can cause several disorders. Some of them are discussed below.

Myasthenia Gravis

Myasthenia Gravis Definition: Myasthenia gravis is an autoimmune disorder that affects the neuromuscular junction at the post-synaptic level.

Myasthenia Gravis Symptoms:

• It is characterised by weakness, fatigue and paralysis of voluntary muscles.
• In some patients, myasthenia gravis may affect only the muscles of the eye, while others experience multi-system involvement.
• Muscle weakness in the eye may lead to blurred or double vision (diplopia). Difficulty in chewing, speaking, or swallowing may also be the initial symptoms.

Myasthenia Gravis Cause: It is an autoimmune disease. It is caused by the production of antibodies produced by one’s own body against nicotinic acetylcholine receptors (AchR) of the muscles.

In the case of myasthenia gravis, antibodies attack and destroy the acetylcholine receptors needed for muscle contraction. As a result, the muscles become weak.

Myasthenia Gravis Prevention: A high amount of cortisol hormones are used to treat this disease

Tetany

Tetany Definition: Tetany is a disorder marked by rapid muscular spasms, caused by malfunction of the parathyroid gland and consequent hypocalcemia.

Tetany Symptoms:

• It is neuromuscular activity and associated sensory disturbance.
• Tetany is characterised by muscle cramps, spasms or tremors. These occur within the muscles when muscle contracts uncontrollably.
• Tetany may occur in any muscle of the body, such as in the face, fingers or calves. The muscle cramping associated with tetany can be long-lasting and painful.

Tetany Cause: A common cause of tetany is very low levels of calcium in the body. Other common causes of tetany include—alcohol abuse, alkalosis (elevated pH of the blood), hypoparathyroidism, malnutrition, pancreatitis, vitamin D deficiency, etc.

Tetany Prevention: It can be treated by introducing a proper dose of calcium.

Muscular Dystrophy

Muscular Dystrophy Definition: Muscular dystrophy is a rare hereditary muscle disease, characterised by progressive damage of skeletal muscle.

Muscular Dystrophy Symptoms:

• Weakness and wasting (atrophy) of various voluntary muscles of the body are clinical features of this disease.
• Depending on the muscular dystrophy subtype, the disorders affect different muscles and have different effects with respect to age, severity and pattern of inheritance.

Muscular Dystrophy Cause: It is a genetic disorder. The absence or defective structure of dystrophin, a muscle cell structural protein, is the basic cause of this disorder.

Muscular Dystrophy Prevention: Presently, no cure for muscular dystrophy is available. Physiotherapy and physical exercise may be helpful to patients suffering from this disorder.

Arthritis

Arthritis Definition: Arthritis is a disease caused by inflammation of one or more joints, leading to pain and difficulty in movement.

Arthritis may be of different types.

Osteoarthritis form of arthritis or osteoarthrosis many bone joints of the body may undergo degenerative changes. This disease is very common in elderly persons.

Arthritis Symptoms:

Painful and swollen joints are the main symptoms of this disease. The pain may reduce temporarily after resting.

Generally, the cartilaginous covering of bones within the joint gets damaged, partially or completely.

This causes friction between the osteoarthritis of arthritis or osteoarthrosis many bone joints of the body may undergo degenerative changes. This disease is very common in elderly persons.

Arthritis Symptoms:

• Painful and swollen joints are the main symptoms of this disease. The pain may reduce temporarily after resting.
• Generally, the cartilaginous covering of bones within the joint gets damaged, partially or completely. This causes friction between the bones. This flattens the bones at the articulation.
• Due to the loss of the cartilage, the joint loses its flexibility and joint becomes stiff. Some patients experience a grating sensation when they move the joint or walk.
• The synovial membrane swells up due to continuous friction, causing excess synovial fluid to be collected within the joints. This may result in swelling of the joints.

Arthritis Cause:

1. Osteoarthritis begins in the cartilage.
2. In obese people of age 65 or above, the cartilage begins to get damaged slowly.
3. This causes frictional erosion of two opposing bones which results in inflammation of the outer capsule of the joint.

Arthritis Prevention:

1. Osteoarthritis cannot be cured.
2. It can be controlled by regular exercise, reducing body weight and changing lifestyle.
3. In severe cases, joint replacement through a surgical process may be done to get relief.

Rheumatoid Arthritis

It is the most common type of autoimmune disease which is triggered by faulty immune systems.

Rheumatoid Arthritis Symptoms:

• The patient often finds that the same joint on both sides of the body becomes painfully swollen and stiff.
• The smaller joints are usually noticeably affected. The joints of fingers, arms, legs and wrists are the most affected ones.
• The joint is tender to touch.

Rheumatoid Arthritis Cause:

• It is an autoimmune disease.
• It is caused when a special antibody or rheumatoid factor attacks the tissues.
• It damages the synovial membrane at the joints. It is more common in females than males.

Rheumatoid Arthritis Prevention: Presently several disease-modifying antirheumatic drugs (DMARDs) are used to stop the progression of the disease and to protect joints under the supervision of doctors

Osteoporosis

Osteoporosis Definition: A bone disorder, in which the density and strength of bones get reduced, is known as osteoporosis.

Osteoporosis, literally meaning porous bone, is a severe disorder of bones.

Osteoporosis Symptoms:

1. Bones become so weak that they are no longer able to support the body weight.
2. Bones can break, even under slight pressure.
3. Chronic back pain is another symptom caused by osteoporosis. This pain can worsen even when a person makes very less movements, such as during regular activities, or while coughing, laughing and sneezing.

Osteoporosis Cause:

1. Osteoporosis occurs when bone tissues and minerals are lost faster than they are replaced.
2. In females, at the menopause stage, oestrogen secretion reduces gradually.
3. This can cause osteoporosis.
4. In males, with increasing age secretion of testosterone reduces gradually, which can cause osteoporosis.

Osteoporosis Prevention:

1. A proper diet is essential to prevent osteoporosis.
2. Vitamin D and a calcium-rich diet are essential to prevent as well as to treat this disease.

Fracture And Dislocation

Fracture: It is a medical condition where the continuity of the bone has been broken.

It can occur due to high force or stress. When a bone breaks into two pieces it is known as a simple fracture. When it breaks into more than two pieces then it is known as a compound fracture.

Dislocation: It is the separation of bones from a joint. In this condition, bones are no longer in their normal position.

Gout

Gout Definition: Gout is an ancient and common form of inflammatory arthritis caused by to accumulation of uric acid crystals within the bone joints.

Gout Symptoms:

1. It is the most common inflammatory arthritis among men. Gout causes sudden painful joint inflammation, usually in one joint.
2. Joint inflammation causes pain and redness of the joint.

Gout Cause: Gout is a rheumatic disease, resulting from the accumulation of monosodium urate (sodium salt of uric acid) crystals at the joint. Uric acid is a byproduct formed due to the breakdown of purines.

Hyperuricemia includes heredity, obesity, certain medications such as diuretics and chronic kidney malfunction.

Gout Prevention: Chronic gout can be treated by using medications that lower the uric acid level in the body.

Locomotion And Movement Notes

1. Autoimmune disorder: A disorder in which the immune system of our body recognizes our healthy cells as foreign and destroys them.
2. Dystrophin: It is a rod-shaped cytoplasmic protein which connects the cytoskeleton of a muscle fibre to the surrounding extracellular matrix.
3. Holotrichous cilia: Possesing cilia all over the cell surface.
4. Hypocalcemia: Low calcium in blood.
5. Hypoparathyroidism: A condition when the parathyroid gland is not able to produce enough parathyroid hormone.
6. Hyperuricemia: Excess uric acid level in blood.
7. Immune response: It is the reaction of the cells and the fluids due to the presence of unknown, foreign substances in the body.
8. Iris: The thin circular structure of the eye which is responsible for controlling the diameter of the pupil and thus the amount of light reaching the eye.
9. Pancreatitis: It is the inflammation of the pancreas.
10. Sella turcica: It is the small cavity present in the sphenoid bone of the cranium. The pituitary gland lies in this cavity.

Points To Remember

1. The study of locomotion and movement is known as kinesiology.
2. The study of bones is known as osteology.
3. The study of muscles is known as mycology.
4. The minimum stimulus required for the contraction of a muscle is known as the threshold stimulus.
5. Stapes are the smallest bone present in the human body.
6. About 260 bones are present in newborns. Some of those fuse with age and the number of bones become 206 in adults.
7. There are 8 cranial bones—1 frontal, 2 parietal, 1 occipital, 2 temporal, 1 sphenoid and 1 ethmoid bone.
8. The bone, which forms the upper jaw, is known as the maxilla and the bone which forms the lower jaw, is known as the mandible.
9. The density and strength of bones decrease due to a deficiency of calcium and phosphorus in bones. This disorder is known as osteoporosis.
10. Osteopetrosis is an inherited disorder of bones. In this disorder, bones become harder and denser.
11. An abnormal condition, where a person can feel his or her missing body part attached in its proper position in the body, is known as phantom limb In deficiency of ATP after death the cross-bridges formed between actin and myosin fibres do not break and the dead body becomes stiff. This condition is known as rigour mortis.
12. The tension that occurs during the muscle contraction is known as muscle tension.
13. The minimal stimulus of infinite duration which results in muscle contraction is known as rheobase.
14. The minimum time, needed for a stimulus to double the strength of the rheobase to cause the muscle contraction, is known as chronaxie.
15. The muscles, which are light in colour due to lack of myoglobin, are known as white muscle. The rate of contraction is high in these muscles, hence known as fast muscles.
16. The muscles, which are dark in colour due to a huge amount of myoglobin, are known as red muscles. The rate of contraction is low in these muscles, hence they are known as slow muscles.
17. When a stimulus is introduced to the muscles, the muscle first contracts and then relaxes. The contraction and relaxation together are known as muscle twitch.
18. If a muscle fibre is introduced to a stimulus continuously for a certain period of time, then the rate of stimulation will be so high that the muscle fibre will be unable to relax between two stimuli. In this condition, the muscle twitches are converted to smooth, sustained contractions, known as tetanus.
19. During excess muscular activity or when a muscle fibre is stimulated continuously without any break, the contracting ability of the muscle gradually decreases and ultimately the muscle fails to contract for some time. This condition is known as muscle fatigue.
20. The cyclic pathway, in which the lactic acid produced by glycolysis in the muscles is transported to the liver, converted to glucose, again returned to muscles and metabolised into lactic acid, is known as the Cori cycle.
21. The method of evaluating and recording the electrical activity of the skeletal muscles is known as electromyography.
22. The Y-shaped bones found at the ventral side of the tail in reptiles, such as lizards, snakes, etc., are known as chevron bones. Its main function is to protect nerves and blood vessels in the tail.
23. The tibia or shin bone is the strongest bone of the body.
24. The two bones which help the birds to fly are pectoralis major and pectoralis minor.
25. The disorder related to inflammation of joints is known as arthritis.
26. In some people, an extra floating rib is found, this is known as gorilla rib.
27. A short band of tough, flexible fibrous connective tissue which connects two bones together is called a ligament and a flexible but inelastic cord of strong fibrous collagen tissue attaching a muscle to a bone is called a tendon.

Chemical Coordination Introduction

So far you have studied the different organ systems of the human body. Different organs constituting these systems, carry out different functions.

All these functions are in turn controlled and coordinated by two I organ systems. One of them is the nervous system.

The other one about which you shall learn in this chapter is the 1 endocrine system.

The two systems mentioned above, though similar, show some differences as well. Differences between these two systems (on the basis of their process of coordination) have been discussed below.

All organisms have the ability to coordinate their activities with constant changes in the environment. In doing so, organisms use a number of pathways that receive and process different signals.

Read and Learn More: WBCHSE Notes for Class 11 Biology

These signals originate from cells within the organism, as well as from the external environment.

This maintains a balance between the various metabolic activities of the body, This is the functional coordination of the body which is necessary for maintaining homeostasis within the body, The Endocrine system involves different glands that contain several specialized cells.

These cells synthesize and secrete some specific biochemical substances, called hormones. Hormones are released directly into the bloodstream by these glands. They act as chemical messengers in the body and flow with the blood to reach the target organs, upon which they act.

Endocrine Glands

The ductless gland that secretes specific chemical substances and releases them directly into the blood or lymph is called an endocrine gland

The characteristics of endocrine glands are—

• These glands are mostly present in humans and higher groups of vertebrates. They do not contain ducts, hence called ‘ductless’.
• They synthesize and secrete hormones.
• As their secretions are carried by the blood and lymph, these glands are surrounded by a network of blood capillaries.

Examples: Pituitary gland, thyroid gland, etc.

• The glands which release their secretions within or outside the body through ducts are called exocrine glands.
• The products of these glands are of different types, e.g., enzymes, saliva, mucus, bile, sweat, sebum, milk, etc.

Examples: Salivary glands, mammary glands, sweat glands, sebaceous glands, etc.

Mixocrine Or Heterocrine Or Mixed Glands

The glands which are both exocrine and endocrine in nature are called merocrine glands.

These glands contain both exocrine and endocrine cells. Examples: Pancreas, testes, ovary, etc.

Hormones

Hormones Definition: The biochemical substances synthesized and secreted by endocrine glands or specialized cells, are called hormones.

In 1902, M. Bayliss and Ernest H. Starling isolated a specific substance from the mucous cells of the duodenum.

This specific substance was actually a secretin hormone that stimulates the secretion of pancreatic juice.

In 1905, Starling named this specific substance a hormone. The term ‘hormone’ has been derived from a Greek word, ‘harakiri, which means ‘to excite or to set in motion’.

Hormones Characteristics: Important characteristics of hormones are as follows—

Nature: Biochemically hormones are protein or polypeptide, lipoprotein, glycoprotein, steroid, etc.

Site of secretion: They are mainly synthesized and secreted by endocrine glands. But organs that contain endocrine cells, like testes, ovaries, etc., can also produce hormones.

Storage: They can be stored only in the cells they are secreted from.

Effect on target organs: Hormones often exert their influence on target organs or tissues, located at some distance from the gland. However, certain hormones like secretin, gastrin, etc., act only in the adjacent regions of its secretion. So, they are also known as local hormones.

Transportation: They are carried to the target organs by blood or lymph.

Chemical messenger: They do not initiate new functions, but only regulate already occurring cellular processes. They carry chemical messages to target organs. Hence, hormones are also called chemical messengers.

Chemical coordinators: They respond to immediate physiological requirements of the body.

They are involved in maintaining the functional coordination of the organs. Hence, the hormones are also known as chemical coordinators.

Quantity: They are very specific and influence unique cellular responses at very low concentrations.

Feedback mechanism: Hormones may trigger a cascade of amplifying mechanisms. A small amount of a hormone induces the release of a larger amount of another hormone and/or intracellular metabolites In the target cells. This mechanism is called the feedback mechanism.

Fate of hormone: Hormones can be degraded in their target cells. It occurs by internalization of the hormone-receptor complex followed by lysosomal degradation of the hormone.

Types of Hormones

The following types of hormones are found in the body—

Harmones Functions

1. Hormones affect the functioning of other glands or tissues. Hormones regulate the metabolism of cells, body growth, and development of various parts of the body.
2. Hormones are also responsible for the regulation of the immune system and reproductive functions.
3. The appearance of secondary sexual characteristics (external characters that distinguish sexes) is also stimulated by hormones.
4. In some cases, hormones may cause permanent changes, such as sex differentiation in humans. These changes persist even when the hormone is absent.
5. Hormones coordinate the responses to stress. They control blood volume and pressure by regulating the sodium and water balance. This, in turn, maintains the condition of homeostasis.
6. To maintain cell membrane integrity and intracellular signaling, hormones regulate calcium and phosphate balance.

Classification Of Hormones

Based on their solubility in water and fat

1. Water-soluble: Peptide hormones.
2. Fat-soluble: Steroid hormones, hormones secreted by the thyroid gland.

[Based on the types of recptors]

Intracellular receptor: Steroid hormones and thyroxine.

Extracellular receptors: Insulin, glucagon, parathormone, etc.

[Based on their interaction with one another]

1. Antagonistic: When a hormone produces the opposite effect of another hormone, it is antagonistic in nature, e.g., insulin and glucagon on blood glucose levels.
2. Synergistic: When two or more hormones act together to produce a greater effect, they are called synergistic in nature, e.g., testosterone and FSH on sperm production.
3. Permissive: When a hormone enhances the effect of another hormone secreted later, it is called permissive in nature, e.g., estrogen and progesterone in the uterine cycle.

Human Endocrine System

The endocrine system is composed of a variety of different endocrine cells and glands. Endocrine cells or glands include—

The hypothalamus, pituitary glands, pineal gland, thyroid gland, parathyroid gland, adrenal glands, pancreas, reproductive organs (testes and ovary), thymus gland, and placenta.

Hypothalamus

The hypothalamus of the brain contains two sets of neurosecretory cells whose hormonal secretions regulate the activity of the pituitary gland.

Hypothalamus Location: The hypothalamus is located superior to the brain stem and inferior to the thalamus.

Hypothalamus Structural and functional features: Hypothalamus is composed of nervous tissue.

White matter is present in the center of the hypothalamus, with grey matter in the periphery.

The distribution of the white matter as islands in the grey matter leads to the formation of multiple nuclei. Specialized neuron clusters of the hypothalamus are called neurosecretory cells.

Secretions produced by these cells are called neurohormones. The network of capillaries extending from the hypothalamus joins to form the hypophyseal vein.

It enters the posterior lobe of the pituitary gland and again divides into capillaries. Axons of some of the neurosecretory cells extend into the pars nervosa region of the neurohypophysis.

This path between the hypothalamus and neurohypophysis is called the hypothalamo-hypophyseal tract.

Neurons Of Hypothalamus

Magnocellular neurons are predominantly located in the paraventricular and supraoptic nuclei of the hypothalamus. They produce large quantities of the neurohormones—oxytocin and antidiuretic hormone (ADH).

Parvocellular neurons have projections that terminate in the median eminence, brain stem, and spinal cord.

These neurons are located within the paraventricular nucleus. They release small amounts of neurohormones (hypophysiotropic hormones) that control anterior pituitary function.

Hormones secreted by the hypothalamus: The hormones secreted by the hypothalamus get carried by the blood to the hypophyseal portal vein. They are mainly glycoprotein in nature. The hormones include—

Thyrotropin-releasing hormone (TRH): TRH stimulates the anterior pituitary gland to secrete thyroid-stimulating hormone.

Growth hormone-releasing hormone (CHRH): GHRH stimulates the anterior part of the pituitary gland to secrete growth hormone.

Growth hormone-inhibiting hormone (GHIH): GHIH inhibits growth hormone secretion from the anterior pituitary.

Gonadotropin-releasing hormone (GnRH): GnRH stimulates the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Corticotropin-releasing hormone (CRH): CRH stimulates the anterior pituitary to secrete adrenocorticotropic hormone (ACTH).

Oxytocin and antidiuretic hormone (ADH): Oxytocin and ADH produced by the hypothalamus are transported to the posterior pituitary where they are stored.

Prolactin-releasing hormone (PRH): PRH stimulates the secretion of prolactin from the anterior pituitary.

Prolactin-inhibiting hormone (PIH): PIH inhibits the secretion of prolactin from the anterior part of the pituitary gland.

MSH-releasing hormone: MSH-RH stimulates the intermediate lobe of the pituitary gland which secretes melanocyte-stimulating hormone.

MSH-inhibiting hormone: MSH-IH inhibits the secretion of melanocyte-stimulating hormone from the intermediate lobe of the pituitary gland.

Pituitary Gland

The Pituitary Gland is also called hypophysis. The name is derived from the Greek words hypo-under and physis-to grow.

Pituitary Gland Location: It is located in a depression in the sphenoid bone, called sella turcica. It is connected below the hypothalamus by the infundibulum (a stalk-like structure).

Tropic hormones

Hormones that are carried to target organs far off from their secretory cells and affect the secretion of other hormones are called tropic hormones. E.g., FSH, LH.

Tropic hormones Structure: The pituitary gland is small in size. It weighs about 0.5-0.6 g in adult males. It is slightly bigger in the case of adult females. In females, it weighs about 0.6-0.7 g.

The pituitary gland is divided into two regions— the posterior pituitary (neurohypophysis) and the anterior pituitary (adenohypophysis).

Adenohypophysis (Anterior pituitary)

Adenophysis is the anterior part of the pituitary gland which secretes hormones like GH, TSH, etc.

Parts of the Adenohypophysis: It is divided into three parts—

Parts of the adenohypophysis: It is divided into three parts—

Pars distaiis: It appear as clusters or cords of cells.

Pars tuberalis: It surrounds the infundibulum of the neurohypophysis.

Pars intermedia: In the human fetus it appears as a lobe between the anterior and posterior pituitary but is reduced in adults.

Hormones Secreted by Anterior Pituitary:

The anterior pituitary releases four important hormones that act as tropic hormones.

Following are the types of hormones secreted by the anterior pituitary—

Thyroid-stimulating hormone (TSH)

Source: TSH is synthesized and secreted from basophilic cells, called thyrotrophs, of the pars distaiis region of the anterior pituitary gland.

Chemical nature: TSH is a glycoprotein in nature.

Target organs: As its name suggests, it is a tropic hormone responsible for the stimulation of the thyroid gland. Hence, the thyroid gland is the target organ.

Hormones Physiological functions:

It stimulates the secretion of thyroid hormones such as thyroxine or tetraiodothyronine (T₄) and triiodothyronine (T₃), from the thyroid gland,

It is an important regulator of metabolic activity in the body,

In addition, it acts as a growth factor for the thyroid gland.

Adrenocorticotropic Hormone (ACTH)

Source: It is produced by the basophilic corticotroph cells of the pars distalis region of the anterior pituitary.

Chemical nature: It is a polypeptide, made up of 39 amino acid residues.

Target organs: It acts upon the cortex region of the adrenal or suprarenal glands located at the top of each kidney.

Hormone Physiological Functions

The release of ACTH stimulates the growth of the cortex region of adrenal glands,

ACTH stimulates the hormone secretion from the adrenal cortex. ACTH stimulates the production and release of glucocorticoids (cortisol), mineralocorticoids (aldosterone), and sex corticoids or sex steroids from the adrenal cortex.

Gonadotropic Hormone (GTH)

Gonadotropic Hormone (GTH) Source: In response to stimulation by gonadotropin-releasing hormone (GnRH), GTHs (both FSH and LH) are synthesized. These hormones are secreted by basophilic cells. FSH and LH are secreted from cells called gonadotrophs. These cells are located at the pars distalis region of the anterior pituitary.

Chemical nature: Both FSH and LH are glycoproteins in nature.

Target organs: FSH and LH have major effects on the human reproductive organs—testes (in males) and ovaries (in females).

Gonadotropic Hormone Physiological functions:

1. Both GTHs (FSH and LH) are important for the growth and development of gonads,
2. FSH stimulates the growth and maturation of the ovarian follicles. It promotes estrogen synthesis and secretion in adult females
3. FSH along with LH stimulates spermatogenesis (sperm production) in males,
4. In females, LH is responsible for ovulation (release of a secondary oocyte from the Graafian follicle) and luteinization (formation of corpus luteum). It also regulates estrogen and progesterone hormone levels in adult females,
5. LH in adult males stimulates interstitial cells (in the testes)   to synthesize and secrete testosterone hormone. In males, LH is termed as interstitial cell-stimulating hormone (ICSH).

Growth hormone (GH) or somatotropic hormone (STH)

Growth Hormone Source: GH is synthesized and secreted from the acidophilic cells, called somatotrophs. These are present in the pars distalis region of the anterior pituitary.

Growth Hormone Chemical nature: GH is a 191 amino acid containing a single-chain peptide hormone.

Growth Hormone Target organs: It is not effective over any specific target organ, rather it affects all the tissues.

Growth Hormone Physiological Functions:

1. Human growth hormone (HGH) affects different organs by stimulating their growth and development,
2. This hormone plays an important role in the metabolism of carbohydrates, fats, and proteins.
3. It also induces cell division in the body. lt also induces calcium retention in the intestine. It also maintains the level of potassium, phosphorus, and calcium in the body.

Prolactin (PRL)

Prolactin (PRL) Source: Prolactin is synthesized and secreted by acidophilic cells, called lactotrophs. These cells are located in the pars distalis region of the anterior pituitary gland.

Prolactin (PRL) Chemical nature: Prolactin is a type of polypeptide.

Prolactin (PRL) Target organs: Alveolar cells of the mammary glands.

Prolactin (PRL) Physiological functions:

1. Prolactin stimulates the growth and development of the mammary gland. It also stimulates the synthesis and secretion of milk.
2. Prolactin increases glucose and amino acid uptake. It stimulates the synthesis of the milk proteins, 8 casein, and ar-lactalbumin, the milk sugar (lactose), and milk fats from the mammary epithelial cells,
3. Prolactin inhibits GnRH release, progesterone biosynthesis, and growth of luteal cells during pregnancy.

Melanocyte-stimulating Hormone (MSH)

Source: MSH is produced mainly in the pars intermedia of the pituitary gland by the proteolytic cleavage of POMC (Pro-opiomelanocortin).

Melanocyte-stimulating Hormone Function:

• Specialized skin cells called melanocytes produce a pigment called melanin. This pigment is responsible for the pigmentation found in the skin, hair and eyes,
• Melanin protects cells from DNA damage which may lead to skin cancer (melanoma).
• Neurohypophysis (Posteriorpituitary) Neurohypophysis is the posterior part of the pituitary gland in which the axons of the neurosecretory cells are present.

Parts of neurohypophysis: Neurohypophysis is divided into pars nervosa and infundibulum.

Pars nervosa: It is composed principally of nerve cell processes, glial cells, and capillaries.

Infundibulum: It is a stalk-like structure that attaches the pituitary gland to the bottom of the hypothalamus.

Hormones secreted by posterior pituitary:

The hormones secreted by the posterior pituitary are as follows—

Oxytocin

Oxytocin Source: The neuropeptide, oxytocin, is secreted by non-myelinated neurons of the paraventricular nucleus of the hypothalamus. It is stored in the pars nervosa region of the neurohypophysis and released in the blood from there.

Oxytocin Chemical nature: It is a peptide hormone, composed of 9 amino acids.

Oxytocin Target organs: Lactating mammary glands and the uterus (during pregnancy).

Oxytocin Physiological Functions:

• Oxytocin stimulates milk ejection by controlling the contraction of the myoepithelial cells (present in glandular epithelium). These myoepithelial cells line the alveoli and ducts in the mammary gland.
• Oxytocin produces rhythmic contractions of uterine smooth muscles during childbirth. It promotes regression of the uterus following the delivery of the baby.
• It induces contractions in internal organs such as the gall bladder, small intestine, urinary bladder, etc.
• It helps in the movement of sperm through the female genital tract.

Antidiuretic hormone (ADH)

Antidiuretic Hormone (ADH) Source: ADH is secreted by the non-myelinated neurons (neurosecretory cells) of the supraoptic nucleus of the hypothalamus. It remains stored within the pars nervosa of the neurohypophysis and is secreted when required.

Antidiuretic Hormone Chemical nature: ADH, also known as arginine vasopressin (AVP), is a peptide containing 9 amino acids.

Antidiuretic Hormone Target organs: Mainly Kidneys.

Antidiuretic Hormone Physiological Functions:

Certain neurons in the hypothalamus are sensitive to the salt-water balance of the blood. When the blood becomes concentrated, these cells send signals to the pituitary gland.

As a result, antidiuretic hormone (ADH) is released from the posterior pituitary. Upon reaching the kidneys, ADH enhances the permeability of the walls of the distal convoluted tubule (DCT) and the collecting duct of a nephron, to water.

This causes excess absorption of water into kidney capillaries from DCT and the collecting duct of the nephron.

As a result, the normal concentration of blood is restored and concentrated urine is produced,

ADH causes constriction of blood vessels, increasing the pressure within them. Hence, it is also known as vasopressin,

ADH helps the smooth muscles to contract and thereby helps in the contraction of the stomach, urinary bladder, etc.

Master Gland

The pituitary gland is known as the master gland. This is because the hormones secreted by the pituitary gland influence the synthesis and secretion of various hormones from other major endocrine glands.

Pineal Gland Or Epiphysis Cerebri

The pineal gland is a small endocrine gland located below the epithalamus of the diencephalon region of the brain.

Pineal Gland Location: The pineal gland or epiphysis is attached to the roof of the third ventricle by its stalk. This gland is located in the diencephalon region of the forebrain in humans.

Pineal Gland Structure: The shape of the pineal gland is like a pine cone. It is about 5-8 mm in length and 3-5 mm in breadth. It is attached to the epithalamus on the roof of the third ventricle of the brain by a short and hollow stalk. It is generally made up of modified nerve cells, called pinealocytes.

The pinealocytes are also called chief cells or parenchymal cells. The pineal gland also contains interstitial cells along with pinealocytes. They are also called glial cells.

Hormones secreted by pineal gland: The hormones of the pineal gland are as follows—

Melatonin

Melatonin Source: Melatonin is secreted by the pineal gland.

Melatonin Chemical nature: Melatonin is an indolamine hormone. It is produced, from the amino acid, tryptophan. Chemically, it is N-acetyl-5- methoxytrypsin.

Melatonin Physiological functions:

1. Melatonin regulates circadian and circannual rhythms,
2. It also helps to regulate the growth of reproductive organs,
3. Its antioxidant properties play a role in immune function and cell proliferation,
4. It helps to prevent pigmentation of the skin,
5. Melatonin increases the growth and activity of other endocrine glands.

Serotonin

Serotonin Source: Serotonin is synthesized and secreted by the pineal gland.

Serotonin Chemical nature: Serotonin belongs to the tryptamine family. Chemically, it is 5-hydroxytryptamine.

Synthesis of serotonin and melatonin After the synthesis of serotonin, N-acetyl serotonin is formed under the influence of the N-acetylating enzyme.

This N-acetyl serotonin produces melatonin, under the influence of hydroxy indol-O-methyl transferase (HIOMT).

Serotonin Physiological Functions:

• Serotonin acts as a neurotransmitter and helps in the conduction of nerve impulses,
• It also helps in constriction of blood vessels, thereby reducing their diameter.

Thyroid Glands

The thyroid gland is a bilobed endocrine gland that is situated in the upper part of the trachea in the neck region.

Thyroid Glands Location: The thyroid gland is located at the upper part of the trachea, slightly inferior to the thyroid cartilage. It surrounds the larynx.

Thyroid Glands Structure: The thyroid gland generally consists of two lateral lobes connected by an isthmus.

This isthmus is present anterior to the 2nd, 3rd, and 4th tracheal rings. Each lobe of the gland extends from the middle of the thyroid cartilage to the 7th or 8th tracheal ring.

Each of the lobes measures about 5 cm in length, 2 cm in breadth, and 2 cm in height. The thyroid glands generally weigh 20-25g. Thyroid follicles are the structural and functional units of the gland.

Within these follicles, a fibrous connective tissue layer is present, that is rich in blood vessels.

This gland is located near the thyroid cartilage, so it is known as the thyroid gland.

This gland is formed of two types of cells—follicular cells and parafollicular cells.

Follicular cells and parafollicular cells Follicular cells are the principal cells of the thyroid.

They form the continuous epithelial lining of the follicles. Typically, they are cuboidal in shape, with spherical nuclei.

Parafollicular cells or C cells are also associated with the follicles. However, these cells are completely separated from the colloid by follicular cells.

Most parafollicular cells synthesize and secrete the hormone, calcitonin. Therefore, they are frequently referred to as clear cells or C cells.

Hormones secreted by the thyroid gland: The different hormones secreted by the thyroid gland have been discussed below.

Thyroxine or tetraiodothyronine (T₄) and triiodothyronine (T₃)

Thyroxine Source: The follicular cells of the thyroid gland secrete thyroxine and triiodothyronine.

Thyroxine Chemical nature: Both T₃ and T₄ are formed by the combination of iodine atoms with the amino acid tyrosine. T₄ contains 4 atoms of iodine while T₃ contains 3 atoms of iodine.

Role of iodine in T₃ & T₄ synthesis

Iodine is required to synthesize the thyroid hormones. It is obtained from water, iodized salt, leafy vegetables, fruits, etc.

It is absorbed by the thyroid follicles from the blood. Inside the follicles, the iodine molecules combine with tyrosine (an amino acid), present in thyroglobulin, to synthesize T₃ and T₄ hormones.

Thyroxine Physiological functions:

1. Carbohydrate metabolism by T₃ and T₄ produces energy. So, they are called calorigenic hormones. They also increase the absorption of 02 into the cells,
2. Thyroxine increases basal metabolic rate (BMR) by stimulating different metabolic activities in most tissues,
3. Thyroxine stimulates the absorption of glucose in the small intestine. Gluconeogenesis and glycogenolysis are also stimulated by thyroxine,
4. Thyroid hormones increase the synthesis of proteins and RNA in the cells.
5. T₄ and T₃ help in the synthesis of cholesterol and other lipids. Thyroxine also increases the concentration of fatty acids by increasing fat mobilization.
6. Oxidation of fatty acids in many tissues is also enhanced by thyroid hormone,
7. These hormones help in the overall growth of the body,
8. Thyroxine affects the heart by causing an increase in heart rate, cardiac contractility, and cardiac output,
9. Thyroxine increases milk production and concentration of fats in the milk.

Calcitonin Or Thyrocalcitonin

Calcitonin Or Thyrocalcitonin Source: Calcitonin is secreted by the parafollicular cells of the thyroid gland.

Calcitonin Or Thyrocalcitonin Chemical nature: Calcitonin is a peptide hormone, composed of 32 amino acids.

Calcitonin Physiological functions:

Calcitonin helps in the absorption of calcium into the matrix of bones and thereby, reduces the concentration of calcium ions in the blood.

Calcitonin is known to slow the breakdown of bone by inhibiting osteoclast function. This function is opposite to that of T₃ and T₄ hormones. Hence, calcitonin is antagonistic to T₃ and T₄.

Calcitonin also regulates the transport and metabolism of phosphates in the body,

It regulates the concentration of alkaline phosphatase in the blood.

Parathyroid Gland

Parathyroid glands are four, small, oval-shaped, yellowish-brown glands that secrete parathyroid hormone (PTH).

Parathyroid Gland Location: The parathyroid glands are located at the upper and lower portions of the posterior borders of the lateral lobes of the thyroid gland.

Parathyroid Gland Structure: The parathyroid glands consist of four separate glands. Each of these glands is small and oval-shaped. They measure about 6 mm in length, 3 mm in breadth, and 2 mm in height.

Generally, each weighs about 140 mg. These glands are richly supplied with blood capillaries. Parathyroid glands are composed of chief cells and oxyphil cells.

Hormones secreted by parathyroid glands:

PTH or parathyroid hormone is the primary hormone of the parathyroid gland. It is discussed below.

Parathyroid Hormone (PTH)

Parathyroid hormone (PTH) Source: PTH is produced by the chief cells of the parathyroid glands.

J. B. Collip was the first scientist to isolate this hormone. Hence, it is also known as Collip’s hormone.

Parathyroid Hormone Chemical nature: It is a polypeptide, made up of 84 amino acid residues.

Parathyroid Hormone Physiological functions:

1. The antagonistic actions of calcitonin and parathyroid hormone maintain the blood calcium level within normal limits,
2. PTH indirectly stimulates osteoclasts to reabsorb calcium from the bone matrix, which then enters into the circulation. In addition, PTH also increases renal excretion of phosphate. Due to this, osteoclasts degrade by the process of osteolysis.
3. PTH prevents P043-, Na+, and HC03- ion reabsorption in the PCT of a nephron.
4. It increases the reabsorption of Ca2+ in the ascending and descending limbs of the loop of Henle.
5. PTH stimulates the synthesis of vitamin D or calciferol. It also converts vitamin D into an active hormone. This activated vitamin D helps to absorb calcium in the intestine.

Adrenal Gland

Adrenal Glands are a pair of triangular-shaped endocrine glands. Each gland is present at the apex of each kidney.

Adrenal Gland Location: The right adrenal gland looks like a small pyramid. It is present at the apex of the right kidney and next to the vena cava. The left adrenal gland looks more like a crescent.

It is present at the apex of the left kidney and next to the aorta, colon, stomach, and spleen.

Adrenal Gland Structure: The adrenal glands (or suprarenal glands) are paired glands, each about 4-6 cm in length and 3-4 cm in breadth. The glands are covered by a layer of fibrous connective tissue, called a capsule.

There are several glandular cells within the adrenal glands. Each adrenal gland consists of an inner portion called the adrenal medulla and an outer portion called the adrenal cortex.

However, these portions do not have any physiological connection with one another. The adrenal glands are richly supplied with blood vessels. Different parts of the adrenal glands have been discussed below under separate heads.

Adrenal Cortex

The peripheral region of the adrenal gland constitutes the adrenal cortex.

Adrenal Cortex Structure: It extends between the capsule and the medullary region of the gland. It is richly supplied with blood capillaries, ft consists of glandular epithelial cells.

The cells of the cortex are arranged in three particular zones. They are—Zona glomerulosa, Zona fasciculata, Zona reticularis.

Hormones secreted by the adrenal cortex: Mainly glucocorticoids, mineralocorticoids, and sex steroids are secreted by the adrenal cortex. All these hormones are steroid in nature.

Their physiological functions are described below.

Glucocorticoids

Glucocorticoids Source: Glucocorticoids are secreted by zona fasciculata of the adrenal cortex.

Adrenal Gland Chemical nature: Chemically these are steroid hormones with 21 carbon atoms.

Adrenal Gland Physiological Functions: Cortisol is the main glucocorticoid secreted by the adrenal cortex. Other than this, it secretes cortisone and corticosterone.

We shall study the physiological functions of cortisol, mainly—

Carbohydrate metabolism:

• Cortisol stimulates gluconeogenesis (or neoglucogenesis) in the body. This leads to the production of carbohydrates from non-carbohydrate molecules,
• It promotes glycogenesis in the liver and muscles. This leads to increased synthesis of glycogen,
• It also stimulates the production of enzymes required in gluconeogenesis.
• It increases glucose absorption in the small intestine

Protein metabolism:

• Cortisol stimulates the breakdown of proteins into amino acids,
• It helps the amino acids to be transported to the liver, where they are transformed into glucose,
• The reduced synthesis and enhanced depletion of proteins result in a decrease in stored protein content in cells,

Lipid metabolism:

• Cortisol increases the absorption of lipids in the small intestine,
• It promotes the breakdown of adipose tissues into fatty acids,
• It declines the rate of synthesis of fats from carbohydrates,
• It stimulates the transport of fatty acids to plasma where they are used up as a source of energy,
• This hormone increases the oxidation of fatty acids in cells,

Anti-inflammatory effect:

1. Cortisol raises the level of production of anti-inflammatory proteins,
2. It also counteracts the inflammatory function of the leucocytes, thereby preventing phagocytosis, chemotaxis (movement of a cell in response to a chemical stimulus), etc.
3. It also decreases the secretion of inflammatory chemicals like interleukin 1 (IL-1), prostaglandins, leukotrienes, etc., by injured or infected cells. This prevents heat generation during infections,
4. This hormone reduces the production of antibodies, the number of leukocytes, and the amount of lymphoid tissue. All these actions result in immunosuppression of the body.

Mineralocorticoids

Mineralocorticoids Source: The cells of zona glomerulosa secrete mineralocorticoid hormones.

Mineralocorticoids Chemical nature: These are steroid compounds with 21 carbon atoms.

Mineralocorticoids Physiological functions: Aldosterone is the main mineralocorticoid in humans.

Its physiological functions are as follows—

• Aldosterone maintains the concentration of NaCI and water in blood and tissue fluid. It stimulates the absorption of Na+ and cr in kidney tubules,
• Aldosterone increases sodium and water reabsorption in the DCT of nephrons,
• It stimulates the excretion of potassium and phosphate through urine,
• It influences the salt and water balance of the extracellular fluid.

Mineralocorticoids Sex steroids: Sex steroids produced by the adrenal cortex include androgens—dehydroepiandrosterone (DHEA), DHEA sulfate (DHEAS), some amount of estrogen, and progesterone.

These hormones are also known as sex corticoids or gonadocorticoids.

Mineralocorticoids Source: These hormones are produced by zona fasciculata of the adrenal cortex. Zona reticulata also produces androgens.

Mineralocorticoids Chemical nature: These hormones are steroid in nature. The main androgen produced here is DHEA, which is a 19-carbon compound.

Mineralocorticoids Physiological functions:

Androgens control the growth and development of reproductive organs in males. Estrogen and progesterone control the functioning of the female reproductive system,

The sex steroids also regulate the appearance Of male or female secondary sexual characteristics.

Adrenal Medulla

The inner portion of the adrenal gland constitutes the adrenal medulla.

Adrenal Medulla Structure: This region is supplied with blood capillaries and glandular cells. Based on histological staining reaction, adrenal medullary cells are known as chromaffin cells.

These cells are named such because they can be stained with chromium-based dye. These
cells are columnar in shape and contain granular cytoplasm.

Chromaffin cells are of two types—

1. E-cells have homogeneous small granules.
2. NE cells have larger granules. These granules contain hormones secreted by these cells

Hormones secreted by adrenal medulla:

The adrenal medulla secretes two catecholamine hormones—epinephrine (E) and norepinephrine (NE). These are also called adrenaline and noradrenaline respectively.

Epinephrine or Adrenaline

Source: The E-type chromaffin cells produce this hormone.

Adrenal Medulla Chemical nature: This hormone is a catecholamine.

Physiological functions:

1. This hormone alters the blood flow by constricting some blood vessels while dilating others.
2. It decreases the lumen of blood vessels by contracting their smooth muscles. This increases the blood pressure within the blood vessels,
3. Adrenaline increases the blood supply to cardiac and skeletal muscles due to dilation of
   arterial vessels supplying them,
4. Adrenaline increases the rate of heartbeat which in turn increases cardiac output,
5. It dilates the trachea and thus, increases the rate of respiration,
6. It regulates BMR (Basal Metabolic Rate), by inducing glycogenolysis and neo-glucogenesis.
7. It plays an important role as a neurotransmitter in the transmission of impulses in the synapse region,
8. It dilates the pupil and stimulates the secretion of tears,
9. It relaxes the smooth muscles of the stomach, urinary bladder, etc. But in the case of ureters, gall bladder, etc., it contracts the smooth muscles.

Adrenaline As The Emergency Hormone

When the body is under any emergency conditions or emotional stress like anger, fear, etc., secretion of this hormone increases.

Hypersecretion of adrenaline regulates the physiological and metabolic functions according to the state of the body. So, it is also known as an emergency hormone.

Norepinephrine Or Noradrenaline

1. Source: The NE-type chromaffin cells produce this hormone.

Chemical nature: This hormone is also a catecholamine.

Adrenal Medulla Physiological functions:

1. This hormone acts as a vasoconstrictor (constricts blood vessels). This results in an increase in blood pressure inside the blood vessels,
2. It increases cardiac output. It also acts as a bronchodilator. Hence, it increases respiratory rate,
3. It increases blood glucose levels and enhances lipid metabolism to produce fatty acids.
4. Norepinephrine also acts as a neurotransmitter. It helps to transmit nerve impulses in the synapse region.

Difference Between Neurotransmitter And Hormone

The difference between a neurotransmitter and a hormone does not lie in the chemical nature of the regulatory molecule.

The difference lies in the way it is transported to its target cells and its distance from the target cells.

A chemical regulator called norepinephrine, for example, is released as a neurotransmitter by sympathetic nerve endings and is also secreted by the adrenal medulla as a hormone.

Pancreas

Pancreas Location: The pancreas is located just below the stomach, adjacent to the spleen. It is connected to the duodenum of the small intestine by the pancreatic duct.

Pancreas Structure: The pancreas has both exocrine and endocrine regions. Therefore, it is a mixed gland.

Exocrine region of the pancreas: The exocrine cells form clusters called acini or alveolar structures.

Endocrine region of the pancreas: The endocrine part of the pancreas consists of isolated islands of cells called islets of Langerhans. These cells are endocrine in nature and remain embedded within the pancreatic acini. These cells are highly vascular.

Around 1-2 million Islets of Langerhans are present in the pancreas. The cells of the islets are polygonal and are closely spaced with sinusoidal capillary networks.

Mainly Two Types Of Cells are found in the islets: or and cells. 6 cells and PP cells are also present in the islets, but A and cells are the primary ones.

Hormones secreted by the pancreas:

The hormones secreted by the pancreas are as follows—

Insulin

1. Source: It is produced by beta cells of islets of Langerhans.
2. Chemical nature: It is a polypeptide hormone and is produced from proinsulin. It is made up of two peptide chains—A chain and B chain.
3. Physiological Functions:

Carbohydrate metabolism:

1. Insulin lowers elevated blood glucose levels after a meal,
2. Insulin activates the glucose transporters and helps the target cells to take up excess glucose circulating in the blood.
3. Insulin promotes glycogenesis. Thus, excess glucose is stored as glycogen in the liver and skeletal muscles,
4. Insulin prevents glycogenolysis (conversion of glycogen into glucose), (e) Insulin also prevents gluconeogenesis (conversion of proteins and fats into glucose).
5. These actions lower the concentration of glucose in the blood. Hence, insulin is also called Hypoglycemic Hormone

Lipid metabolism:

1. Insulin induces the synthesis of fats from glucose and fatty acids, within the adipose tissues,
2. It is known as an anabolic hormone because it prevents the oxidation of fats.
3. It prevents the production of ketone bodies, thus, known as an antiketogenic hormone,

Protein metabolism:

• Insulin induces more reabsorption of amino acids, leading to more protein synthesis.
• It prevents the catabolic breakdown of proteins and the removal of amino acids.

Glucagon

Source: It is secreted by a -cells of the islets of Langerhans.

Glucagon Chemical nature: It is a polypeptide of 29 amino acid residues. Glucagon is synthesized from proglucagon.

Glucagon Physiological functions:

• Glucagon stimulates glycogen breakdown by glycogenolysis.
• It prevents the conversion of glucose into glycogen. Hence, it increases the glucose concentration in the blood,
• It also prevents the oxidation of glucose in the tissue fluid. Hence, glucagon is known as the ‘hyperglycemic hormone’.
• Glucagon acts as an antagonist to insulin,
• In the adipocytes, glucagon activates hormone-sensitive lipase. This enzyme then breaks down triglycerides (stored fat) into diacylglycerol and free fatty acids. It also helps to release them into the circulation.

Somatostatin

Somatostatin Source: Somatostatin is released from the 5 cells of the islets of the pancreas. It is also secreted by the hypothalamus and the mucous lining of the small intestine.

Somatostatin Chemical nature: Somatostatin is a 14 amino acid-containing peptide hormone.

Somatostatin Physiological functions:

Somatostatin has a generalized inhibitory effect on pancreatic exocrine and endocrine functions. It inhibits the secretion of both glucagon and insulin. Therefore, it acts as a local hormone (acts on the organ from where it is produced).

Outside the pancreas, it decreases the digestive activity of the digestive tract. It slows down the absorption of nutrients,

A very small amount of somatostatin is also produced in the hypothalamus which is transported to the anterior pituitary. Here it acts as a neurohormone to inhibit the release of growth hormone and thyrotropin,

It slows down the secretion of gastrointestinal hormones as well as the contractions of the stomach and duodenum.

Pancreatic Polypeptide

Pancreatic polypeptide Source: Pancreatic polypeptide is secreted from PP cells of pancreatic islets.

Pancreatic polypeptide Chemical nature: It is a peptide hormone containing 36 amino acids.

Pancreatic polypeptide Physiological functions: It is responsible for the inhibition of pancreatic exocrine secretion, gall bladder contraction, modulation of gastric acid secretion, and gastrointestinal motility.

Gonads

Gonads are mixed organs that produce male and female gametes as well as hormones in males and females respectively.

The female gonad is the ovary and the male gonad is the testis. They are responsible for producing sex hormones in humans. These hormones regulate the formation of gametes.

They also determine the secondary sexual characteristics of adult males and females.

Gonads Testes

The testis (plural: testes) is the primary reproductive organ in males and is responsible for the production of male gametes or sperms.

Gonads Location: The testes are located inside the scrotum, outside the pelvic cavity, between the two legs.

Gonads Structure: The testis consists of numerous lobules. Each lobule is made of convoluted tubes, called seminiferous tubules.

The Leydig cells or interstitial cells, present in the connective tissue between the sperm-producing seminiferous tubules, are the endocrine cells. These cells are responsible for the production of testosterone, the principal male hormone.

Hormones secreted by testes: The hormones secreted by testes are called androgens or male sex hormones. The principal androgen is testosterone.

Testosterone

Testosterone Source: Testosterone is synthesized and secreted by the interstitial cells of the testes under the influence of interstitial cell-stimulating hormone (ICSH) or luteinizing hormone (LH).

The ICSH is secreted from the anterior pituitary gland. Some amount of testosterone is also secreted by the adrenal glands.

Testosterone Chemical nature: Chemically testosterone is a 19-carbon steroid.

Testosterone Physiological functions:

1. Testosterone helps in the development of primary reproductive organs and accessory reproductive organs such as the epididymis, prostate gland, etc.
2. Testosterone regulates the appearance of secondary sexual characteristics in males such as pubic hair, axillary hair, facial hair, etc.
3. During adulthood, testosterone regulates normal sperm development.
4. It increases energy metabolism, thereby increasing the BMR.

Testosterone promotes the synthesis of mucoproteins and collagen. This leads to greater accumulation of calcium ions in bones, making them longer and stronger. However, they prevent the lengthening of bones after adolescence,

It also helps in the personality development of males.

Ovary

The ovary is the primary female reproductive organ that is responsible for the production of female gametes or ova.

Ovary Location: They are located in the lower abdominal cavity, on both sides of the uterus. These are attached to the inner lining of the body, by folds of the peritoneum, called mesovarium.

Ovary Structure: The ovary is almost oval-shaped. Its peripheral part is lined by epithelial cells. This layer is known as germinal epithelium.

This layer is followed by another layer called tunica albuginea, followed by the stroma. The stroma consists of an outer cortex layer and an inner medulla layer.

The outer cortex contains sac-like structures, called follicles of different sizes. These remain scattered in the connective tissue of the outer cortex. The inner medulla contains blood vessels, lymph vessels, etc.

The mature Graafian follicle within the ovary contains the ovum prior to ovulation (release of the ovum). Graafian follicle ruptures to release the secondary oocyte.

Under the influence of LH hormone, the disintegrated Graafian follicle is transformed into corpus luteum

Hormones secreted by ovary: Hormones secreted by the ovary have been discussed below.

Oestrogen

Oestrogen Source: The parts of the ovary that secrete estrogen are—

Granulosa cells of Graafian follicle and corpus luteum. Secretion of this hormone is controlled by FSH, secreted from adenohypophysis of the pituitary gland.

Other than the ovaries, estrogen is also secreted by the adrenal cortex, testes (mainly Sertoli cells), and placenta (only during pregnancy).

Corpus luteum

• Graafian follicle contains the oocyte, in mammalian ovaries. After maturation, it releases the oocyte and itself degrades into an endocrine gland. This is known as the corpus luteum.
• It contains yellow-colored lutein granules. This endocrine gland secretes progesterone.
• This hormone is secreted when fertilization occurs and is necessary for preparing the uterus for the conception of the embryo.
• On the other hand, when fertilization does not take place, the corpus luteum gets degraded and progesterone is no longer secreted.

Corpus luteum Chemical nature: Oestrogen is a steroid compound.

Corpus luteum Physiological functions:

1. Oestrogen is responsible for the growth and development of female reproductive organs,
2. It is responsible for the onset of puberty and the appearance of secondary sexual characteristics, such as breast development, growth of pubic hair, etc., in females,
3. Oestrogen stimulates the growth of the uterus and vagina during puberty. It is necessary for the maturation of the primordial follicle (primary stage of the follicle) into the Graafian follicle,
4. The changes that take place within the ovaries and the uterus, during the early phases of the menstrual cycle (monthly cycle of changes that occur in the female reproductive system), are triggered by this hormone.
5. Oestrogen causes the thickening of the uterine lining during the menstrual cycle in order to prepare for pregnancy,
6. It maintains bone density and also the elasticity of skin and vaginal lining,
7. It increases fat deposition in the subcutaneous layer under the skin.

Progesterone

Progesterone Source: Under the influence of luteinizing hormone (LH), the corpus luteum within the ovary synthesizes and secretes this hormone. Other than the ovaries adrenal cortex and placenta also secrete progesterone hormone.

Progesterone Chemical nature: Progesterone is a natural steroid.

Progesterone Physiological functions:

• Progesterone decreases contractility of the uterine smooth muscles and prepares the endometrium of the uterus for implantation of embryo,
• During the implantation of the embryo and embryonic development, progesterone helps to develop the placenta,
• Progesterone helps in the growth of the embryo throughout the gestation period (the duration of carrying an embryo or fetus inside the mother’s womb),
• It prevents the menstrual cycle as well as ovulation during pregnancy,
• It also helps in the growth of the mammary glands and the enlargement of breasts.

Relaxin

Relaxin Source: During pregnancy period, ovaries, placenta, and uterus produce this hormone.

Relaxin Chemical nature: Relaxin is a water-soluble polypeptide.

Relaxin Physiological functions: It causes the pelvic ligaments and pubic symphysis to relax. It helps in the relaxation of the cervix to facilitate the delivery of the baby.

Other hormones secreted by the ovaries

1. Inhibin is secreted by the granulosa cells, present in the ovaries. This hormone inhibits the secretion of FSH from the anterior pituitary gland. Inhibins regulate LH and FSH release through endocrine feedback regulation at the anterior pituitary.
2. Activin is also produced by the granulosa cells. It promotes the proliferation of granulosa cells. It also increases the number of FSH receptors on granulosa cells and modulates steroidogenesis (synthesis of steroids) in the ovarian follicles.
3. Follistatin is a single-chain glycoprotein hormone found in ovarian follicular fluid. It is known to inhibit FSH release from the anterior pituitary.

Prostaglandin (PG)

The biologically active, lipid compounds that are synthesized from unsaturated fatty acids such as arachidonic acid, are called prostaglandins (PG).

It helps to carry out several physiological processes, such as—contraction and relaxation of the smooth muscles in the uterus (especially while receiving the ovum), secretion of HCl in the gastric juice, etc.

Placenta

The temporary organ present in the uterus of a pregnant woman, that maintains the mechanical and physiological connection between the mother and the developing embryo, is called the placenta.

Placenta Location: It is present within the inner walls of both the growing fetus and the mother.

Placenta Hormones secreted: There are several hormones secreted by the placenta that are involved in the maintenance of pregnancy and the growth of the embryo.

The hormones secreted by the placenta are—estrogen, progesterone, relaxin, chorionic thyrotropin, human chorionic gonadotropin, and human chorionic somatomammotropin.

Human chorionic gonadotropin (HCG)

Chemically, it is a glycoprotein in nature.

Its physiological functions are as follows—

• It helps to maintain the structure and growth of the corpus luteum. Therefore, it regulates the secretion of estrogen and progesterone.
• HCG stimulates the secretion of testosterone in the male fetus. Due to this influence, the sex organs develop and the testes descend into the scrotum, at the layer period of gestation.
• Human chorionic somatomammotropin (HCS) or Human placental lactogen (HPL)
  Chemically, it is a polypeptide hormone.

Its physiological functions are as follows—

• HCS reduces the glucose concentration in the mother’s body, keeping the glucose available for the fetus.
• It also releases the fats from the fatty acids stored in the mother’s body.
• HCS increases the production of proteins and helps the fetus to grow.

Thymus gland

Thymus gland Location: This gland is located behind the sternum below the thyroid gland. It is situated partly in the neck region and partly in the thorax.

Thymus gland Hormones secreted: This gland secretes several hormones, such as thymosin, thymopoietin, thymulin, and thymic humoral factor. This gland is most active in children and adolescents. It gradually shrinks and becomes less active in adults.

Thymus gland Biological role:

• The hormones of the thymus gland regulate the development of T-lymphocytes.
• Thymosin and thymic humoral factors, increase immune responses against pathogens. The Thymus gland is called the Throne of immunity.
• Thymosin also stimulates the secretion of certain pituitary hormones (growth hormone, LH, gonadotropin-releasing hormone, ACTH, etc.)

Mechanism Of Hormone Action

• Hormones are responsible for transferring messages related to various physio-biochemical activities of the cells.
• They bind to hormone-specific receptor molecules at the target cells to form a hormone-receptor complex. This initiates a series of events that lead to the generation of second messengers within the cell (the hormone is known as the first messenger).
• The second messengers then trigger a series of molecular interactions that alter the physiological state of the cell. The entire process is called signal transduction.

Receptors

Receptors Definition: The cellular structures that recognize and bind to hormones are called receptors.

But in the current view, receptors can recognize and bind to a variety of other extracellular regulatory signaling molecules, such as growth factors and neurotransmitters.

Ligands

• Ligands are extracellular signaling molecules that bind to receptors. The receptor has the ability to specifically recognize a ligand among the different molecules present in the environment surrounding the cells.
• Ligand-receptor binding results in the activation of intracellular signaling pathways. As a result, amplification of the signal occurs which affects gene expression, i.e., synthesis of proteins according to the genetic information.

Receptors are classified into two main groups according to their location in the cells—

• Extracellular or cell surface receptors, located on the plasma membrane of the target cell (mainly for non-steroid hormones).
• Intracellular or nuclear transcription factor receptors are located inside the cell (mainly for steroid hormones).

Receptors are classified into two main groups according to their location in the cells—

• Extracellular or cell surface receptors, located on the plasma membrane of the target cell (mainly for non-steroid hormones).
• Intracellular or nuclear transcription factor receptors are located inside the cell (mainly for steroid hormones).

Extracellular Or Cell Surface Receptors

The hormones that are protein or polypeptide in nature (e.g., oxytocin, relaxin, etc.) are insoluble in lipids. Hence, they cannot move directly across the cell membrane of the target cell.

These hormones bind to specific receptor molecules located on the surface of the cell membrane.

These cell surface receptors are usually glycoproteins with three defined structural and functional domains—

• Extracellular domain: It contains the ligand binding site of the receptor and is hydrophilic in nature.
• Transmembrane domain: It anchors the receptor into the plasma membrane
• Intracellular domain: It contains amino acids, which are targets for phosphorylating enzymes that regulate receptor activity.

The hormone-receptor complex has two mechanisms of hormone action—

By cAMP formation, by changing the permeability of the plasma membrane.

Mechanism Of Hormone Action By cAMP Formation

• When the hormone binds to the receptor, a conformational change occurs in the cytoplasmic domain of the receptor.
• This leads to the stimulation of an intracellular signaling pathway of guanosine nucleotide binding protein called G-protein. G-protein activates adenylyl cyclase.
• cAMP is synthesized from ATP by the action of adenylyl cyclase. This cAMP, after synthesis, triggers the activation of other enzymes and stimulates the metabolic processes in the cells. Here hormone acts as the first messenger and cAMP as the second.

Example:

• The action of epinephrine on liver cells may serve as an example where cAMP acts as a second messenger. It converts glycogen stored in the liver into glucose.
• The binding of the hormone to the receptor brings about changes in the receptor. As a result, GTP binds to G-protein at the intracellular domain of the receptor.
• One of the G-protein subunits dissociates, which is called active G-protein. This active G-protein then interacts with adenylyl cyclase.
• When activated by the G-protein subunit, adenylyl cyclase catalyzes the
  formation of cAMP from ATP.
• The cAMP formed at the inner surface of the plasma membrane diffuses within the cytoplasm.
• Then it binds to and activates protein kinase-A. This enzyme, in the presence of Mg2+ and ATP, activates inactive phosphorylase kinase.
• Active phosphorylase kinase converts inactive glycogen phosphorylase-b into active glycogen phosphorylase-a.
• Active glycogen phosphorylase-a converts glycogen into glucose-l-phosphate, which is further converted into glucose-6-phosphate.
• Finally, after the release of the phosphate group, it is converted to glucose.

 

Mechanism of hormone action by changing the permeability of plasma membrane

• In this case, the hormone-receptor complex changes the permeability of the cell membrane of the target cell.
• This allows the metabolic substance to enter the target cell which increases cellular metabolism.

Example:

1. Insulin changes the permeability of the sarcolemma of muscle cells or muscle fibers. Due to this, glucose molecules can easily enter the muscle cell or fiber, from circulation.
2. The receptor for insulin, present in the cell membrane, has two -subunits and two -subunits. Insulin binds to the binding sites of outer a-subunits, to form the hormone-receptor complex.
3. This changes the conformation of the subunits. As a result, activated subunits change into active tyrosine kinase, which further activates G-proteins present in the cell membrane.
4. Active G-proteins further activate the phosphodiesterase enzyme. It converts phosphatidylinositol 4,5-biphosphate (PIP2), into inositol triphosphate (IP3)
   and diacylglycerol (DAG). These compounds function as second messengers
5. Glucose Transporter like GLUt-4 can flow through the cytoplasm, with the help of IP3, towards the cell membrane.
6. It binds to the cell membrane protein, changing its permeability. Thus the channels produced in the cell membrane, allow glucose to pass into the muscle cells.

 

Nuclear Receptors Or Intracellular Receptors

Nuclear or intracellular receptors are the receptors of steroid hormones. These are present in the cell cytoplasm. These receptors have binding sites for both hormones (ligands) and DNA. These receptors are transcription factors regulated by ligand binding.

Members of this family include the receptors for—

1. Progesterone and estrogen,
2. Glucocorticoids and mineralocorticoids,
3. Vitamin D and thyroid hormone,
4. Retinoic acid and 9-cis-retinoic acid,
5. Orphan receptors (a group of receptors with unknown ligands).

Mechanism of hormone action involving intracellular receptors

This mechanism is described as follows—

• Steroid hormones and the active steroid derivative of vitamin D are lipid-soluble and hydrophobic. Therefore, they cross the plasma membrane to bind to intracellular hormone receptors. This forms a hormone-receptor complex.
• The binding of hormones to the receptor produces a conformational change in the structure of the receptor. It allows the hormone-receptor complex to enter the nucleus and bind to specific DNA sequences.
• This results in either activation or repression of gene transcription.
• Within the nucleus, the synthesis of mRNA (transpiration) increases. The mRNA is translated into Proteins in the Production Of Enzymes, Stimulating Metabolic Activities within the Cell.

Hormones As Chemical Messengers

• Hormones serve as chemical messengers in the body and help to maintain the internal chemical balance i.e., homeostasis of the body.
• Hormones regulate and coordinate physiological and metabolic functions by acting on receptors located on or inside target cells.
• They act on receptors of target cells located far away from the secretory cells or glands. They are carried to their target cells by plasma or lymph. These hormones stimulate the metabolic reactions within the target cells.

Example:

• Adenohypophysis of the pituitary gland secretes TSH hormone. It diffuses into the network of blood capillaries.
• It is carried to the location of the thyroid glands. It stimulates the thyroid glands to secrete the hormones T₃ and T₄. Thus, the hormone TSH acts as a chemical messenger.

Hormones As Regulators

• Hormones regulate the concentration of different components within the body fluids.
• They also regulate different metabolic processes in the body. This maintains the internal chemical balance.

Example:

• The islets of Langerhans of the pancreas, secrete hormones like insulin and glucagon.
• Both the hormones regulate the glucose concentration in the blood. When the concentration of glucose rises in the blood, insulin converts excess glucose into glycogen which gets stored in the liver.
• This lowers the blood glucose level. Glucagon, on the other hand, acts when the blood glucose level falls below normal.
• It converts glycogen proteins and fats into glucose. This increases the glucose concentration in the blood.

Role Of Hormomes In Maintaining Homeostasis

• Hormones are secreted by the endocrine glands. They are carried to their target cells by blood. They maintain equilibrium inside the cells.
• To maintain internal equilibrium, the secretion of hormones needs to be regulated according to the need.
• The decline or rise in the secretion of a hormone depends on its concentration in the blood. This mechanism is called feedback control.

This is of two types—

• Negative and
• Positive feedback control.

Negative Feedback Control

In this type of control mechanism, the synthesis of a hormone is reduced or entirely stopped when its amount in the blood is above normal.

Example:

• The normal concentration of T₄ in human blood or serum is about 4-12 pg/lOOmL and that of T₃ is about 80-200 ng/lOOmL.
• When the concentrations of these two hormones rise in the blood, a negative signal is generated and sent to the hypothalamus.
• This prevents the synthesis and secretion of TSH-RH from the neurosecretory cells of the hypothalamus. As a result, secretion of TSH from the thyrotroph cells of the adenohypophysis also decreases.

Positive Feedback Control

In this system, the synthesis of a hormone increases if its amount in the blood is below normal.

Example: When The Concentrations of these two hormones— T₄ and T₃ decrease in blood, a positive signal is generated and sent to the hypothalamus.

• This induces the synthesis and secretion of TSH-RH from the neurosecretory cells of the hypothalamus.
• Secretion of TSH-RH further stimulates the thyrotroph cells leading to increased secretion of TSH.
• Uterine contraction at the onset of labor in pregnant women releases oxytocin hormone from the posterior pituitary.
• The presence of this hormone intensifies the contractions. Increased uterine contraction causes more oxytocin to be released and the cycle goes on until the baby is born.
• The birth halts the release of this hormone, thus, ending positive feedback control.

Disorders Related To Endocrine Glands

• Various factors affect the secretion of hormones, causing several disorders diseases, or syndromes in the body.
• Oversecretion of the hormones is called hypersecretion, while less secretion of the hormones is called hyposecretion. Some of the disorders caused by hyper and hyposecretion of hormones have been discussed below.

Disorders Related To Pituitary Gland

The disorders related to the pituitary gland are as follows—

Dwarfism

Dwarfism is a disorder characterized by shorter-than-normal skeletal growth.

Dwarfism Causes: Two major causes of this disorder are achondroplasia and growth hormone deficiency (hyposecretion)

Achondroplasia: It is a disorder caused due to the presence of a defective allele in the human genome. It accounts for 70% of dwarfism cases. It often leads to increased spinal curvature and distortion of skull growth.

Growth Hormone Deficiency: It is a medical condition in which the body produces insufficient growth hormone. It may result in mutations of specific genes, damaged pituitary gland, Turner’s Syndrome (a disorder due to the absence of an X chromosome in females), poor nutrition, or even stress.

Growth Hormone Deficiency Symptoms:

• Children with growth hormone deficiency have a slow growth rate and have a short body build. The height may range up to 3 feet.
• More severe forms of dwarfism are associated with abnormal functioning of other organs, such as the brain or liver.
• Bones also get affected due to this disorder. Early degenerative joint disease, lordosis, or scoliosis (curvature in the vertebral column) can cause pain and disability.
• Improper growth and development of the; primary sex organs occur, leading to infertility, The Appearance of secondary sexual characteristics may also get delayed.

Growth Hormone Deficiency Treatment:

• Early diagnosis and treatment can prevent some of the problems associated with dwarfism.
• People with dwarfism due to growth hormone deficiency can be treated with growth hormone.
• However, the treatment should begin before adolescence. After adolescence, the treatment will become ineffective.

Gigantism

Gigantism is the condition of abnormal growth in height and elongation of long bones in childhood.

Gigantism Causes: Gigantism may result due to—

• Hypersecretion of STH or GH in childhood
• Tumour in the acidophilic cells or adenoma in the adenohypophysis region, before the onset of adolescence.

Gigantism Symptoms:

The skeletal system and muscles show increased growth than normal. The hands and feet become larger.

1. The height of the body reaches 7-8 ft.
2. Growth of the different internal organs gets stimulated.
3. Basic Metabolic Rate (BMR) increases.
4. The growth of different endocrine glands or endocrine cells is comparatively less. The Thymus gland increases in structure.
5. The glucose concentration in the blood increases.

Gigantism Treatment: This disorder can be treated by surgery, radiotherapy, etc. However, the treatment must be started at the early stage.

Acromegaly

Acromegaly is caused by hypersecretion of growth hormone (GH) in adults, resulting in an abnormal increase in bone and soft tissue growth.

Acromegaly  Causes: The causes of acromegaly are—

Hypersecretion of growth hormone in adult tumors in the acidophilic cells of the adenohypophysis of the pituitary gland.

Acromegaly  Symptoms:

1. Excess GH secretion leads to disproportionate skeletal growth. The appearance becomes gorilla-like with a lower jaw protruding forward and a forehead slanting forward.
2. Soft tissue swelling acromegaly Acromegaly is caused by hypersecretion of growth hormone (GH) in adults, resulting in an abnormal increase in bone and soft tissue growth.

Causes: The causes of acromegaly are—

• Hypersecretion of growth hormone in adults
• Tumour in the acidophilic cells of the adenohypophysis of the pituitary gland.

  Symptoms:

Excess GH secretion leads to disproportionate skeletal growth. The appearance becomes gorilla-like with the lower jaw protruding forward and the forehead slanting forward.

Soft tissue swelling kidney, etc., results in the weakening of muscles in these organs. Weak muscles around vocal cords lead to thick, deep voices and slow speech patterns.

Treatment: Current treatment options include surgery, radiotherapy, etc. Other than these, treatment with somatostatin analogs (stop GH production) and dopamine agonists like bromocriptine (reduce the size of the tumor and control GH-induced hyperglycemia) are useful.

Diabetes Insipidus

This disease is caused by the hyposecretion of ADH hormone from the posterior pituitary gland.

Disorders Related To Thyroid Gland

• The disorders related to the thyroid gland are as follows—:% Cretinism Cretinism is a disorder caused by to hyposecretion of thyroid hormones.
• It is characterized by stunted physical and mental growth in children. It is also known as congenital hypothyroidism.

Thyroid Gland Causes:

• Reduced secretion of thyroid hormones (hypothyroidism) is the major cause of cretinism in children.
• Deficiency of iodine in maternal nutrition may cause this disorder in the newborn.

Thyroid Gland Symptoms:

1. Reduced growth of the body. The limbs are shorter than normal.
2. The tongue thickens and protrudes from the mouth involuntarily. Excess saliva secretion occurs. Short and wide neck, pot belly, scanty hair, and swollen eyelids are observed.
3. Glucose, iodine, and thyroxine concentrations decrease in the blood.
4. The growth of primary sex organs and the appearance of secondary sexual characteristics get delayed. Treatment: Hormone therapy is the common approach for treating hypothyroidism. Daily intake of thyroxine hormone tablets can counter this disorder.

Myxoedema

Myxoedema is caused due to the deficiency of thyroxine hormone in adults. It is more common in females than in males.

Myxoedema Causes:

• Hyposecretion of thyroxine in adults causes myxoedema. This occurs due to the deficiency of iodine. Removal of the thyroid gland by surgery, use of antithyroid drugs, or ionizing radiations may cause myxoedema.
• Autoimmune thyroiditis (an inflammatory disorder of the thyroid gland) may lead to myxoedema.
• Hashimoto’s disease or autoimmune thyroiditis
• It is an autoimmune disease and occurs when our immune system attacks the thyroid tissue.
• It results in the inflammation of the thyroid gland (thyroiditis) and may cause hypothyroidism.

Myxoedema  Symptoms:

1. The limbs and neck become swollen. This occurs due to the accumulation of mucus
   within the cells in this region. So it has been named myxoedema (edema caused by to accumulation of mucus).
2. Loss of memory takes place.
3. Anorexia (eating disorder) is observed among the patients.
4. BMR decreases.
5. The rate of heartbeat decreases.
6. Due to the swollen nature of the face, the eyes appear tiny.
7. Levels of carbohydrates, iodine, and thyroxine in the blood, decrease.

Myxoedema  Treatment: Introducing thyroxine hormone into the body, through the process of hormone therapy, may be helpful in treating myxoedema.

Goitre

Goitre is characterized by the enlargement of the thyroid gland. It may be caused due to hypo or hyperthyroidism.

Goitre is generally of two types—

Simple goiter: The condition of an enlarged thyroid gland, due to the hyposecretion of thyroxine is called simple goiter.

Goitre Causes:

• It is caused due to deficiency of iodine in the diet which results in low thyroxine secretion.
• In response to this, there is reduced negative feedback inhibition of TSH. The elevated TSH secretion stimulates the thyroid to enlarge.
• People living in hilly regions suffer from endemic goiter because the soil in those regions does not contain iodine.
• The secretion of T₃ and T₄ hormones may be less due to genetic defects.

Goitre Symptoms: This disease is characterized by the enlargement of the thyroid gland. This is manifested as a swelling of the front part of the throat. In severe cases, an enlarged thyroid can exert pressure on the trachea and esophagus.

This can lead to—

Breathing trouble which may lead to the condition of dyspnoea,

1. Cough,
2. Hoarseness of voice,
3. Difficulty in swallowing.

Goitre Treatments: The regulated use of iodized salt allows the thyroid gland to producer the thyroid hormones and therefore helps to prevent simple goiter. It can also be treated by consuming iodine-rich food like marine fish.

Exophthalmic Goiter or Grave’s disease: The condition in which the thyroid gland becomes overactive, in the case of hyperthyroidism, is called exophthalmic goiter.

Exophthalmic Goiter  Causes: Increased TSH receptor stimulation leads to thyroid hyperplasia (follicles enlarge in size), causing enlargement of the thyroid gland. This leads to hypersecretion of thyroxine hormone.

Exophthalmic Goiter  Symptoms:

1. The eyes protrude because of edema in the tissue of the eye socket. Eye muscles swell and cause less blinking of the eyelids.
2. The person may not be able to close his eyes while sleeping. This causes drying up and infection of the conjunctiva.
3. The patient usually becomes hyperactive, nervous, irritable, and suffers from insomnia.
4. The BMR increases.
5. The heartbeat also increases.
6. Hypersecretion of thyroid hormones results in osteoporosis in the patients.

Exophthalmic Goiter  Treatments:

Hyperthyroidism can be treated with propylthiouracil, methimazole, etc. These drugs prevent the bond formation between iodine and tyrosine.

This prevents the formation of thyroxine. Removal or destruction of a portion of the thyroid by means of radioactive iodine is sometimes effective therapy in this condition.

Disorders Related To Parathyroid Gland

The disorders related to the parathyroid gland are as follows

Hypoparathyroidism

The disorder caused by to hyposecretion of parathyroid hormone from the parathyroid gland is called hypoparathyroidism.

Hypoparathyroidism Causes:

1. Hypoparathyroidism is caused by the hyposecretion of parathyroid hormone (PTH).
2. Hyposecretion of PTH may be due to inflammation or a tumor in the parathyroid gland.

Hypoparathyroidism Symptoms:

• The calcium level in the blood decreases, i.e., hypocalcemia occurs.
• The contraction-relaxation mechanism of the muscles decreases, and spasms occur within the muscles.
• When this condition becomes severe, hands, fingers, etc., become distorted leading to the condition of tetany.
• In the case of women, the menstrual cycle is painful.
• The skin becomes dry and nails become brittle.
• Fatigue, weakness, headache, mental depression, etc., are also some important symptoms of the disease.

Hypoparathyroidism Treatment: During the initial stages of the disease, introducing calcium and vitamin D supplements within the body, may be helpful.

Hyperparathyroidism

• The disorder caused by to hypersecretion of PTH hormone from the parathyroid gland is called hyperparathyroidism.
• Hyperparathyroidism Causes: The main cause of this disorder is excess PTH secretion. Due to the presence of a tumor in the parathyroid gland, secretion of PTH increases.

Hyperparathyroidism Symptoms:

• The concentration of calcium in the blood increases i.e., hypercalcemia is observed.
• Renal calculi may occur due to excess calcium deposition.
• The bones may become soft and distorted due to the leaching of calcium. Cysts may be found within the bone. This abnormality of bones is called osteitis fibrosa cystica or brittle bone disease.
• The muscular layer of the inner lining of the left atrium may get thickened, known as left ventricular hypertrophy. This leads to malfunctioning of the heart.

Hyperparathyroidism Treatment: The tumor containing the parathyroid gland has to be removed by surgery.

Tetany

The disorder involves involuntary contraction of muscles or muscle spasms. It is caused by to malfunction of parathyroid glands and consequent deficiency of calcium.

Tetany Causes: The various causes of tetany are as follows— Hypocalcemia (deficiency of calcium),

• Hypoparathyroidism,
• A low level of magnesium and a higher level of phosphate in the blood,
• Deficiency of vitamin D and calcium in the diet, for a long period of time,
• Infection with Clostridium tetani.

Tetany Symptoms:

• Due to sudden contraction of the facial muscles, the nose and lips become distorted.
• Due to muscular spasms, wrists, fingers, joints, etc., become distorted. Especially the thumb bends towards the palm.
• In some cases, paresthesia may occur, and a piercing or burning sensation is felt.
• Treatment: For the treatment of tetany, Ca and vitamin D supplement is essential. This restores the calcium concentration in the blood.

Disorders Related To Pancreas

The disorders related to the pancreas are as follows—

Diabetes mellitus

Diabetes mellitus is a disorder characterized by increased blood glucose levels.

In general, there are two forms of diabetes mellitus—

Type 1 (TIDM) and Type 2 (TIIDM).

Type 1 Diabetes Mellitus: It is caused due to reduced secretion of insulin. Thus, type 1 Diabetes mellitus is also called insulin-dependent diabetes (IDDM)

Type 1 Diabetes Mellitus Cause: Low insulin secretion due to autoimmune destruction of the islet cells is the major cause of type I diabetes mellitus.

This results in impaired entry of glucose into the cells and hence, reduced accumulation of glucose in the blood.

Type 1 Diabetes Mellitus Symptoms:

• Accumulation of glucose in the blood (hyperglycemia) leads to increased plasma osmolarity (glucose concentration more than 80-120 mg/100 mL).
• This is accompanied by excess loss of water and sodium (polyuria).
• Excess glucose in blood leads to the discharge of excess glucose through urine (glycosuria).
• The resulting dehydration triggers compensatory mechanisms such as thirst (polydipsia).
• The inability of the cells to utilize glucose creates a state of cellular starvation.
• It also leads to the accumulation of ketone bodies in the blood (ketoacidosis). Diabetic ketoacidosis is an acute pathologic event characterized by elevated blood glucose levels, ketone bodies, and metabolic acidosis.

Type 1 Diabetes Mellitus Treatment: By taking proper medical advice, the glucose level in the blood can be kept under control. In severe cases, insulin therapy may be used.

Type 2 Diabetes Mellitus: Type 2 diabetes mellitus is also called Insulin-independent or non-insulin-dependent diabetes mellitus (NIDDM). It is much more common than type 1, accounting for about 90% of cases.

Type 2 Diabetes Mellitus Causes: Type 2 diabetes mellitus is caused due to a decreased responsiveness of insulin receptors in the peripheral tissues to insulin. This inhibits the action of insulin (insulin resistance) and causes a decrease in the absorption and oxidation of glucose in the cells. As a result, glucose concentration rises in the blood.

Type 2 Diabetes Mellitus Symptoms:

• Symptoms include hyperglycemia, glycosuria, dehydration, etc., which are also observed in the case of IDDM.
• Other than these, the retina may get damaged (retinopathy), which can lead to blindness.
• Diabetes mellitus is associated with increased risk factors for coronary heart disease (CHD).
• Some long-term complications include nephropathy (damage of nephrons) that leads to renal failure, and neuropathy (damage of neurons).
• Treatment: Treatment aims at the control of blood sugar, and is monitored by prevailing levels of glycosylated hemoglobin.
• Regular medical check-ups must be conducted to keep the blood glucose under control.

Hypoglycemia due to over-secretion of insulin

Due to the over-secretion of insulin by the beta 3 cells of islets of Langerhans, the glucose level decreases in the blood (about 50 mg/100 mL of blood). This is called hypoglycemia.

This condition leads to weakness. In severe cases, a person may suffer from muscle contractions, convulsions, and even coma.

Disorders Related To Adrenal Cortex

The disorders related to the adrenal gland are as follows—

Addison’s disease

Addison’s disease is caused by low secretion of adrenal hormones, cortisol and aldosterone also occurs due to antibodies produced against the cortical cells, and microbial infection of glands or tumors.

Addison’s Disease Symptoms:

• The main symptoms of Addison’s disease include loss of appetite, weakening of muscles, decrease in BMR, and decrease of storage of proteins in the body.
• Lack of aldosterone results in loss of sodium and water and retention of potassium. This leads to low blood pressure and possibly severe dehydration.
• Without cortisol, glucose concentration decreases in the blood, causing hypoglycemia.
• As ACTH causes a buildup of melanin deposition, hence, hyposecretion of ACTH leads to the darkening of the skin.
• Mental stress is also seen.

Addison’s Disease Treatment: Hormone replacement therapy is the standard treatment for Addison’s disease.

Cushing’s Syndrome

This disorder is caused by the abnormally high levels of cortisol.

Cushing’s Syndrome Causes: Cushing’s syndrome is caused by high levels of cortisol secreted from the adrenal cortex in the body.

This may occur due to a tumor in the adrenal cortex. Even tumors in the pituitary gland can lead to such diseases. Over-secretion of ACTH and prolonged ACTH therapy may cause Cushing’s syndrome.

Cushing’s Syndrome Symptoms:

1. Muscle protein is metabolized, leading to muscle degeneration.
2. Excess subcutaneous fat is deposited especially around the midsection of the body due to the over-secretion of cortisol. For this reason, the trunk appears obese, while the arms and legs appear thinner.
3. The depletion of calcium from the bones leads to osteoporosis (brittle bones).
4. An excess of aldosterone and reabsorption of sodium and water by the kidneys leads to a basic blood pH and hypertension.
5. Hyperglycemia along with mental stress and depression are seen.
6. The face becomes moon-shaped due to swelling. This is known as the moon face.

Cushing’s Syndrome Treatment: Treatments include surgery, radiation, chemotherapy, or the use of cortisol-inhibiting drugs.

Transsphenoidal adenomectomy (surgical removal of the tumor) is the most widely used treatment.

Chemical Coordination Notes

• Acinus: Cluster of cells resembling many-lobed berry.
• Anorexia: An eating disorder characterized by weight loss and or lack of appetite
• Catecholamine: Organic compounds containing catechol and a side chain amine (Collectively epinephrine non-epinephrine).
• Hyperfunctioning: Increased activity of an organ system or its parts
• Neurohormones: Hormones Released By Neurons
• Signal: Message Transmitted On Binding Of A molecule (like Hormone) To Its Receptor.
• Target Organs: The organ That Acts As The site Of a Hormone.

Points Of Remember

1. The nervous system and the endocrine system both, work together, to maintain the coordination among different organs while regulating their functions.
2. Glands are the most important part of the endocrine system.
3. Glands may be of three types—exocrine, endocrine, and exocrine.
4. Exocrine glands secrete enzymes, that are carried by ducts from the synthesizing cells to the target cells.
5. Exocrine glands are of three types on the basis of their modes of secretion—merocrine, apocrine, and holocrine.
6. Endocrine glands secrete hormones, that are carried by blood or lymph to the target cells situated away; from the synthesizing cells.
7. Exocrine glands have both exocrine and endocrine properties.
8. The study of endocrine glands, hormones, and their associated effects is called endocrinology.
9. Thomas Addison is known as the ‘Father of Endocrinology1.
10. Some of the endocrine glands include— the hypothalamus, pituitary, pineal gland, thyroid gland, parathyroid gland, adrenal gland, pancreas, gonads, thymus, and placenta.
11. Tropic hormones regulate the functioning of the other endocrine glands.
12. Neurohormones are secreted by nerve cells. These include ADH and oxytocin.
13. The hypothalamus serves as the main connection between the endocrine and nervous systems.
14. Herring bodies are large swellings at the terminal end of the axons at the posterior pituitary. These temporarily store ADH and oxytocin hormones.
15. FSH is also known as the gametokinetic factor.
16. The presence of hCG hormone in the urine of females is a determining factor, used in a pregnancy detection kit.
17. Hormones bind to their specific receptors, thereby forming hormone-receptor complex.
18. The receptors may be of two types—extracellular and nuclear or intracellular receptors.
19. Myasthenia gravis is a disease caused by the hyposecretion of the hormone, thymosin.
20. Several endocrine cells are also present in the gastrointestinal tract. These cells secrete gastrointestinal hormones, which help in digestion.