Sainavle

Cardiac Output

Cardiac Output Definition: Cardiac output is the volume of blood pumped by each ventricle per minute.

Cardiac output depends on the frequency of heartbeat as well as the volume of blood ejected in one contraction.

It is expressed as minute volume or litres of blood per minute. The usual values of cardiac output for adults when at rest are 5 to 6 L/min or approximately 8% of body weight per minute.

With strenuous activity, an adult’s cardiac output can increase to almost 25 L/min to satisfy the body’s demands for nutrients and oxygen. Cardiac output is a product of heart rate (beats/ minute) and stroke volume.

Read and Learn More Class 11 Biology Notes

The amount of blood pumped by each ventricle with each heartbeat is called stroke volume.

In a healthy adult at resting condition, the stroke volume is 70 ml of blood. Cardiac output is calculated by the formula— Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR) So, the cardiac output is 70 x 72 = 5040 ml min’1 or approximately 5 litres min’1.

The cardiac output represents the volume of blood that is delivered to the body. It is an important factor in the determination of the effectiveness of the heart to deliver blood to the rest of the body.

Cardiac Index And Stroke Volume Index

Normal, resting cardiac output differs among individuals. Therefore, in clinical practice, measured values for cardiac output are often expressed as a flow (L/min) per body surface area (m2) which is called cardiac index.

It is the volume of blood released by each ventricle of the heart per minute to the body surface.

The surface area is estimated from calculations based on body weight and height. Cardiac index normally ranges from 2.6 to 4.2 L/min/m2.

The stroke volume index is the volume of blood released during each heartbeat with respect to the surface area of the body. The average value of stroke volume index in a healthy adult human is 47ml.

In 1870, German physiologist Adolf Fick contrived the first method for measuring cardiac output in animals and human beings.

The basis of this method is called Fick’s principle. This principle states that the amount of any substance received by an organ per unit of time is equal to the arterial level of the substance minus the venous level times the blood flow.

So Blood flow through an Organ In a given time =

Amount of substance removed from or added to blood during the same time period/ A- V difference of concentration of the substance/ml. of blood.

Over the past decades, many methods for cardiac output measurement have been developed like thermodilution method, oesophageal Doppler method, transoesophageal echocardiography, pulse contour cardiac output, lithium dilution cardiac output measurement, etc.

Factors Controlling Cardiac Output

Maintaining and regulating cardiac output is one of the most intricate functions of the circulatory system.

The cardiac output is the product of stroke volume and heart rate. Therefore, factors regulating stroke volume and/or heart rate are able to control it.

The key factors regulating cardiac output are given below—

Sympathetic stimulation: Increased sympathetic stimulation (due to fright, anger, etc.) increases the heart rate. It also increases stroke volume by increasing contractility, which results in a more complete ejection of blood from the heart.

Hence, cardiac output increases which is again brought to normal by increased parasympathetic activity after sympathetic stimulation.

Venous return: Cardiac muscle fibres are stretched by increased blood volume returning to the heart.

Increased stretch results in a greater force of contraction, which increases stroke volume and thereby cardiac output. Decreased venous return decreases cardiac output.

Heart rate: Increased heart rate decreases stroke volume but increases cardiac output. Excessive increase in the heart rate decreases cardiac output.

Myocardial contractility: Increased myocardial contractility results in increased cardiac output.

The strength of the force of myocardial contraction depends on the initial length of cardiac muscle fibres, nutrition and availability of oxygen, concentration of H+ ions, etc.

Peripheral resistance:

• The pressure against which ventricles pump blood is called afterload.
• This is determined by the resistance of arterioles to blood flow, which is called peripheral resistance.
• Increased peripheral resistance will reduce the volume of blood flow through the arteries and will in turn increase afterload.
• This will lower the stroke volume and thereby decrease cardiac output. Thus, peripheral resistance is inversely proportional to cardiac output.

Exercise: Exercise activates the sympathetic nervous system, increasing heart rate and contractility of the myocardium. Both of these actions increase venous return, contributing to increased stroke volume. Hence, cardiac output is increased.

Blood pressure

Blood pressure can be defined as the lateral pressure exerted by the flowing blood on the walls of the blood vessels (especially the arteries).

Types of blood pressure are—

Systolic blood pressure: The peak value of pressure exerted by the blood on the walls of the arteries during ventricular contraction or systole, is known as systolic blood pressure. Normal systolic blood pressure in an adult is in a range of 105-135 mm Hg (average 120 mm Hg) during each cardiac cycle.

Diastolic blood pressure: The minimum pressure exerted by the blood on the walls of the arteries during ventricular relaxation or diastole, is known as diastolic blood pressure. Normal diastolic blood pressure in an adult is about 80mm Hg (60 – 90 mm/Hg).

Pulse pressure: The term pulse refers to the alternating surges of pressure (expansion and then recoil) in an artery that occurs with each contraction and relaxation of the left ventricle. This difference between systolic and diastolic pressure is called pulse pressure.

Normally the pulse pressure is SBP-DBP = (120-80)= 40mm Hg.

Mean arterial pressure: Mean arterial pressure (MAP) is the average pressure during the cardiac cycle. The value of MAP varies at different levels of the circulatory system.

Blood pressure can be measured directly or indirectly

Direct measurement of blood pressure is done by cardiac catheter with the help of a pressure transducer or by putting a needle in an artery and connecting with a pressure transducer or simply a Hg manometer.

Indirect measurement of blood pressure involves auscultatory palpatory or oscillatory method by using a sphygmomanometer.

commonly called a blood pressure cuff, is an instrument used to obtain blood pressure readings by the auscultatory method.

Electrocardiogram [ECG]

Electrocardiogram Definition: The electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart recorded during the cardiac cycle.

Discovery: In 1887, Augustus D Waller recorded the first electrocardiogram. After that in 1903, Willem Einthoven studied the details of ECG and he is known as the father of ECG. After 1930, many advanced forms of ECG have been developed.

Description: The electrical activity of the heart is monitored and recorded on the ECG. The process of recording ECG is known as electrocardiography. An ECG configuration may consist of waves, and intervals recorded as voltage against time. A single waveform begins and ends at the baseline.

Electrocardiograph

Cardiac impulse, generated from the SA node, is transported to all tissues of the heart as electrical impulse via the AV node, a bundle of His, Purkinje fibre, etc., and all over the body through cells and blood.

So, the electrical activity of the heart can be observed by using electrodes at different parts of the body. The instrument used to measure these activities is known as electrocardiograph.

Einthoven’s triangle

During electrocardiography, an imaginary triangle is formed with three limb leads or electrodes (two shoulders and one left leg) by keeping the heart at the centre. This is known as Einthoven’s triangle

Methods of ECG recording: There are various methods for ECG recording. The electrodes that are used for the procedure, are placed in different parts of the body. Generally, there are two types of methods for electrocardiography—unipolar and bipolar ECG methods.

Waves Of Electrocardiogram

The ECG recorded on graph paper is composed of five types of waves— P, Q, R, S and T.

The P wave: The normal cycle of electrical activity in the heart begins with atrial depolarisation and is recorded as the P wave.

The shape of the P wave is usually symmetrical and upright. The normal duration of the P wave is 0.08 to 0.11 seconds.

The QRS complex: The QRS complex represents ventricular depolarization.

The QRS complex consists of three separate waveforms—

Q wave, R wave, and S wave. The first negative deflection (below the baseline) after the P wave is the Q wave.

The next tall positive deflection (above the baseline) is the R wave. The S wave is the small negative deflection (below the baseline) that follows the R wave. Under normal conditions, the duration of the QRS complex is less than 0.10 seconds.

The ST segment: The ST segment represents the time between ventricular depolarization and repolarization.

It begins at the end of the QRS complex (called the J point) and ends at the beginning of the T wave. Normally, it measures 0.12 seconds or less.

The T wave: The T wave represents ventricular repolarization, rest, and recovery. Its duration normally measures 0.20 seconds or less.

T waves are sensitive T indicators for the presence of a number of abnormalities.

The U wave: The U wave follows the T wave and has the same polarity (deflection) as the T wave. Its origin and mechanism are not known. Due to its low voltage, it is usually flat and almost not visible.

The PR interval: The PR interval starts at the beginning of the P wave and ends at the beginning of the QRS complex.

The normal duration of the PR interval is 0.12 to 0.20 seconds. The PR interval represents the total atrial (supraventricular) electrical activity prior to the activation of the bundle of His, ventricular branches, and Purkinje fibre system.

The QT interval: The QT interval is measured from the beginning of the QRS complex to the end of the T wave.

The QT interval represents total ventricular activity. The normal QT interval measures about 0.38 seconds and varies in males and females and with age.

Significance Of ECG

Electrocardiography is an indispensable tool in the diagnosis, prognosis and therapeutic aspect most of cardiac disorders.

From the ECG tracing, the following information can be determined—

1. Heart rate and heart rhythm,
2. Conduction abnormalities,
3. A prior heart attack,
4. Chances of coronary artery disease,
5. Abnormal thickness of heart muscle.

An electrocardiogram captures the electrical activity of the heart, so it can give information about the following conditions—

1. Myocardial Infarction,
2. Re-entry pathways in the electrical system,
3. Location of the heartbeat initiator, types of cardiac arrest (ventricular fibrillation, a systole, etc.) and other different changes in the electrical activity of the heart.

Double Circulation

Double Circulation Definition: When pathways, blood flows through called double the heart circulation.

Double circulation involves the movement of blood through the heart twice during each trip of the body.

The two pathways involved in this area—

Pulmonary circulation (i.e., transportation of blood from the heart to the lungs) and systemic circulation (i.e., transportation of blood from the heart to the rest of the body).

A double circulation must have a three or four-chambered heart. The right side of the heart receives deoxygenated blood from all over the body and pumps it to the lungs.

In the lungs, the blood loses CO₂ and receives O₂. The left side of the heart receives oxygenated blood from the lungs and transports it to all the body parts.

Amphibians, reptiles, birds and mammals have double circulation. Fishes have two-chambered hearts. They have a single circulation pathway.

Mechanism Of Double Circulation

The double circulation in human beings can be divided into— pulmonary circulation and systemic circulation.

Pulmonary Circulation

Pulmonary Circulation Definition: Pulmonary circulation is the movement of blood from the heart to the lungs, and then from the lungs back to the heart again. The pulmonary system begins with the pulmonary trunk and ends in the left atrium.

Pathway of circulation:

The pulmonary trunk originating from the right ventricle, divides into right and left pulmonary arteries which enter the right and left lung respectively.

These pulmonary arteries carry deoxygenated blood and divide into smaller arteries, arterioles and capillaries around the lung alveoli.

After gaseous exchange, oxygenated blood from the alveolar capillaries is carried by four pulmonary veins to the left atrium.

Systemic circulation

Systemic circulation Definition: The circulation, in which blood flows from the left ventricle to the body and again returns to the right atrium is known as systemic circulation.

Pathway of circulation:

The aorta originates from the left ventricle and then forms the arch of the aorta. It moves through the lungs as the dorsal aorta. This dorsal aorta crosses the diaphragm and moves into the abdominal cavity. Here, it is known as the abdominal aorta.

The aorta divides into many branches which form capillaries to supply oxygenated blood to different parts of the body.

The visceral capillaries join together to form venules and veins.

Veins combine together and form two main veins—superior vena cava and inferior vena cava. They drain deoxygenated blood into the right atrium from the visceral capillaries.

Regulation Of Cardiac Activity

Cardiac activity in humans is regulated mainly by—neural control and hormonal control.

Neural control: Neural control of the cardiovascular system involves two nerve centres— the cardioinhibitory centre (located in the medulla oblangata of the brain) and the cardioacceleratory centre (located in the vertebral column).

Parasympathetic nerves in the brain and sympathetic nerves in the spinal cord are involved in this regulation.

Stimulation of parasympathetic nerve fibres originating from the cardioinhibitory centre reduces heart rate and blood pressure. This nerve fibre is known as the vagus nerve or 10th cranial nerve.

This nerve fibre is distributed in the SA node, AV node and atrial muscles. Stimulation of sympathetic nerve fibres originating from the cardio accelerator centre of the spinal cord increases heart rate, blood pressure and contractility of cardiac muscles.

These nerve fibres are distributed in the SA node, AV node and the cardiac muscles.

Baroreceptors are a type of sensory neurons that are excited by the stretch of the blood vessel or increase in blood pressure.

They are present in different parts of the body. Due to their stimulation, the produced nerve impulse moves to the brain through the nerves.

As a result, the impulses from the cardioinhibitory centre come to the heart through the vagus nerve. This, in turn, reduces heart rate.

Hormonal control: Various hormones play a role in the control of the cardiovascular system.

Important sites of hormone secretion include the adrenal medulla, posterior pituitary gland, kidney, and atrium of the heart.

The major hormonal role is played by epinephrine and norepinephrine. Other hormones such as thyroxine, arginine, vasopressin and erythropoietin have some effect on the cardiovascular system.

Atrial muscle cells produce atrial natriuretic peptide which plays an important role in this.

Epinephrine and norepinephrine: When the sympathetic nervous system is activated, the adrenal medulla releases epinephrine (>90%) and norepinephrine (<10%), which circulate in the blood.

Epinephrine and norepinephrine have similar direct effects on the heart, but norepinephrine elicits a powerful baroreceptor reflex because it causes systemic vasoconstriction and increases mean arterial pressure.

Myogenic heart

• A myogenic heart is capable of generating an action potential, depolarization and consequent contraction from within the muscle itself instead of receiving any stimulus from a nerve.
• A healthy heart will perform myogenic rhythms that will allow it to beat 60 to 100 times per minute. Myogenic heart is found in animals like molluscs and vertebrates.

Neurogenic heart

The neurogenic heart refers to the highly specialized muscle cells that require neural input to initiate and maintain contractions. The neurogenic heart is found in animals like annelids and arthropods.

Atrial natriuretic peptide (ANP): Atrial natriuretic peptide is a 28-amino acid polypeptide synthesised and stored in the atrial muscle cells. They are released into the bloodstream when the atria are stretched. It decreases blood volume by increasing sodium excretion.

Other hormones: Thyroxine hormone directly affects the SA node of the heart and increases the heart rate.

Arginine vasopressin (AVP) exerts its major effect on the cardiovascular system by causing the retention of water by the kidneys, which regulates blood volume. Erythropoietin regulates RBC synthesis by stimulating bone marrow.

The decrease in hematocrit stimulates its release, which stimulates RBC synthesis.

Therefore, the increase in plasma volume is balanced with a larger RBC mass.

Disorders Related To Cardiovascular System

There are different types of abnormalities found in the function of the heart and in the circulation of blood.

Hypertension or high blood pressure

Hypertension is a transitory or sustained elevation of both the systemic and pulmonary arterial blood pressure to a level that induces cardiovascular damage.

If the blood pressure lies between 120/80 mm Hg and 139/89 mm Hg, it is called prehypertension (to denote increased risk of hypertension).

A blood pressure of 140/90 mm Hg or above is considered as hypertension.

Cause: Though the exact causes of hypertension are usually unknown, there are several factors including smoking, obesity, lack of physical activity, sedentary lifestyle, etc., which may be responsible for this condition.

Types of hypertension: There are two broad categories of hypertension—

Primary or essential hypertension and Secondary hypertension.

Primary hypertension: It may develop as a result of environmental or genetic causes. It accounts for 90% of the total cases.

It is further divided into two types—

Benign or chronic hypertension: In this condition, blood pressure is usually higher than normal over a long time.

It increases the risk of heart attack, stroke, heart failure, or kidney disease.

Malignant or accelerated hypertension: It is caused by extremely high blood pressure. Immediate medical attention is required in malignant hypertension as it can quickly cause serious damage to the eyes, kidneys and brain.

Causes of malignant hypertension include any form of secondary hypertension, complications of pregnancy, use of cocaine, monoamine oxidase inhibitors (MAOIs), oral contraceptives, etc.

Several diseases such as cerebral oedema, and heart failure can be caused by this condition.

Malignant hypertension can be treated by using medicines such as anti-hypertensive drugs.

Secondary hypertension: High blood related to any other disorder or cause is known as secondary hypertension.

Diabetes, renal disease, renovascular hypertension, Cushing’s syndrome, obesity, and hormonal disorders are some common conditions related to secondary hypertension. Secondary hypertension can also occur during pregnancy.

Effects of hypertension: Most people with high blood pressure have no signs or symptoms. Although a few people with early-stage high blood pressure may experience dull headaches, dizzy spells, etc. Hypertension mainly destroys the heart, kidney, blood vessels and brain.

Effect on heart: Hypertension increases the rate and intensity of the contraction of the heart. This may increase the chances of ventricular hypertrophy and failure. It can also increase the risk of clotting of blood in the lungs and ischemic heart diseases. This affects the circulation of blood and O₂ in the body.

Effect on brain: Hypertension may cause haemorrhage, stroke, paralysis and ultimately death.

Effect on kidney: Hypertension causes high blood pressure in the kidneys. As a result, the renin-angiotensin system activates and ultimately causes kidney failure.

Prevention and treatment:

• This can be mainly prevented by changing lifestyle.
• It can also be prevented by avoiding the consumption of fatty food, cigarettes, alcohol, etc.
• The patient should take anti-hypertensive drugs.

Mainly, two types of medicines are used in case of hypertension—

Vasodilator drugs: These drugs increase the blood flow to the kidneys. This results in the elimination of excess water, thereby reducing blood pressure (by reducing blood volume).

Natriuretic or diuretic drugs: These drugs decrease the absorption of salt and water in the renal tubules and reduce blood pressure.

Hypotension

Abnormally low arterial blood pressure is known as hypotension. For an adult, less than 90 mm Hg systolic pressure and less than 60 mm Hg diastolic pressure describes hypotension.

Hypotension may result in loss of consciousness, nausea, fatigue, lightheadedness, dizziness, abnormalities in blood circulation, etc.

Circulation And Circulatory System

The Physiological process, by which digested food materials, oxygen, etc., are distributed throughout the body and hormones, enzymes, waste products, etc., are taken to their target sites through a particular fluid known as circulation.

The circulatory system is the organ system found in multicellular organisms that are concerned with the circulation of blood and consist of blood vessels and the heart.

Significance of animals do circulation: Multicellular not have most of their cells in contact with the external environment. Hence, they have developed oxygen, important metabolic substances and metabolic wastes. In the circulatory system, a fluid (blood) is driven through tubes (arteries, veins and capillaries) by a muscular pump (heart).

Transportation of useful substances to cells: Circulatory systems transport nutrients, waste products, hormones, respiratory gases, blood cells, antibodies and cells involved in the immune system in order to maintain homeostasis.

Transportation of respiratory gases: Circulation helps in the transportation of respiratory gases between the lungs and all the cells of the body.

When deoxygenated blood passes through the respiratory surfaces of the lungs, gaseous exchange takes place, i.e., carbon dioxide leaves the blood and oxygen enters into the blood.

The oxygen-rich blood goes back to the heart and is pumped from the heart into the blood vessels, that transport blood throughout the body. Oxygen reaches all the cells of the body through blood.

Transportation of excretory substances: It transports all the excretory substances like carbon dioxide, and non-protein nitrogenous waste materials and helps in their elimination from the body.

Transportation of hormones: Some cells synthesise substances like hormones which regulate different metabolic processes. The circulatory system transports these substances to their target cells.

Transportation of nutrients or other materials to storage organs: It helps to transport substances from the site of production to organs where they are stored.

Maintenance of internal equilibrium: It regulates internal temperature and contributes to thermal regulation by distributing heat from heat-producing sites
to areas of the body that cannot keep themselves warm.

Types of circulatory system: Depending upon the type of circulating fluid, there are two types of circulatory system— blood circulatory system and lymphatic system. These are discussed under separate heads.

Blood Circulatory System

The blood circulatory system is of two types—open and closed circulatory system.

Open blood circulatory system

The blood circulatory system in which the heart pumps blood (haemolymph) Into the Haemocoel (body cavity) So That Blood Directly Bathes The Living Cells, Is Called an Open Blood Circulatory System.

Examples: Open circulatory systems are found in animals such as arthropods, molluscs, and some other invertebrate groups.

Advantages of an open circulatory system:

1. Direct exchange of gases, nutrients, and waste products takes place between the living cells of the organiser Hence, less amount of energy is required.
2. Animals with an open circulatory system (with the exception of insects that carry out gaseous exchange independently from the circulation) are generally slower and have a low metabolic rate.
3. Blood pressure remains low.

Disadvantage of open circulatory system

This system is not very effective, because fluid is pumped under low pressure, so it circulates slower than in a closed circulatory system.

Closed Blood Circulatory System

The circulatory system in which the heart pumps the blood in the vessels, is called a closed blood circulatory system.

Lymphatic system

The lymphatic system is a network of tubular vessels or ducts and nodes through which lymph flows.

The lymphatic system is made up of the following structures—

Lymphatic capillaries: Lymphatic capillaries are tube-like structures. They have thin walls made of only a single layer of endothelial cells resting on a basement membrane.

They are closed at one end (blind end). They remain interwoven with blood capillaries but are not connected to them. They are wider than blood capillaries.

The lumens of these capillaries are filled with lymph and walls are permeable to interstitial fluid constituents, including protein Lymph vessels or lymph ducts Lymphatic capillaries join to form lymphatic vessels.

These vessels are structurally similar to the cardiovascular veins, though they have thinner walls and more valves. These valves prevent the backward flow of lymph as lymph flows toward the thoracic cavity.

Smaller lymphatic vessels unite to form larger lymphatic vessels. These larger vessels unite to form the two main lymphatic vessels or trunks.

They are called the thoracic duct and the right lymphatic duct.

The thoracic duct receives lymph from the entire body The thoracic duct receives lymph from the entire body The thoracic duct receives lymph from the entire body lymphatic duct.

This duct joins to the left subclavian vein near the left side of the heart. The right lymphatic duct is present on the right side of the thoracic region. It collects lymph from the right hand, throat and thorax.

This duct opens into the right subclavian vein Cistema chyli: A small, dilated sac found near the lower area of the thoracic duct in the lumbar region of the body is called the cistema chyli or receptaculum chyli. It is part of the lymphatic system into which lymph from the intestinal trunk and two lumbar lymphatic trunks flow.

Lymph glands or lymph nodes: The lymph nodes are small bean-shaped structures that look like tiny bulbs and are found at intervals along the lymph vessels. They are masses of collagen and elastin fibres surrounded by a capsule made of fibrous tissue.

The afferent lymphatic vessels enter and efferent lymphatic vessels leave the lymphatic nodes.

Lymph nodes are especially abundant in specific regions such as armpits, neck and groins.

Axillary nodes, intercostal nodes, and inguinal nodes are some of the lymphatic nodes.

Besides these, some other glands such as the spleen, tonsils, etc., also consist of lymphatic tissues. The spleen is considered to be the largest lymphatic gland in the human body

Function Of Lymph Nodes

1. Lymph nodes filter different foreign particles such as bacteria, etc., from the lymph.
2. Lymphocytes are produced from the lymphoid cells present in the lymph glands. These lymphocytes are transported to the blood vessels through lymphatic vessels. They develop the immunity power of the body by producing antibodies.
3. Several antitoxins and antibiotics are produced from lymph nodes. These substances destroy the toxins that enter our body.

Spleen

1. It is the largest lymph gland present in our body. Some part of it is composed of lymphoid tissues.
2. It is located between the diaphragm and the fundus region of the stomach.

Function of spleen

1. Spleen is the graveyard of RBCs. The worn-out RBCs are phagocytised by the macrophages present in the spleen.
2. It works as a blood reservoir as it can store some amount of RBCs during rest.
3. Lymphocytes originate in the spleen.
4. It produces antibodies and antitoxins.

Lymph

Lymph Definition: The transparent, colourless or pale yellow tissue fluid that contains white blood cells (particularly modified lymphocytes) is called lymph.

This fluid circulates throughout the lymphatic system and returns to the venous blood by the thoracic and right lymphatic ducts. It transfers materials from blood to the body cells and vice versa.

Origin: In arteries, blood flows at a higher pressure than in veins. Due to this, blood plasma filters out from the arterial capillaries and moves into the intercellular space to form interstitial fluid.

This fluid contains water, glucose, amino acids, oxygen, carbon dioxide and several other electrolytes.

The exchange of different substances between blood and cells takes place via interstitial fluid.

Hence, its composition changes continuously. Some of this fluid is reabsorbed by the blood capillaries but most of it enters the lymph capillaries.

Read and Learn More Class 11 Biology Notes

This fluid collected inside the lymph capillaries is called lymph. At the time of formation, lymph is a clear watery liquid which has the same composition as that of interstitial fluid.

Composition Of Lymph

Lymph have a similar composition to blood plasma. Depending on the destination, lymph contains different concentrations of lymphocytes, proteins and fats. It has a large number of leucocytes (mostly lymphocytes) ranging from 500-75000/mm3. The lymph does not contain platelets.

The non-cellular portion of lymph contains water (94%) and solids (6%). The organic components of lymph include various proteins, fats, glucose and other non-protein nitrogenous substances urea, creatinine, enzymes, etc.).

Total protein content varies with the region from which the lymph drains. In fasting conditions fat content is low but after a fatty diet, it becomes higher. Inorganic components such as calcium, phosphorus, and chlorides are also present in lymph.

Functions Of Lymph

Transportation: Lymph is modified interstitial fluid, that is drained from the tissue and moved back to the circulatory system via the lymphatic network.

The lymphatic network transports important materials such as vitamins, digested food components, O₂ etc., in the body. They also take away the waste products such as CO₂, nitrogenous and non-nitrogenous wastes, etc., from the cells.

Removal of excess tissue fluid: Lymph returns protein and excess interstitial fluid to peripheral circulation.

Absorption: There are several small finger-like projections present on the inner wall of the small intestine, known as villi. Each of these villus consists of a lymphatic vessel, known as lacteal. Lacteal absorbs the digested fat and transports it to the bloodstream.

Disease prevention: The lymphatic system helps the body fight against diseases as the system contains a high concentration of lymphocytes in lymph nodes. The invading microorganisms and foreign particles are destroyed in the lymph nodes.

Human Circulatory System

The human circulatory system consists of the muscular heart, the network of blood vessels and blood. Blood is the circulating fluid which flows through the blood vessels after being pumped by the heart.

Types: Two types of circulatory systems are found in human beings— blood and lymphatic systems.

Heart

The heart is the main organ of the circulatory system. In vertebrates, it is located on the ventral side of the oesophagus. The human heart is more complex and the Function of lymph nodes advanced as compared to other organisms.

Heart Definition: A multi-chambered muscular organ, that pumps blood throughout the body in a rhythmic process, is known as the heart.

Location: The heart is situated in between the lungs in the middle of the thoracic cavity, slightly tilted to the left. The heart rests on the superior surface of the diaphragm, posterior to the sternum.

Read and Learn More Class 11 Biology Notes

Shape and size: The size of the heart is closely related to the size of the organism’s body.

The base of the heart is broad and flat, about 9 cm in breadth. The length of the heart of an adult is about 10-12 cm. Externally, the heart appears as a cone-shaped structure.

Shape and size: The size of the heart is closely related to the size of the organism’s body.

The base of the heart is broad and flat, about 9 cm in breadth. The length of the heart of an adult is about 10-12 cm. Externally, the heart appears as a cone-shaped structure

Wall of the heart

The wall of the heart is composed of the following layers—

Pericardium: The heart is enclosed in a double-layered sac, called pericardium. It is the fibrous outer wall made of dense connective tissue.

The pericardium is further divided into two layers —

Fibrous pericardium: It is the outer fibrous transparent sac that covers the heart.

Serous pericardium: The inner side of the pericardium is formed of a serous layer and is known as serous pericardium.

This serous pericardium is again divided into two layers, the inner layer—visceral pericardium and the outer layer—the parietal pericardium.

Visceral Pericardium: This layer is also called the epicardium. It is the innermost layer of the pericardium. It is composed of a single sheet of squamous epithelial cells overlying delicate connective tissue. This layer adheres to the heart.

Parietal Pericardium: This layer lies between the visceral pericardium and the fibrous pericardium.

The pericardial cavity lies between the visceral pericardium and the parietal pericardium. This cavity is filled with pericardial fluid which serves as a shock absorber and reduces friction between the pericardial membranes.

The primary function of the pericardium is to—

1. Protect The Heart
2. Anchor The Heart To The Surrounding Structures, Such As The Diaphragm And The Great Vessels; And
3. Prevent The Heart From Overfilling.

Myocardium: The myocardium is a thick contractile layer present just below the visceral pericardium.

It is composed of muscle cells, known as myocardiocytes. This layer is more thick in the ventricles than in the atria. This layer of the left ventricle is thicker than the right ventricle.

Endocardium: The endocardium is the innermost layer of the heart wall and lines the heart’s chambers.

It is made of squamous epithelium that rests on a thin connective tissue layer. It is continuous with the endothelium of the great blood vessels—the inferior and superior vena cava.

Chambers Of The Heart

The heart is a hollow, four-chambered, muscular organ that consists of the upper right and left atria (singular: atrium) and the lower right and left ventricles.

The atria are separated by a thin muscular wall called the interatrial septum and the ventricles are separated by a thick muscular wall called the interventricular septum.

There is a translucent, depression present in the septum between the right and left atrium, known as fossa ovalis. In the embryonic stage, there is an opening in place of the fossa ovalis.

It is known as foramen ovale. The heart actually functions as two separate pumps.

The right atrium and ventricle act as one pump to propel deoxygenated blood to the lungs. At the same time, the left atrium and ventricle act as another pump to propel oxygenated blood collected from the lungs towards the heart throughout the systemic circulation.

The right atrium and right ventricle: The inner surfaces of both atria are mostly smooth except for a low network of muscular ridges called musculi pectinati. The right atrium receives deoxygenated blood through the openings of the superior vena cava, inferior vena cava & coronary sinus.

The right atrium is connected to the right ventricle through an opening, known as the right atrioventricular orifice or aperture.

The right atrioventricular orifice is guarded by the right atrioventricular valve or tricuspid (formed of three flaps or cusps) valve.

This valve regulates unidirectional blood flow from the right atrium to the right ventricle and prevents the backflow of blood.

There are numerous muscular ridges on the inner wall of the right ventricle, known as columnae carneae.

They contain some finger-like muscular projections, known as papillary muscles. The cusps or leaflets of the tricuspid valve are attached to the wall of the right ventricle by strong, elastic threads, known as chordae tendineae. The opening and closing of the tricuspid valve are controlled by the papillary muscles through the chordae tendineae.

The pulmonary artery originates from the right ventricle. There is a seminular valve present at the juncture of the pulmonary artery and right ventricle.

Left atrium and left ventricle: The left atrium and left ventricle are also connected to each other through an opening known as the left atrioventricular orifice or aperture.

The left atrioventricular orifice is guarded by the bicuspid (formed of two cusps) or mitral valve.

The left atrium receives four pulmonary veins, two from the left and two from the right lung.

These pulmonary veins bring oxygenated blood from the lungs. They do not have any valve to regulate the blood flow. Blood flows obliquely from the atrium to the ventricle which is regulated by the mitral valve.

The left ventricle which is larger than the right ventricle, has much thicker walls. Like the right ventricle, it also possesses columnae carneae and papillary muscles. Chordae tendineae are also seen.

The lower part of the left ventricle receives blood from the left atrium. The upper part, which has mainly fibrous walls, is termed the aortic vestibule, and it ends in the aorta, the largest artery of the body.

The junction of the vestibule and aorta contains dense fibrous tissue that encircles the aortic valve and is continuous with the cardiac skeleton.

Cardiac grooves

• All the different chambers of the heart are separated from each other by grooves. These grooves are known as cardiac grooves.
• The left and right atria are separated from each other by an interatrial groove. Atria and ventricles are separated from each other by the coronary sulcus or atrioventricular groove.
• The grooves by which two ventricles are separated from each other are known as anterior interventricular sulcus and posterior interventricular sulcus.

Junctional Tissues Of Heart

Junctional Tissues Of Heart Definition: The tissues of the cardiac muscles, responsible for the origin and conduction of cardiac impulses (waves of electric potential generated by stimulation of cardiac muscles by specific stimulus), are known as junctional tissues of the heart.

Junctional Tissues Of Heart Types:

1. Sino-atrial (SA) Node,
2. Atrioventricular (AV) node,
3. Bundle of His,
4. Purkinje fibres,
5. Internodal tract.
6. Sinoatrial (SA) node

Location: Situated on the right wall of the right atrium, near the entrance of the superior vena cava.

Characteristics: The SA node is a small mass of muscle tissue, about 3.0 mm in width.

Function: This is known as the “Natural Pacemaker” or heartbeat regulator. It initiates impulses automatically at a more rapid rate than any other
part of the conduction system.

Its rate of discharge of impulses determines the rate of heartbeat. The electrical signal spreads out over the walls of the atria, causing them to contract.

Atrioventricuiar (AV) node

Location: It is located in the right posterior portion of the interatrial septum.

Characteristics: Muscles of AV nodes are cardiac muscle fibres. It contains a very small amount of myofibrils.

AV muscle fibres are comparatively smaller than the SA node. The AV node and the conduction tissue surrounding it are known as the AV junctional tissue.

The AV node is also called the ‘reserve pacemaker of heart’ because during functional inactivity of the SA node, it can initiate heartbeats, but at a slower rate.

Function: The action potential spreading through the right atrium causes depolarization of the AV node.

When the impulses reach the AV node, there is a slight delay that allows the atria to finish their contraction before the ventricles begin their contraction.

The AV node then conducts impulses to the muscles of the ventricles through a bundle of His and Purkinje fibres.

Bundle of His and its branches

Location: It is a band of nerve fibres that originates from the posterior part of the AV node, and then passes along the interventricular septum to the ventricles.

Characteristics: A Swiss physician, Wilhelm His, first described this tissue. It divides into right and left branches which proceed along the right and left sides of the interventricular septum to the tips of the two ventricles. It is also called an AV bundle.

Function: These specialised fibres conduct the impulses at a very rapid velocity (about 2 m/sec). The bundle of His constitute the only electrical link between the atria and the ventricles.

Purkinje fibres

Location: The Purkinje fibres, named after their discoverer Johannes von Purkinje, are the tree-like terminal branchings of right and left bundles of His.

Characteristics: Cells of these fibres consist of high amount of glycogen-containing sarcoplasm. There are thousands of myofibrils present around the cells.

Function: Purkinje fibres conduct the impulses at high velocity (about 4 m/sec) throughout the ventricles. During the ventricular contraction, they carry the impulses from both the left and right branches of the bundle of His to the myocardium of the ventricle.

Internodal tract

Location: These are located between the SA node and the AV node.

Characteristics: There are three internodal tracts connecting the SA node and AV node—the anterior tract of Bachman, a middle tract of Wenckebach and the posterior tract of Thorel.

Function: Their main function is to carry impulses from the SA node through the right atrium to the AV node.

As the impulses pass along the internodal tracts, the right atrium contracts, pushing the blood it contains, through the tricuspid valve into the right ventricle.

Myocardial tissue has four main characteristics that integrate the heart’s electrical and mechanical activity

Automaticity: It is the ability to initiate an impulse or stimulus. The pacemaker cells (SA node) of the cardiac conduction system spontaneously depolarize in the absence of external stimulation. The AV-junctional tissue and the His-Purkinje network also possess the property of automaticity.

Excitability: It is the ability to respond to an impulse or stimulus. The cells are electrically irritable because of an ionic imbalance across the cell membranes.

Cells thus respond to external stimuli from chemical, mechanical or electrical sources. Atrial and ventricular myocardial fibres respond to the impulse generated by the pacemaker cells of the cardiac conduction system by depolarization and repolarization.

Conductivity: It is the ability to transmit impulses to other areas of the heart. Cells of the conduction system and the myocardial muscle fibres, both have this property.

Contractility: It is the ability of cardiac muscles to shorten, responding to stimuli with mechanical action. Myocardial fibres respond mechanically to electrical stimulation by contracting.

The course of circulation of blood through the human heart The circulation of blood through the human heart occurs through periodic systole and diastole.

They are discussed below—

Atrial diastole: This is the relaxation time of both the atria. During this time deoxygenated blood enters into the right atrium through superior and inferior vena cava and oxygenated blood from the lungs enters into the left atrium through pulmonary veins.

The mixing of the blood is prevented due to the presence of an interatrial septum between the atria.

Auricular or atrial systole: This is contraction time for both atria. The deoxygenated and oxygenated blood move from the right and left atria respectively to the corresponding ventricles.

The tricuspid and bicuspid valves present at the atrioventricular grooves open to guide the flow of blood.

Ventricular diastole: This is the relaxation time of the ventricles. During this time, the left ventricle receives oxygenated blood from the left atrium and the right ventricle receives deoxygenated blood from the right atrium.

Ventricular systole: It is the time for contraction of both the ventricles. The deoxygenated blood in the right ventricle flows to the lungs through the pulmonary artery.

The oxygenated blood from the left ventricle flows to different parts of the body through the aorta.

The bicuspid and tricuspid valves close during this time and prevent the backflow of the blood to the atria. At the same time the semilunar valves open and guide the blood flow to the respective blood vessels and prevent the backflow of the blood to the ventricles.

Erythropoietin

It is a glycoprotein hormone which controls erythropoiesis. It is produced in the interstitial fibroblast cells closely associated with peritubular capillary and proximal convoluted tubule in the kidney (90%), liver and other organiser:

Rouleaux formation

Sometimes RBCs are stacked on each other and form a structure like stacked dinner plates. This condition is known as rouleaux formation.

These stacked structures can move easily through broad blood vessels but cannot move through narrow blood vessels, and sometimes become barriers in blood circulation.

Site of rupture: After completing their lifespan, RBC rupture in the liver and spleen. The spleen is known as the graveyard of RBC.

Haemolysis and fate of RBCs: At the end of their lifespan, the plasma membrane of the RBC ruptures and releases their content including haemoglobin in the surrounding fluid. This process is known as haemolysis.

Functions of RBC: These are as follows—

Transportation of O₂ and CO₂: RBCs collect oxygen from the lungs and release it to the cells and tissues.

They take up carbon dioxide from the cells and tissues and release it to the lungs.

Haemoglobin found in the red blood cells helps in the transport of oxygen and carbon dioxide between the lungs and the cells.

Regulation of acid-base balance: Haemoglobin present in RBC acts as a buffer and regulates the carbonate ion concentration.

Regulation of viscosity of blood: RBCs help to regulate the viscosity of the blood.

Maintenance of ionic balance: In the RBCs, carbon dioxide either binds to haemoglobin or reacts with water.

In reaction with water, it produces carbonic acid, which when dissociates, produces hydrogen ions. The deoxygenated haemoglobin is a stronger base and is capable of taking up these hydrogen ions. Hence, they are buffered by haemoglobin.

Other functions: RBCs carry the blood group antigens and Rh factor.

Erythrocyte sedimentation rate (ESR) The erythrocyte sedimentation rate (ESR) is the rate at which red blood cells settle down in a tube containing blood under standardized conditions and is a measure of inflammation or abnormal proteins in the body.

ESR commonly increases with any condition that gives rise to inflammation, such as infection, arthritis, or cancer.

It plays a very important role in the detection of any disease. According to the Wintrobe method, the normal ESR value is 0-9mm/h for males and 0-20mm/h for female

Haemoglobin: The red, oxygen-carrying, iron-containing pigment, comprising of a haem group and globin protein, that is found in the red blood cells of vertebrates is called haemoglobin.

Structure of haemoglobin: Haemoglobin (Hb) is a molecule made up of four subunits. Each subunit contains a haem moiety conjugated to a polypeptide. There are two pairs of polypeptides in each haemoglobin molecule.

Haem is an iron-containing porphyrin derivative. The polypeptides are collectively referred to as globin.

Instruments for measuring haemoglobin

The concentration of haemoglobin can be measured by using different instruments and methods. This includes Helden’s haemoglobinometer and Gauhar’s haemoglobinometer. Nowadays, a spectrophotometer is used to measure the concentration of haemoglobin.

RBC ghost

After hypotonic haemolysis of RBCs, all the cytoplasm along with the haemoglobin comes out from the cells. If these cells are observed under a microscope, then a spectrin-rich endoskeleton is found in these cells. These cells are known as RBC ghosts.

Location: Haemoglobin is found in the RBCs of human beings and other vertebrates.

Types: Normally there are two types of haemoglobin found in human beings, these include HbA and HbF.

HbA: HbA (Adult haemoglobin) is found in adult human beings. It has 2 alpha and 2 beta polypeptide chains and comprises 97% of normal adult haemoglobin.

HbA is again of two types: HbA₂ and HbA₂ The globin portion of HbA-! consists of four polypeptide chains—2ar chains and 2/3 chains.

Each chain contains 141 amino acid molecules. Each chain contains 146 amino acid molecules.

Thus, HbA is designated as α2δ2. Not all the haemoglobin in the blood of normal adults is HbA HbA₂ consists of two a and two 6 chains.

Thus, HbA2 is designated as α2δ2. The β chains also contain 146 amino acid molecules, but 10 individual residues differ from those in the chains.

About 98% of the total haemoglobin in adults is HbAi and 2-2.5% is HbA₂.

HbF: HbF is normal foetal haemoglobin. It has 2 alpha and 2 gamma polypeptide chains mainly found in the embryonic stage.

Variants of normal haemoglobin are formed when there are genetic changes in the globin genes.

Amount: The amount of haemoglobin present in 100 ml of blood is 14-17g in healthy adult males and 12-16g in healthy adult females.

Chemical nature: Haemoglobin is formed of iron-containing pigment haem (4%) and globin protein (96%). Haem consists of Fe²⁺ or ferrous ions in the centre of heterocyclic organic compounds, called porphyrins.

The molecular weight of haemoglobin is 68000 Da.

Fate of haemoglobin: Old RBCs are destroyed by the macrophages. Breakdown of haemoglobin occurs by the action of the enzyme haem oxygenase.

This generates haem and globin molecules. The haem is then converted to bilirubin and biliverdin. These pigments get released in the bile. The iron group of the haem portion is reused in the formation of haemoglobin.

Functions of haemoglobin: The functions of haemoglobin are discussed below.

Helps to transport respiratory gases: During respiration in vertebrates and in higher animals, the inhaled oxygen combines with haemoglobin to form oxyhaemoglobin. This oxyhaemoglobin moves through blood capillaries and reaches the tissues.

Similarly, the carbon dioxide produced in the tissues combines with haemoglobin and form carbaminohaemoglobin, which reaches the alveoli of the lungs through the blood capillaries.

In this way, haemoglobin helps in the transportation of oxygen and carbon dioxide.

Maintains the acid-base balance in blood: Haemoglobin acts as a buffer and maintains the acid-base balance of the body.

Produces pigment materials: The haemoglobin found in dead RBCs forms pigments such as biliverdin, bilirubin, sercobilinogen, etc.

These give colour to the faeces. Haemoglobin also produces the pigment urochrome, which gives a straw colour to the urine.

White Blood Corpuscle or Leucocyte or WBC

White Blood Corpuscle or Leucocyte or WBC Definition: White blood cells (WBCs) are colourless nucleated blood cells which lack haemoglobin and participate in reactions against invading pathogens or foreign particles in the body.

Structural characteristics:

1. WBC do not contain any pigment so, they are colourless.
2. They contain cytoplasm and a persistent nucleus. The cytoplasm of WBC can be granular or smooth (agranular). The nucleus can be of different shapes, such as spherical, kidney-shaped, etc.
3. All the white blood cells are capable of amoeboid movement.
4. WBCs are rich in nucleoprotein. Their cytoplasm consists of lipids, glycogen, cholesterol, vitamin C and different enzymes.
5. Mitochondria, Golgi apparatus and other organelles are present in the cytoplasm of WBCs.

Size: WBCs do not have a fixed diameter. Their average diameter is 8-18 pm.

Lifespan: The average lifespan of WBC is 1-15 days.

Number: The total WBC count is normally about 4,000 – 11,000 per mm3 of blood.

Origin of WBC: WBC originates in bone marrow, spleen and lymph nodes. White blood cells are produced from pluripotent haemopoietic stem cells—myeloblast, lymphoblast and monoblast.

Types: Depending on the presence and absence of granules in the cytoplasm, WBCs are categorised into two main groups— Granulocytes (including neutrophils, eosinophils and basophils) and Agranulocytes (including lymphocytes and monocytes).

This classification depends on whether granules can be distinguished in their cytoplasm using a light microscope and conventional staining methods. WBCs can also be divided into two broad groups—the phagocytes and the immunocytes.

Granulocytes, which include three types of cells— neutrophils, eosinophils and basophils, together with monocytes comprise the phagocytes.

The lymphocytes, their precursor cells and plasma cells make up the immunocyte population. The types of WBCs (granulocytes and agranulocytes) are discussed under separate heads.

Granulocytes: The white blood cells that contain granules in their cytoplasm are known as granulocytes.

According to the characteristics of granules, granulocytes are categorised into three types— neutrophil, eosinophil and basophil.

The granulocytes have lobed nuclei which have varying shapes from cell to cell. Hence granulocytes are also known as polymorphonuclear leucocytes.

Neutrophil: The granular WBCs which can be stained with neutral stain are called neutrophils.

Structural features:

The cytoplasm of the neutrophils consists of numerous granules and they can be stained with neutral stain. They get stained with both basic (methylene blue) and acidic (eosin) dyes. Hence, they appear purple in colour.

• The nucleus appears purple due to staining and is divided into two to five segments or lobes.
• The lobes are divided into two to five segments or lobes.
• The lobes are connected by a thin strand or filament of nuclear material,

Neutrophils have 3 types of granules—

• Azurophilic or primary granules,
• Specific or secondary granules,
• Gelatinase-containing granules or tertiary granules.

Diameter: 10-12 pm

Lifespan: A few hours to a few days (2-4 days).

Number: Neutrophils are the most abundant type of WBCs in our body. They make up 50-70% of the total amount of WBCs. There are about 3000-6000 neutrophils present per mm3 of blood.

Functions: Neutrophils act as the first line of defence against pathogens. They are motile and phagocytic (destroy damaged tissues). They contain enzymes like protease, elastase, metalloproteinase, NADPH oxidase and antibody-like substances, defensins.

These antimicrobial peptides are active against bacteria and fungi. They secrete Platelet Aggregation Factor (PAF) which accelerates the aggregation of platelets at the injured site of blood vessels.

Reasons for the increase in the neutrophil count

The following reasons are responsible for the increase in the neutrophil count—

1. Infection,
2. Destruction of cells and tissues due to burns, myocardial infarction, etc., © leukaemia,
3. Excess smoking, taking contraceptive pills, etc.

Eosinophil: The granulated WBCs where the granules can be stained with acidic dye (eosin) are known as eosinophils.

1. Structural features:
   1. Usually, eosinophils have a bilobed nucleus. The cytoplasm is packed with large, coarse, retractile, orange-red granules containing histamine. The granules are referred to as eosinophilic because they take up the acidic dye, eosin.
   2. Eosinophils are produced in the bone marrow and circulate in the bloodstream for about 6 hours before migrating to the tissues.
2. Diameter: 10-12 pm
3. Lifespan: A few days (8-12 days).
4. Number: There are 150-300 eosinophils per mm3 of blood. They constitute about 1-4% of the total WBCs.
5. Functions:
   1. Eosinophils respond to chemotactic stimuli and are phagocytic. They can kill ingested organisms,
   2. Eosinophils also take part in allergic reactions. They also produce histaminase and aryl sulphatase B. These are the enzymes that inactivate two inflammatory agents released by mast cells,
   3. Eosinophils take part in dissolving blood clots in blood vessels.

Basophil: The granular WBCs where the granules are stained with basic dyes (methylene blue) are known as basophils.

1. Structural features:
   1. The nucleus of basophil is 2-3 lobed. The cytoplasm has fewer number of coarse granules,
   2. The granules are called basophilic because they take up basic stains (such as methylene blue). Upon staining the granules take up bluish purple color against pale blue cytoplasm.
2. Diameter: 8-10 pm.
3. Lifespan: 12-15 days.
4. Number: They are the rarest type of white blood cell, making up only 0.4% of the total white blood cells, found in a blood smear. 0-100 basophils are present per mm3 of blood.
5. Functions: They have a role in allergic and inflammatory responses.

Basophil granules release some important substances such as—

1. Proteases: Exaggerate inflammation,
2. Hyaluronic acid: Necessary for deposition of ground substances in the basement membrane,
3. Heparin: Prevents intravascular blood clotting,
4. Histamine: Mediate’s acute hypersensitivity reaction—vascular changes, increased capillary permeability
5. Serotonin: Acts as vasoconstrictor.

Agranulocytes: The white blood cells that do not have any granules in their cytoplasm are known as agranulocytes.

Agranulocytes are mainly of two types—

1. Monocytes and
2. Lymphocytes.

Monocyte: The large agranulated cells, capable of phagocytosis are known as monocytes.

1. Structural features:
   1. Monocytes are amoeboid in appearance. The nucleus is round or oval-shaped when cells are young.
   2. But as they grow older, the nucleus becomes convoluted, kidney-shaped or horse-shoe-shaped.
   3. The cells have a large amount of dear non-granular cytoplasm, maybe, with vacuoles in it.
   4. These cells are mainly formed in the bone marrow.
2. Diameter: About 16 – 18 pm in diameter.
3. Lifespan: About 2-4 days.
4. Number: 300-600 monocytes per mm3 of blood.
5. Functions:
   1. Monocytes in the circulation are precursors of tissue macrophages that are actively phagocytic.
   2. Monocytes circulate in the blood for 1-3 days and then migrate to body tissues, where they transform into macrophages.
   3. They destroy dead cells and bacteria by the process of phagocytosis,
   4. They secrete some signalling proteins like interleukin 1 (IL-1), which increase the body temperature during infections and help in the production of T-lymphocytes.

Lymphocyte: The granulated white blood cells which produce antibodies are known as lymphocytes.

1. Structural features:
   1. Lymphocytes form an essential component of the immune system and are derived from haemopoietic stem cells.
   2. On the basis of nuclear size, the lymphocytes are of two types. These are—
      1. Small lymphocytes: These are more common in normal blood. They have a large, dense, round nucleus which occupies the major portion of the cell.
      2. Large lymphocyte: The nucleus may be round and indented. The cytoplasm is basophilic and thicker than that of the small lymphocyte and forms a wider zone around the nucleus.
2. Diameter: A small lymphocyte is slightly larger than a red blood cell and the diameter is about 7.5pm. The diameter of large lymphocytes is about 12 p.m.
3. Lifespan: About 2-3 days.
4. Number: These are the second most common WBC (25-35%), and are easy to find in blood smears 1500-4000 lymphocytes are present per mm3 of blood.
5. Functions:
   1. They produce immunoglobulins or antibodies,
   2. Lymphocytes are mainly concerned with the production and transport of antibodies and they take an active part in the response to a wide range of antigens,
   3. They attack viruses, cancer cells, etc.
   4. Certain lymphocytes are part of the generalised response of the innate immune system, responding quickly to threats. Others act against specific pathogens or infected cells and are part of the adaptive immune response,
   5. Lymphocytes of the adaptive immune system are specific to a particular antigen—substances foreign to the body

General functions of WBCs

1. Destruction of germs: The neutrophils and monocytes destroy the ingested pathogens and foreign materials by phagocytosis. They help to protect the body from harmful agents.
2. Production of antibodies: The lymphocytes increase the defensive mechanism of the body by producing antibodies.
3. Prevention of allergies: Eosinophils play an important role in allergic reactions by producing histamine.
4. Secretion of heparin: Basophils prevent intravascular blood coagulation by secreting heparin.
5. Trephone production: From plasma proteins, the monocytes produce a chemical substance, known as trephone. It provides nourishment to the tissues.

Thrombocyte Or Platelet

Definition: Platelets or thrombocytes are small, granulated and anucleated blood corpuscles, which are involved in blood clotting. (Greek. Thrombos= lump or clot and cyta= vessel; the word cute is derived from it and used to name these cells)

Platelets are membrane-bound fragments of cells that form when larger cells in the bone marrow break apart. Platelets do not contain nuclei and help in coagulation and clotting of blood at the wound.

Structural features:

1. Platelets are actually not true cells but merely circulating fragments of cells.
2. They are non-nucleated, round or oval, plate or disc-shaped structures, which is the reason behind their nomenclature.
3. Platelets are formed from the terminal end of the pseudopodia of megakaryocytes (large bone marrow cells will lobed nuclei).
4. They are surrounded by selectively permeable membranes.
5. They appear somewhat smaller in the microscope. This is because their cytoplasm is divided into two zones—an outer hyalomere, which hardly stains, and an inner granulomere, which contains granules, that appear bluish in colour when stained.

Size: The mean diameter of platelets varies in different individuals, ranging from approximately 2-4 m.

Lifespan: These break down in the blood within 7 to 10 days after their formation.

Number: The platelet count in the circulating blood is normally between 2,00,000 and 5,00,000 per mm3 of blood. Newborn babies have a slightly lower level of platelets, which normally attain the adult range within three months from birth.

Functions: These are as follows—

Blood clotting and coagulation: Platelets play a key role in blood clotting, which prevents excessive blood loss after an injury.

Storage of important biomolecules: Platelets also act as the storage for several substances, including serotonin, epinephrine, histamine, and thromboxane and also help in their transportation.

Repairing of wounds: Upon activation, they release specific molecules which initiate constriction of local blood vessels, which facilitates clot formation.

• The Living World
  • The Living World Notes
• Biological Classification
  • Classification System Of Living Organisms Notes
  • Archaebacteria Characteristics and Types Notes
  • Biological Classification Important Notes
• Plant Kingdom
  • Algae – Definition, Characteristics, Types Notes
  • Chlorophyceae – Definition, Distribution and Characteristics Notes
  • Red Algae General Characteristics Notes
  • Bryophyta -Definition and Characteristics Notes
  • Characteristics And Classification Of Gymnosperms
  • Plant Kingdom – Classification and Characteristics Notes
• Animal Kingdom
  • Animal Kingdom Notes
• Structural Organisation in Plants and Animals
  • Types And Functions Of Epithelial Tissues Notes
  • Endothelium Definition and Function Notes
  • Glandular Epithelium – Definition, Structure and Functions Notes
  • Muscular Tissue – Structure, Functions and Types Notes
  • Nervous Tissue – Characteristics, Structure, Function Notes
  • Number Of Longitudinal Tracheal Trunks In Cockroach Notes
  • Haemocytes In Insects Notes
  • Morphology Anatomy Of An Insect Cockroach Notes
  • Structural Organization In Animals Notes
• Morphology of Flowering Plants
  • Plant Stem – Structure and Functions Notes
  • Root System – Roots, Types of Roots and Functions of Roots Notes
  • Morphology of Leaf – Structure, Types, Parts & Modifications
  • Bud – About, Types, Example Notes
  • Phyllotaxy Notes
  • What Is Aestivation ? Describe Various Types Of Aestivation
  • Morphology Of Flowering Plants Notes
• Anatomy of Flowering Plants
  • Shoot Apex Notes
  • Permanent Tissue Notes
  • Anatomy Of Flowering Plants Notes
• Cell:Structure and Function
  • Cell Cycle And Cell Division Notes
• Cell – The Unit of Life
  • Nucleoplasm – Function, Components Notes
  • Erythopoietin Notes
  • Human Circulatory System Notes
  • Lymph Notes
  • Cardiac Output Notes

• Diversity of Living Organisms
  • Biological Classification Multiple Choice Question and Answers
  • Plant Kingdom Multiple Choice Question and Answers
  • Animal Kingdom Multiple Choice Question and Answers
• Structural Organisation in Plants and Animals
  • Morphology of Flowering Plants Multiple Choice Question and Answers
  • Anatomy of Flowering Plants Multiple Choice Question and Answers
• Cell:Structure and Function
  • Biomolecules Multiple Question and Answers
  • Cell Cycle and Cell Division Multiple Choice Question and Answers
• Plant Physiology
  • Transport in Plants Multiple Question and Answers
  • Photosynthesis in Higher Plants Multiple Choice Question and Answers
  • Respiration in Plants Multiple Choice Question and Answers
  • Plant Growth and Development Multiple Choice Question and Answers
• Human Physiology
  • Digestion and Absorption Multiple Choice Question and Answers
  • Breathing and Exchange of Gases Multiple Choice Question and Answers
  • Body Fluids and Circulation Multiple Choice Question and Answers
  • Excretory Products and Their Elimination Multiple Choice Question and Answers
  • Neural Control And Coordination Multiple Choice Questions And Answer
  • Chemical Coordination Multiple Choice Question And Answers