Blood, Heart and Circulation for IB B3.2 and NEET Biology

1 Components and functions of blood

Syllabus link: NCERT XI Ch 15 Body Fluids and Circulation, 15.1 Blood (15.1.1 Plasma, 15.1.2 Formed elements); IB B3.2 Transport (first assessment 2025)

NCERT = statement as given in NCERT (use in NEET/CBSE answers)   IB = IB Biology (first assessment 2025) link   HL = IB Higher Level only

Every cell needs a steady supply of oxygen and food and a way to get rid of waste. In tiny animals diffusion is enough. In large animals like us, a pump and a set of pipes are needed. This chapter on the heart and circulation covers the fluid (blood and lymph), the pump (the heart) and the pipes (blood vessels), and how the whole system is controlled. It is card 3 of our Human Physiology series, after Digestion and Absorption and Gas Exchange.

What does the circulatory system do?

The circulatory system is the transport system of the body. It carries:

• gases: oxygen from the lungs to the tissues and carbon dioxide back;
• nutrients: glucose, amino acids and fats from the gut;
• nitrogenous wastes: urea from the liver to the kidneys;
• hormones from endocrine glands to their target organs;
• antibodies and white blood cells to sites of infection.

It also spreads heat around the body, helps keep the pH steady, and seals wounds by clotting. In an exam, if the options list all of these, the answer is “all of the above”.

Open and closed circulatory systems

Animals move body fluid in one of two ways. In an open system, the heart pumps blood into open spaces or body cavities (sinuses), so the tissues are bathed directly in blood (called haemolymph). Blood does not stay inside vessels, so it flows slowly and at low pressure. In a closed system, blood always stays inside a network of vessels, so it can be pushed faster and its flow can be directed precisely.

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NCERT NCERT: open circulatory systems are present in arthropods and molluscs, and annelids and chordates have a closed circulatory system. Leeches are the annelid exception often quoted in MCQs: their true vessels are reduced and blood flows through haemocoelic channels. Among the usual MCQ options, the annelids are the first group with a true closed system (the ribbon worms, phylum Nemertea, have a simpler one).

Body fluids: blood and lymph

NCERT Blood is the most common body fluid in higher organisms. Lymph is the other. Blood is a special connective tissue: a fluid matrix called plasma with formed elements (cells and cell fragments) suspended in it.

Plasma

Plasma is a straw-coloured, viscous fluid that makes up nearly 55% of blood. NCERT 90 to 92% of plasma is water and proteins make up 6 to 8%.

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Plasma also carries small amounts of minerals (Na⁺, Ca²⁺, Mg²⁺, HCO₃^–, Cl^–), glucose, amino acids, lipids, hormones, urea and dissolved CO₂. Clotting factors are also present in plasma, in an inactive form. Plasma without the clotting factors is called serum.

Formed elements

Erythrocytes (red blood cells), leucocytes (white blood cells) and platelets together are the formed elements. They make up nearly 45% of blood. The next section looks at each one.

2 Red blood cells, white blood cells and platelets

Syllabus link: NCERT XI 15.1.2 Formed elements; IB B3.2 (transport), C3.2 Defence against disease (phagocytes, lymphocytes)

Three kinds of formed elements do the work of blood in the heart and circulation system: red cells carry gases, white cells defend, and platelets seal leaks.

Red blood cells (erythrocytes)

Erythrocytes are the most abundant cells in blood: NCERT a healthy adult has 5 to 5.5 million RBCs per mm³ of blood. They are made in the red bone marrow in adults, from haematopoietic stem cells. In most mammals they have no nucleus and are biconcave, which gives a large surface for gas exchange and room for more haemoglobin.

• They contain the red iron-containing protein haemoglobin: about 12 to 16 g in every 100 mL of blood in a healthy person. Haemoglobin carries most of the oxygen (and some CO₂).
• An average RBC lives about 120 days, after which it is destroyed in the spleen, the ‘graveyard of RBCs’. The liver recycles the iron and turns the haem into bile pigment.
• Without a nucleus and mitochondria an RBC cannot divide, but it also does not use up the oxygen it carries.

White blood cells (leucocytes): a short summary

Leucocytes are colourless because they have no haemoglobin. They are nucleated, relatively fewer in number (NCERT 6000 to 8000 per mm³) and generally short-lived. There are two main groups:

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This is only a summary. For the structure of each cell, how they fight infection, and NEET-style questions on them, read our full guide to white blood cells and their functions.

Platelets (thrombocytes)

Platelets are not whole cells. NCERT They are cell fragments produced from megakaryocytes, special cells in the bone marrow. Blood normally has 1,50,000 to 3,50,000 platelets per mm³. Platelets release a variety of substances that are involved in clotting. A fall in their number can lead to clotting disorders, which cause excessive loss of blood from the body.

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3 Blood groups and blood clotting

Syllabus link: NCERT XI 15.2 Blood Groups (15.2.1 ABO, 15.2.2 Rh), 15.3 Coagulation of Blood; IB C3.2.2 sealing cuts in skin by blood clotting; IB D3.2 (ABO multiple alleles, codominance)

Two practical questions come up whenever blood leaves the heart and circulation: can it be given to someone else, and how does the body stop it leaking out?

ABO grouping

Blood of different people can look the same but may not mix safely. NCERT ABO grouping is based on the presence or absence of two surface antigens (chemicals that can induce an immune response) on the RBCs, namely A and B. The plasma of different people contains two natural antibodies (proteins made in response to antigens): anti-A and anti-B. Karl Landsteiner discovered the ABO system in 1900.

If a person receives red cells carrying an antigen that their own plasma has antibodies against, the donor cells clump together (agglutination) and burst. This can block small vessels and damage the kidneys. That is why blood is matched before every transfusion.

• Group O people are universal donors: their red cells carry neither A nor B antigen.
• Group AB people are universal recipients: their plasma has neither anti-A nor anti-B.
• IB Genetics link: the ABO gene has three alleles, I^A, I^B and i. I^A and I^B are codominant and both are dominant to i, so there are six genotypes but four phenotypes.

Rh grouping

NCERT Another antigen, the Rh antigen, similar to one present in Rhesus monkeys (hence Rh), is also observed on the surface of RBCs of the majority (nearly 80%) of humans. Such people are Rh positive (Rh⁺), and those without it are Rh negative (Rh⁻). An Rh⁻ person does not have anti-Rh antibodies naturally; they make them only after being exposed to Rh⁺ blood. So Rh grouping must also be done before a transfusion.

Erythroblastosis foetalis (Rh incompatibility in pregnancy)

1. An Rh⁻ mother carries an Rh⁺ foetus (the father is Rh⁺). In the first pregnancy the placenta keeps the two bloods separate, so the first baby is usually safe.
2. During delivery of the first child, a small amount of the baby’s Rh⁺ blood can enter the mother’s blood. She starts making anti-Rh antibodies.
3. In a later pregnancy with an Rh⁺ foetus, these antibodies cross the placenta and destroy the foetal red blood cells.
4. This can be fatal to the foetus or cause severe anaemia and jaundice in the baby: erythroblastosis foetalis.
5. NCERT It can be avoided by giving the mother anti-Rh antibodies (anti-D immunoglobulin) immediately after the delivery of the first child. They remove the baby’s Rh⁺ cells before her immune system responds.

Coagulation of blood

When you cut yourself, bleeding usually stops within a few minutes. NCERT Blood shows coagulation (clotting) in response to an injury or trauma: a mechanism that prevents excessive loss of blood from the body. A dark reddish-brown scum forms at the site of the cut. This is a clot or coagulum, formed mainly of a network of threads called fibrin in which dead and damaged formed elements of blood are trapped.

1. An injury exposes the tissue under the vessel lining. Platelets stick to the damaged area and to each other, forming a temporary platelet plug.
2. Platelets and the damaged tissue release clotting factors. A cascade of linked enzyme reactions follows, involving many factors present in plasma in an inactive state.
3. These reactions form thrombokinase (the prothrombin activator).
4. Thrombokinase converts inactive prothrombin into the active enzyme thrombin. Calcium ions play a very important role here.
5. Thrombin converts soluble fibrinogen into insoluble fibrin threads.
6. The fibrin mesh traps blood cells and platelets, and the clot seals the wound. Later the clot shrinks and serum is squeezed out.

IB IB C3.2.2 expects you to outline this in three steps: platelets release clotting factors; a cascade of reactions leads to the rapid formation of thrombin; thrombin converts fibrinogen to fibrin, which forms a mesh that traps cells. The cascade lets a few platelets set off a large response quickly, but it has to be tightly controlled: a clot inside a coronary artery (a thrombus) causes a heart attack.

4 Structure of the heart and blood vessels

Syllabus link: NCERT XI 15.5.1 Human Circulatory System (heart, blood vessels); IB B3.2.1 to B3.2.5 (capillaries, arteries, veins, pulse) and B3.2.14 HL (adaptations of the mammalian heart)

The heart is the pump at the centre of the heart and circulation system; the blood vessels are the pipes. Structure matches function in both.

Position and coverings of the heart

NCERT The heart, the mesodermally derived organ, is situated in the thoracic cavity, in between the two lungs, slightly tilted to the left. It has the size of a clenched fist. It sits in a hollow of the left lung called the cardiac notch, protected by the rib cage. It is protected by a double-walled membranous bag, the pericardium, enclosing the pericardial fluid, which reduces friction as the heart beats.

From outside to inside, the wall and its coverings are:

1. Fibrous pericardium: tough outer sac that anchors the heart.
2. Parietal pericardium: outer serous layer, lining the fibrous sac.
3. Pericardial fluid in the pericardial cavity, secreted by the two serous layers.
4. Visceral pericardium (epicardium): inner serous layer, stuck to the heart.
5. Myocardium: thick layer of cardiac muscle.
6. Endocardium: thin endothelium (squamous epithelium) lining the chambers and valves.

Cardiac muscle is striated, branched and involuntary. Each cell usually has one central nucleus. Cells are joined end to end by intercalated discs containing gap junctions, so ions and electrical impulses pass quickly from cell to cell and the heart contracts as one unit. Cardiac muscle never tires because it has many mitochondria and a rich blood supply.

Chambers and septa

NCERT Our heart has four chambers: two relatively small upper chambers called atria and two larger lower chambers called ventricles. A thin, muscular wall called the inter-atrial septum separates the right and the left atrium, whereas a thick-walled inter-ventricular septum separates the left and the right ventricle. The atrium and the ventricle of the same side are separated by a thick fibrous tissue called the atrio-ventricular septum, which has an opening with a valve.

Auricle or atrium? In a whole-heart drawing, the ear-like flap on each atrium is the auricle (auricle means ‘little ear’). In a longitudinal section, the same region appears as a hollow chamber, the atrium. Many books use the words as synonyms.

Valves

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NCERT The valves in the heart allow the flow of blood only in one direction: from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta. The opening of the superior vena cava and the four pulmonary veins have no valves.

Inside the chambers

• Right atrium receives deoxygenated blood from the superior vena cava, the inferior vena cava and the coronary sinus (blood from the heart muscle itself). Its wall has low muscular ridges called musculi pectinati.
• On the inter-atrial septum there is an oval depression, the fossa ovalis. It is the remains of the foramen ovale, an opening that lets blood pass from the right to the left atrium in the foetus (whose lungs do not work); it closes after birth.
• Left atrium receives oxygenated blood from the lungs through four pulmonary veins, two from each lung.
• Ventricles are the pumping (distributing) chambers. Their inner walls carry muscular ridges called columnae carneae (trabeculae carneae); some project as papillary muscles.
• Chordae tendineae are tendon-like cords joining the papillary muscles to the cusps of the AV valves. When the ventricles contract, the papillary muscles pull on them and stop the cusps from turning inside out into the atria. Chain: columnae carneae → papillary muscles → chordae tendineae → AV valve.
• The left ventricle has the thickest wall, because it pumps blood all round the body against high resistance; the right ventricle pumps only to the nearby lungs.

Blood vessels: arteries, veins and capillaries

Blood leaves the heart in arteries and returns in veins. The order is: aorta → arteries → arterioles → capillaries → venules → veins → venae cavae. Arteries carry blood away from the heart and veins carry it towards it, whether it is oxygenated or not; that is why the pulmonary artery carries deoxygenated blood.

Arteries and veins have three layers: an inner tunica intima (endothelium of squamous cells over elastic tissue), a middle tunica media (smooth muscle and elastic fibres) and an outer tunica externa (mainly collagen fibres). Capillaries have only the endothelium.

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IB IB B3.2.1 to B3.2.5 links each structure to its job:

• Capillaries: a very large total surface area, a wall one cell thick and, in organs such as the kidney and small intestine, fenestrations (pores) let substances exchange quickly with tissue fluid.
• Arteries: elastic fibres stretch when the ventricle pushes blood in and recoil between beats, which keeps blood moving and evens out the pressure; muscle in the wall resists bursting and can narrow the lumen (vasoconstriction) to control flow.
• Veins: pressure is too low to push blood up the legs, so valves stop backflow and the squeezing of nearby skeletal muscles pushes blood towards the heart.
• Pulse rate: each surge of blood in an artery can be felt as a pulse at the wrist (radial artery) or neck (carotid artery). Count beats for 30 s and double, or use a digital monitor.

Why does blood slow down in capillaries?

Resistance to flow rises steeply as a vessel narrows: it is proportional to 1/r⁴ (r = radius). Most of the peripheral resistance is in the arterioles, whose muscular walls can change their radius; this is where blood pressure drops the most. Each capillary is narrower still, but there are billions of them side by side, so their total cross-sectional area is about 1000 times that of the aorta. The same volume of blood spread over a much wider bed moves slowly (about 0.03 cm/s), which gives time for exchange.

Major arteries from the aorta

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Blood from the head and brain returns in the jugular veins, from the arms in the subclavian veins; these join to form the superior vena cava. The inferior vena cava collects blood from the lower body. For a clear animation of the route through the heart, see the US National Heart, Lung, and Blood Institute page on how blood flows through the heart.

5 Cardiac cycle and regulation of heart rate

Syllabus link: NCERT XI 15.5.1 (nodal tissue), 15.5.2 Cardiac Cycle, 15.5.3 Electrocardiograph (ECG), 15.7 Regulation of Cardiac Activity; IB B3.2.15 HL (stages of the cardiac cycle), C3.1 HL (control of heart rate by the medulla)

Every heartbeat follows the same cardiac cycle of contraction and relaxation. This section covers how the beat starts, the stages of the cardiac cycle, heart sounds, the ECG, and how the heart rate is controlled.

A heart that starts its own beat

A heart is myogenic when the impulse for each beat starts in the heart muscle itself, and neurogenic when it comes from nerves (ganglia) outside the muscle. Vertebrate hearts, including ours, and the hearts of molluscs (cephalopods such as the octopus are the usual exam example) are myogenic. Many arthropods (such as crabs and lobsters) and annelids have neurogenic hearts. A myogenic heart keeps beating for a while even when removed from the body.

Nodal tissue: the conducting system

NCERT A specialised cardiac musculature called the nodal tissue is distributed in the heart:

1. Sino-atrial node (SAN): a patch in the right upper corner of the right atrium, near the opening of the superior vena cava. It makes the most action potentials, about 70 to 75 per minute, so it sets the rhythm: it is the pacemaker.
2. Three internodal pathways (anterior, middle and posterior) carry the impulse across the right atrium to the AV node, while it also spreads over both atria.
3. Atrio-ventricular node (AVN): in the lower left corner of the right atrium, close to the atrio-ventricular septum and the opening of the coronary sinus. It delays the impulse by about 0.1 s so the atria finish emptying before the ventricles contract.
4. AV bundle (bundle of His): a single bundle that passes from the AV node through the atrio-ventricular septum into the top of the inter-ventricular septum. There it divides into right and left bundle branches, which run down the septum.
5. Purkinje fibres: the branches end in a network of fibres spreading through the walls of both ventricles, from the apex upwards, so the ventricles contract from the bottom and squeeze blood up into the arteries.

Speeds are very different along this path: about 0.05 m/s in the AV node (the delay), about 0.3 to 1 m/s in atrial muscle, and up to about 4 m/s in Purkinje fibres, the fastest in the heart.

Action potential in heart muscle

• Resting (polarised) state: outside of the membrane positive, inside negative. Resting potential is about −90 mV in ventricular muscle (about −60 mV in SA node cells). It is measured in millivolts, displayed on a cathode ray oscilloscope (CRO).
• Depolarisation: Na⁺ ions rush in and the inside becomes positive. This happens just before the muscle fibre contracts.
• Repolarisation: K⁺ ions move out (efflux) and the inside becomes negative again.
• The sodium-potassium pump restores the original ion distribution: it pumps 3 Na⁺ out and 2 K⁺ in for each ATP used.
• SA node cells have no stable resting potential; they slowly depolarise on their own until they fire. That is what makes them a pacemaker.

The cardiac cycle

NCERT The sequential events in the heart which are cyclically repeated are called the cardiac cycle; it consists of systole (contraction) and diastole (relaxation) of both the atria and ventricles. The heart beats about 72 times per minute, so one cycle lasts 60/72 = about 0.8 s.

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Heart sounds

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The interval from lub to dub is the ventricular systole (0.3 s); from dub to the next lub is 0.5 s. A murmur is an extra whooshing sound from turbulent flow, for example when blood leaks back through an incompetent (leaky) valve or squeezes through a narrowed one. A doctor listens with a stethoscope.

Stroke volume and cardiac output

• NCERT Stroke volume (SV): the volume of blood pumped out by each ventricle during a cardiac cycle, about 70 mL.
• NCERT Cardiac output (CO): the volume of blood pumped out by each ventricle per minute. CO = SV × heart rate = 70 mL × 72 = 5040 mL, about 5 L per minute.
• Athletes have a much higher cardiac output during exercise (it can rise five-fold), mainly by increasing stroke volume.
• Cardiac index = cardiac output / body surface area = 5 L/min ÷ 1.7 m² ≈ 3 L/min/m². It lets doctors compare people of different sizes.
• Frank-Starling law: the more the ventricles are filled (and stretched) at the end of diastole, the more forcefully they contract, so stroke volume rises with venous return.

The electrocardiogram (ECG)

NCERT An ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. A patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) for a standard record; a 12-lead ECG gives more detail.

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Counting the QRS complexes in a set time gives the heart rate. Atrial repolarisation does not show as a separate wave: it is tiny and happens at the same time as the QRS complex.

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In complete heart block no impulses reach the ventricles, which then beat at their own slow ‘escape’ rhythm of about 20 to 40 per minute. Sudden pauses can make the person faint: these episodes are called Stokes-Adams attacks. The treatment is an artificial pacemaker: a small battery-powered device implanted under the skin below the collarbone, with a lead (electrode) passed through a vein into the right ventricle (and often the right atrium) to deliver regular impulses.

Regulation of heart rate

NCERT Normal activities of the heart are regulated intrinsically (auto-regulated by specialised muscles: nodal tissue), hence the heart is called myogenic. A special neural centre in the medulla oblongata can moderate the cardiac function through the autonomic nervous system.

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• Vagal tone: on its own, the SA node would fire about 100 times per minute. The vagus nerve releases acetylcholine all the time, which brings the resting rate down to about 72.
• Baroreceptors in the walls of the aortic arch and carotid sinus detect stretch. When blood pressure rises they fire more, the cardio-inhibitor centre is stimulated, vagal output rises and sympathetic output falls, so heart rate falls. Marey’s law: heart rate is inversely related to arterial blood pressure (when other factors stay the same).
• Chemoreceptors in the aorta, carotid bodies and medulla detect a rise in CO₂ (fall in pH) during exercise, and the medulla raises heart rate.
• Thyroxine raises heart rate indirectly by raising metabolic rate and the tissues’ oxygen demand.
• Ions: excess K⁺ slows the heart and weakens contraction; Ca²⁺ strengthens contraction, and a very high level can stop the heart in contraction.
• Age, sex and temperature: a newborn’s heart beats about 120 to 140 times per minute, an adult’s about 72; women have a slightly higher resting rate than men; heart rate rises with body temperature (fever).

6 Double circulation and coronary circulation

Syllabus link: NCERT XI 15.5 Circulatory Pathways, 15.6 Double Circulation; IB B3.2.13 HL (single circulation of bony fish versus double circulation of mammals)

Double circulation is the key design feature of heart and circulation in mammals and birds. Comparing vertebrate hearts shows how it evolved.

How the vertebrate heart evolved

NCERT All vertebrates have a muscular chambered heart. Fishes have a 2-chambered heart with an atrium and a ventricle. Amphibians and reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle, whereas crocodiles, birds and mammals have a 4-chambered heart with two atria and two ventricles.

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Amphibians cope with mixed blood because they are cold-blooded (low metabolic rate) and their blood picks up extra oxygen through the skin (cutaneous respiration). Birds and mammals are warm-blooded with a high basal metabolic rate, so their cells need a large, steady oxygen supply: complete separation of oxygenated and deoxygenated blood in a 4-chambered heart makes that possible.

Double circulation in humans

NCERT Blood passes through the heart twice in one complete circuit, so humans have double circulation:

• Pulmonary circulation: right ventricle → pulmonary artery → lungs (oxygenated) → pulmonary veins → left atrium.
• Systemic circulation: left ventricle → aorta → arteries, arterioles, capillaries of the tissues (deoxygenated) → venules, veins → venae cavae → right atrium.
• The systemic circulation supplies nutrients, O₂ and other essential substances to the tissues and takes CO₂ and other harmful substances away for elimination.

Portal systems

Normally blood passes through one capillary bed between an artery and a vein. In a portal system, a vein carries blood from the capillaries of one organ to the capillaries of a second organ before it returns to the heart: a portal vein has capillaries at both ends.

• NCERT Hepatic portal system (all vertebrates): the hepatic portal vein carries blood from the intestine (and stomach, pancreas and spleen) to the liver before it is delivered to the systemic circulation. The liver stores glucose as glycogen, deals with excess amino acids (making urea) and breaks down old red cell products. The hepatic vein then carries blood to the inferior vena cava.
• Hypophyseal portal system: blood from capillaries in the hypothalamus goes to the anterior pituitary, carrying releasing and inhibiting hormones.
• Renal portal system: blood from the hind limbs passes through the kidneys first. It is well developed in fishes and amphibians, reduced in reptiles and birds, and absent in mammals.

Coronary circulation

NCERT A special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature. The heart cannot use the blood inside its chambers; its thick walls need their own supply.

• The right and left coronary arteries are the first branches of the ascending aorta, just above the aortic semilunar valve. They run over the surface of the heart and branch into the myocardium.
• Coronary arteries fill mainly during diastole, because contracting muscle squeezes them shut in systole.
• Deoxygenated blood is collected by cardiac veins into the coronary sinus, which opens into the right atrium (guarded by the Thebesian valve).
• A blockage of a coronary artery cuts off part of the heart muscle from oxygen: the basis of angina and heart attacks (section 7).

7 Blood pressure and cardiovascular disorders

Syllabus link: NCERT XI 15.8 Disorders of Circulatory System; IB B3.2.4 (pulse), B3.2.6 (causes and consequences of occlusion of the coronary arteries)

Blood pressure is the force that drives blood round the heart and circulation system, and most disorders of the heart and circulation involve abnormal blood pressure or blocked arteries.

What is blood pressure?

Blood pressure is the force blood exerts on the walls of the arteries. It is measured with a sphygmomanometer and written as two numbers in mm Hg:

• NCERT Normal blood pressure is 120/80 mm Hg. 120 is the systolic (pumping) pressure, during ventricular systole; 80 is the diastolic (resting) pressure.
• Pulse pressure = systolic − diastolic = 120 − 80 = 40 mm Hg.
• Mean arterial pressure (MAP) = diastolic + 1/3 (pulse pressure) = 80 + 40/3 ≈ 93 mm Hg. It is closer to the diastolic value because the heart spends longer in diastole.

How the reading is taken: the cuff is pumped up until it stops blood flow in the arm artery, then slowly let down while a stethoscope listens below the cuff. The first tapping sound (a Korotkoff sound, from blood spurting through the partly squeezed artery) marks the systolic pressure; the point where the sounds disappear marks the diastolic pressure. These are not the lub and dub of the heart valves.

Disorders of the circulatory system

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Treatment of blocked arteries

• Clot busters (thrombolytics) such as streptokinase and tissue plasminogen activator (tPA) dissolve a fresh clot.
• Angioplasty: a balloon on a catheter opens the artery, and a mesh tube (stent) keeps it open.
• Coronary artery bypass grafting (CABG): a vein from the leg or an artery from the chest is used to route blood around the blockage.
• Daily low-dose aspirin, statins (lower cholesterol) and lifestyle changes reduce the risk of another event.

Other heart problems in MCQs

• Rheumatic heart disease: after a throat infection with Streptococcus pyogenes, antibodies made against the bacterium’s cell wall (M) protein also attack similar proteins in the heart (molecular mimicry), especially the mitral valve. The scarred valve may leak or narrow, and the heart has to work harder.
• Arrhythmias: abnormal heart rhythms. A fast rhythm is a tachyarrhythmia; a slow one is a bradyarrhythmia, for example from SA node failure or AV block, and may need a pacemaker.
• Heart murmurs: abnormal sounds from leaky (incompetent) or narrowed valves.

8 Lymph and its functions

Syllabus link: NCERT XI 15.4 Lymph (Tissue Fluid); IB B3.2.10 to B3.2.12 HL (formation of tissue fluid, exchange with cells, drainage into lymph ducts)

Not all the fluid that leaves the blood comes straight back. The lymphatic system is the drainage network that completes the heart and circulation story.

From plasma to tissue fluid to lymph

NCERT As blood passes through the capillaries in tissues, some water along with many small water-soluble substances move out into the spaces between the cells of tissues, leaving the larger proteins and most of the formed elements in the blood vessels. This fluid released out is called the interstitial fluid or tissue fluid. It has the same mineral distribution as that in plasma. Exchange of nutrients, gases, etc., between the blood and the cells always occurs through this fluid.

The lymphatic system

NCERT An elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins. The fluid present in the lymphatic system is called the lymph.

• Lymph capillaries join into lymph vessels, which pass through lymph nodes (filters full of lymphocytes and macrophages) and join two main ducts.
• The thoracic duct, the largest, begins in the abdomen at a sac called the cisterna chyli (at the level of the first and second lumbar vertebrae). It drains lymph from both legs, the abdomen, the left side of the chest, the left arm and the left side of the head, and empties into the left subclavian vein (where it meets the left internal jugular vein).
• The right lymphatic duct drains the right arm and the right side of the head, neck and chest, and empties into the right subclavian vein.
• Lymph flows only one way: from the tissues towards the heart. There is no pump; valves and muscle movements keep it moving.
• NCERT Fats are absorbed through lymph in the lacteals present in the intestinal villi. Lymph that is milky with absorbed fat is called chyle.
• Lymphoid organs: lymph nodes, spleen, thymus, tonsils and Peyer’s patches.

Functions of lymph

• Returns tissue fluid and leaked plasma proteins to the blood, keeping blood volume steady. Without it, fluid would collect in the tissues (oedema).
• NCERT Lymph is a colourless fluid containing specialised lymphocytes which are responsible for the immune responses of the body.
• Lymph nodes filter out bacteria and debris; lymphocytes there make antibodies.
• NCERT Lymph is an important carrier for nutrients, hormones, etc.
• Carries absorbed fats (as chylomicrons) from the intestine to the blood.

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If lymph vessels are blocked, fluid collects and the tissue swells. In filariasis, Wuchereria worms live in the lymph vessels of the legs and cause a huge swelling called elephantiasis.

9 Key terms

Learn these key terms for blood, the heart and circulation; most one-mark questions test a definition.

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10 Common doubts about blood, heart and circulation

These are the questions students keep asking on Reddit, Quora, Stack Exchange and NEET doubt pages. Each answer is ours; the link shows where the doubt comes from.

O negative is the universal donor, but its plasma has anti-A and anti-B. Why doesn’t that attack the patient’s cells?

Because what is usually transfused is packed red cells, with most of the plasma removed. The rule ‘O is the universal donor’ is about red cell antigens: O⁻ cells carry no A, B or Rh antigen, so the patient’s antibodies have nothing to attack. The small amount of donor antibody left is diluted in the patient’s blood. For plasma the rule is reversed: AB plasma (no antibodies) is the universal plasma. (Doubt seen on reddit.com)

If the pulmonary artery carries deoxygenated blood, why is it called an artery?

Arteries and veins are named by direction, not by the oxygen they carry. An artery carries blood away from the heart; a vein carries it towards the heart. The pulmonary artery takes blood from the right ventricle to the lungs, so it is an artery even though its blood is deoxygenated. The pulmonary veins are the matching exception. (Doubt seen on biology.stackexchange.com)

Why is the left ventricle wall so much thicker than the right?

Both ventricles pump the same volume per beat. The left ventricle has to push blood through the whole body against high resistance (about 120 mm Hg), while the right only pushes it through the nearby lungs (about 25 mm Hg). More muscle means more force. A thick right ventricle would actually damage the delicate lung capillaries. (Doubt seen on quora.com)

Why is the first Rh-positive baby of an Rh-negative mother usually safe?

The mother has no anti-Rh antibodies to begin with; she makes them only after being exposed to Rh⁺ blood. During the first pregnancy the placenta keeps the two bloods apart. The main exposure happens at delivery, when the first baby is already born. Her antibodies then threaten the next Rh⁺ baby, which is why anti-Rh antibodies are given to her right after the first delivery. (Doubt seen on medicneet.com)

Why is there no wave for atrial repolarisation in the ECG?

The textbook answer is that it is hidden by the QRS complex, which happens at the same time. The fuller answer is that atrial muscle is thin, so the repolarisation signal (the Ta wave) is very small and spread out; even when there is no QRS, as in some heart blocks, it is hard to see. For exams, ‘masked by the QRS complex’ is accepted. (Doubt seen on reddit.com)

Capillaries are the narrowest vessels. Why isn’t resistance highest there, and why is blood slowest there?

One capillary has a high resistance, but billions of them lie side by side (in parallel), so their combined resistance is lower than that of the arterioles, which are the main site of resistance and pressure drop. Speed depends on total cross-sectional area: the capillary bed is about 1000 times wider than the aorta, so blood moves slowest there, which gives time for exchange. (Doubt seen on biology.stackexchange.com)

Why does the AV node delay the impulse?

If the ventricles contracted at the same moment as the atria, the atria would not have finished emptying, and the ventricles would push blood back up. The AV node cells are small, have few gap junctions and conduct slowly, giving a delay of about 0.1 s. The PR interval on an ECG measures this delay. (Doubt seen on reddit.com)

If blood has clotting factors, why doesn’t it clot inside our vessels?

The factors circulate in an inactive form, and clotting needs a trigger: damaged tissue or exposed collagen under the vessel lining. Healthy endothelium is smooth and releases substances that stop platelets sticking, and natural anticoagulants such as heparin and antithrombin keep the cascade switched off. When a vessel lining is damaged, as in atherosclerosis, a clot can form inside it: a thrombosis. (Doubt seen on reddit.com)

Are tissue fluid and lymph the same thing?

Almost. They are the same fluid in different places. Tissue fluid is the fluid around cells; once it enters a lymph capillary it is called lymph. Lymph collects more lymphocytes as it passes through lymph nodes, and lymph from the intestine also carries fat. Both have much less protein than plasma. (Doubt seen on medicneet.com)

11 FAQ

Short answers to the questions we are asked most about the heart and circulation.

12 Quick check

Tap each question to reveal the answer and check your grasp of blood, the heart and circulation before trying the full question set.