Blood, Lymph and the Circulatory System

Every cell in the body needs a steady supply of oxygen, nutrients and signalling molecules, and needs its waste carried away just as steadily. Blood is the fluid that does this job, moved by a muscular pump, the heart, through a closed network of vessels. A second, quieter fluid, lymph, drains what blood leaves behind in the tissues and doubles as a route for the immune system. This chapter covers what blood and lymph are made of, how blood is grouped and why that grouping matters clinically, how the heart is built and timed, and why humans run a "double" circulation rather than a single loop. It is one of NCERT's most numbers-heavy chapters, and UPSC leans on exactly that: precise figures, named structures, and the sequence in which events happen.

Blood: plasma and formed elements

Blood is classified as a connective tissue, unusual only in that its matrix is a liquid rather than something solid like bone or cartilage. It has two components. The fluid matrix, plasma, makes up roughly 55 per cent of blood volume; the cellular part, called the formed elements (red cells, white cells and platelets), makes up the remaining 45 per cent. Plasma itself is 90 to 92 per cent water, with proteins contributing 6 to 8 per cent and the rest a mix of dissolved minerals, glucose, amino acids and lipids in transit around the body. Plasma also carries the clotting factors in an inactive form; drain those out and what remains is called serum, a distinction UPSC has tested by asking what separates plasma from serum.

Plasma proteins: three, three jobs

The major plasma proteins are albumins, globulins and fibrinogen, and each does a genuinely different job, which is exactly the kind of one-to-one mapping a statement-based question likes to scramble. Albumins are the most abundant and chiefly maintain osmotic balance, keeping water from leaking out of the bloodstream into the tissues. Globulins are mainly defence proteins; the antibodies (immunoglobulins) that neutralise pathogens are globulins. Fibrinogen is the precursor used in clotting, converted into the fibrin mesh that seals a wound. A useful way to hold the three apart: albumin for pressure, globulin for defence, fibrinogen for clotting.

Erythrocytes and haemoglobin

Erythrocytes, or red blood cells, are the most numerous cells in blood, present at roughly 5 to 5.5 million per cubic millimetre in a healthy adult man. They are produced in the red bone marrow, and in most mammals they mature by ejecting their nucleus, ending up as biconcave discs, a shape that raises the surface area available for gas exchange. Their red colour comes from haemoglobin, an iron-containing protein that binds oxygen in the lungs and releases it in the tissues; a healthy person carries roughly 12 to 16 grams of haemoglobin per 100 millilitres of blood. An individual erythrocyte survives about 120 days before it is broken down, mostly in the spleen, which is why the spleen is nicknamed the graveyard of red blood cells.

Leucocytes: five named types, two families

White blood cells, or leucocytes, are colourless because they lack haemoglobin, retain their nucleus throughout their (comparatively short) life, and are far less numerous than red cells, averaging only 6,000 to 8,000 per cubic millimetre. NCERT sorts the five real leucocyte types into two families based on whether their cytoplasm shows visible granules under a microscope.

The granulocytes are neutrophils, eosinophils and basophils. Neutrophils are the most abundant leucocyte of all, 60 to 65 per cent of the white cell count, and along with monocytes they are phagocytic, literally engulfing invading microorganisms. Eosinophils, only 2 to 3 per cent of the count, resist infections and are heavily involved in allergic reactions. Basophils are the rarest, just 0.5 to 1 per cent, and rather than eating pathogens they release histamine, serotonin and heparin, driving the inflammatory response.

The agranulocytes are lymphocytes and monocytes. Lymphocytes, at 20 to 25 per cent of the count, come in two major forms, B lymphocytes and T lymphocytes, both central to the body's adaptive immune response. Monocytes (6 to 8 per cent) are the other phagocytic cell type here, alongside neutrophils.

The trap UPSC likes: match the cell to its function, not just its name. Phagocytosis belongs to neutrophils and monocytes; allergy and infection resistance to eosinophils; histamine release to basophils; immune-response coordination to the two lymphocyte types.

Platelets and how a clot forms

Platelets, also called thrombocytes, are not whole cells at all but fragments pinched off from megakaryocytes, large cells resident in the bone marrow. A healthy person carries roughly 1.5 to 3.5 lakh (150,000 to 350,000) platelets per cubic millimetre of blood; a fall below this range produces clotting disorders and excessive bleeding.

Clotting itself runs as a cascade rather than a single step. An injury triggers platelets, and damaged tissue, to release factors that convert an inactive plasma protein, prothrombin, into the active enzyme thrombin, via an enzyme complex called thrombokinase. Thrombin then converts fibrinogen (the plasma protein, not the reaction's final product) into fibrin, an insoluble mesh of threads that traps blood cells and forms the visible scab. Calcium ions are essential throughout this cascade, which is why citrate (a calcium-chelating agent) is added to stored blood to keep it from clotting.

Blood groups: the ABO and Rh systems

Blood is typed by which antigens sit on the surface of red blood cells, and which antibodies circulate in the plasma. The ABO system rests on two surface antigens, A and B.

• Group A carries the A antigen and anti-B antibodies in plasma.
• Group B carries the B antigen and anti-A antibodies.
• Group AB carries both antigens and neither antibody, which is why AB individuals can receive blood from any of the four groups and are called universal recipients.
• Group O carries neither antigen but both antibodies, which is why O blood can be given to any of the four groups (donor and recipient Rh status still has to match) and O individuals are called universal donors.

The logic is always the same: never transfuse a red cell antigen into a plasma that already carries the matching antibody, or the donated cells clump together and are destroyed.

The Rh system is a separate, second layer of typing, based on the Rh antigen (named for its similarity to an antigen first found in Rhesus monkeys), present on the red cells of roughly 80 per cent of people, who are called Rh-positive; the remaining, antigen-free 20 per cent are Rh-negative. An Rh-negative person exposed to Rh-positive blood builds antibodies against the Rh antigen. This has one clinically important consequence: if an Rh-negative mother carries an Rh-positive foetus, the placenta keeps the two blood supplies apart during the first pregnancy, but a small amount of the foetus's blood typically crosses over at delivery, sensitising the mother. In any subsequent Rh-positive pregnancy, her existing anti-Rh antibodies can cross into the foetal circulation and destroy foetal red cells, causing severe anaemia and jaundice in the baby, a condition called erythroblastosis foetalis. Giving the mother anti-Rh antibodies immediately after the first delivery prevents this sensitisation from carrying over into later pregnancies.

Lymph: the body's second circulation

As blood passes through capillaries, water and small dissolved substances filter out into the spaces between cells, while the large plasma proteins and most formed elements stay inside the vessel. This filtrate is interstitial or tissue fluid, and it is how nutrients and gases actually reach and leave individual cells, since blood itself never touches most tissue directly. A dedicated network of vessels, the lymphatic system, collects this fluid and drains it back into the major veins; once inside that network, the fluid is called lymph. Lymph is colourless, carries specialised lymphocytes that mount immune responses, and also transports nutrients and hormones. It has one job blood plasma does not: fat absorbed in the intestine enters the lymphatic system directly, through vessels called lacteals inside the intestinal villi, bypassing the portal blood route that carries other digested nutrients to the liver.

The heart: chambers and valves

The human heart sits in the thoracic cavity between the two lungs, tilted slightly to the left, roughly the size of a clenched fist, and enclosed in a protective double-walled sac called the pericardium. It has four chambers: two smaller upper chambers, the atria, and two larger, thicker-walled lower chambers, the ventricles. A muscular inter-atrial septum separates the two atria and a thicker inter-ventricular septum separates the two ventricles, keeping oxygenated and deoxygenated blood from mixing.

Each atrium connects to the ventricle beneath it through an opening guarded by a valve, and the two are named differently on purpose. The right atrium opens into the right ventricle through the tricuspid valve, built from three muscular cusps. The left atrium opens into the left ventricle through the bicuspid valve, also called the mitral valve, built from two cusps. Where the ventricles empty outward, into the pulmonary artery (from the right ventricle) and the aorta (from the left ventricle), the openings are each guarded by a semilunar valve. Every valve in the heart enforces one-way flow, atria to ventricles and ventricles to the great arteries, and stops blood running backward.

The conduction system and the cardiac cycle

The heart is myogenic: it generates its own contraction rhythm rather than waiting on nerve signals, thanks to a specialised patch of tissue called the nodal or conducting tissue. The sino-atrial node (SAN), sitting in the upper wall of the right atrium, fires spontaneously faster than any other part of this tissue, 70 to 75 times a minute, which is why it sets the pace for the whole heart and is called the pacemaker. Its signal spreads across both atria and reaches the atrio-ventricular node (AVN), positioned near the base of the right atrium close to the atrio-ventricular septum, which relays it down the atrio-ventricular bundle (the bundle of His) into the ventricular walls, branching finally into Purkinje fibres that trigger contraction across both ventricles almost simultaneously.

One full cardiac cycle runs through this sequence: the whole heart starts in a relaxed, joint diastole, with the tricuspid and bicuspid valves open and blood draining passively into the ventricles. The SAN then fires, triggering atrial systole, a contraction that tops up ventricular filling by roughly another 30 per cent. As the signal reaches the ventricles they contract in turn, ventricular systole, while the atria simultaneously relax; rising ventricular pressure snaps the tricuspid and bicuspid valves shut (preventing backflow into the atria) and forces the semilunar valves open, ejecting blood into the pulmonary artery and aorta. The ventricles then relax, ventricular diastole, pressure inside them falls, the semilunar valves close, and once atrial pressure exceeds ventricular pressure again the tricuspid and bicuspid valves reopen, returning the heart to joint diastole to begin the next cycle. At a resting rate of 72 beats a minute, one full cycle takes about 0.8 seconds. The two heart sounds heard through a stethoscope, conventionally written as "lub" and "dub", mark the closure of the tricuspid and bicuspid valves and the closure of the semilunar valves respectively, not their opening.

Each ventricle ejects roughly 70 millilitres of blood per cycle, the stroke volume. Multiplying stroke volume by heart rate gives the cardiac output, the volume pumped by each ventricle in a minute, which averages around 5 litres in a resting healthy adult and rises well above that during exertion, since the body can adjust both stroke volume and heart rate. The nodal tissue's autonomy is only the baseline: a centre in the medulla oblongata moderates it through the autonomic nervous system, sympathetic signals raising heart rate, contraction strength and cardiac output, parasympathetic signals lowering all three, with adrenal medullary hormones adding a further boost to cardiac output under stress.

Double circulation, and why it matters

Blood leaving the right ventricle enters the pulmonary artery and is carried to the lungs to be oxygenated, returning through the pulmonary veins to the left atrium: this loop is the pulmonary circulation. Blood leaving the left ventricle enters the aorta and is distributed through arteries, arterioles and capillaries to every tissue, returning through venules, veins and the vena cava to the right atrium: this second loop is the systemic circulation. Because the human heart has fully separate left and right sides, oxygenated and deoxygenated blood never mix at any point in the cycle, unlike the incomplete double circulation seen in most reptiles, where a single, undivided ventricle lets the two streams blend. This separation is what makes double circulation efficient: blood reaches the lungs at full pressure and the rest of the body at full pressure, rather than a single circuit diluting both. Two smaller, specialised circuits run alongside these two main loops: the hepatic portal system, which carries blood from the intestine to the liver before it rejoins the systemic circulation, and the coronary circulation, a dedicated supply to the heart's own muscular wall.

Blood pressure and the ECG

Blood pressure is recorded as two numbers, systolic over diastolic, in millimetres of mercury: the higher, systolic figure is the pressure generated as the ventricles contract and eject blood, and the lower, diastolic figure is the pressure during ventricular relaxation. A reading of around 120/80 mm Hg is treated as normal; a person whose blood pressure repeatedly reads 140/90 mm Hg or higher is classified as hypertensive, a state that raises the risk of heart disease and damages organs such as the brain and kidneys over time.

The heart's electrical activity, rather than its mechanical pumping, can be recorded from the body surface using an electrocardiograph, producing an electrocardiogram (ECG), typically from three standard leads placed on both wrists and the left ankle. A standard ECG trace has three named deflections. The P wave marks depolarisation (electrical excitation) of the atria, which drives atrial contraction. The QRS complex marks depolarisation of the ventricles and the onset of ventricular systole. The T wave marks repolarisation of the ventricles, their return to a resting electrical state, and its end marks the end of systole. Counting QRS complexes over a fixed period gives heart rate directly, and because a healthy ECG has a characteristic, repeatable shape, any distortion in it is a genuine diagnostic signal, which is why the test is a routine part of cardiac screening.

The exam angle

This chapter rewards precision over general understanding, and UPSC's questions on it usually target one of a few recurring confusions. The first is cell-to-function matching: which leucocyte phagocytoses, which resists infection, which drives inflammation, and separately, which plasma protein does what (osmotic balance versus clotting versus defence). The second is valve identity: tricuspid sits on the right, bicuspid or mitral on the left, and semilunar valves are not atrioventricular valves at all but guard the exits into the pulmonary artery and aorta, a distinction that is easy to blur under exam pressure. The third is the systole/diastole sequence itself, since a statement can quietly reverse cause and effect, for instance claiming the semilunar valves open because the ventricles are relaxing, when it is rising pressure during ventricular contraction that forces them open. The fourth is the ABO logic: O is the universal donor precisely because it carries no antigens for a recipient's antibodies to react against, and AB is the universal recipient precisely because it carries no antibodies of its own, a pairing that is often stated backwards in a distractor option. Finally, watch for numbers presented as if interchangeable: the 70 to 75 action potentials the SAN can generate in a minute is not the same figure as the heart's actual resting rate of about 72 beats a minute, and the 0.8-second cardiac cycle is easy to confuse with stroke volume or cardiac output figures in a mixed-statement question.

Quick revision points

• Blood: plasma (about 55 per cent, itself 90 to 92 per cent water) plus formed elements (about 45 per cent).
• Plasma proteins: albumin (osmotic balance), globulin (defence, includes antibodies), fibrinogen (clotting).
• RBCs: 5 to 5.5 million per cubic millimetre, biconcave, no nucleus in mammals, haemoglobin 12 to 16 g/100 mL, lifespan about 120 days, broken down mainly in the spleen.
• WBCs: 6,000 to 8,000 per cubic millimetre. Granulocytes: neutrophils (60 to 65 per cent, phagocytic), eosinophils (2 to 3 per cent, resist infection/allergy), basophils (0.5 to 1 per cent, release histamine). Agranulocytes: lymphocytes (20 to 25 per cent, B and T types, immune response), monocytes (6 to 8 per cent, phagocytic).
• Platelets: 1.5 to 3.5 lakh per cubic millimetre, from megakaryocytes; clotting cascade runs prothrombin to thrombin to fibrinogen to fibrin, calcium-dependent.
• ABO groups: A (anti-B antibody), B (anti-A antibody), AB (universal recipient, no antibodies), O (universal donor, no antigens).
• Rh: about 80 per cent of people are Rh-positive; Rh incompatibility across pregnancies causes erythroblastosis foetalis.
• Lymph: tissue fluid drained by the lymphatic system, carries lymphocytes and nutrients, uniquely transports absorbed fat via intestinal lacteals.
• Heart: four chambers; tricuspid valve (right AV), bicuspid/mitral valve (left AV), semilunar valves (pulmonary artery and aorta exits).
• SAN (pacemaker, right atrium) to AVN to AV bundle/bundle of His to Purkinje fibres; cardiac cycle about 0.8 seconds at 72 beats/min; stroke volume about 70 mL; cardiac output (stroke volume times heart rate) about 5 litres/min.
• Double circulation: pulmonary (right ventricle to lungs to left atrium) and systemic (left ventricle to body to right atrium), kept fully separate.
• Blood pressure: normal about 120/80 mm Hg; hypertension at 140/90 mm Hg or higher, repeatedly.
• ECG waves: P (atrial depolarisation), QRS (ventricular depolarisation, systole begins), T (ventricular repolarisation, systole ends).

Work through the linked questions below to test how UPSC turns these figures and named structures into statement-based and matching traps.