Body Fluids and Circulation — NEET Notes

Blood — a fluid connective tissue

Blood is classified as a connective tissue because it shares the defining property of that tissue class: cells (the formed elements) suspended in an extracellular matrix (the plasma). The matrix is unusual only in being liquid. By volume, plasma makes up about 55 per cent of whole blood and the formed elements about 45 per cent. An adult human carries roughly five litres of this fluid in continuous circulation, completing a full circuit through the body in under a minute at rest.

Plasma — the matrix

Plasma is a straw-coloured, viscous fluid. Of its composition, 90 to 92 per cent is water, while proteins account for 6 to 8 per cent. Three families of plasma proteins do the heavy work, and NEET has tested their roles in a column-match almost verbatim.

Plasma also carries small amounts of ions — Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻ — and the metabolic traffic of the body: glucose, amino acids, lipids, hormones, urea. Coagulation factors travel through plasma in inactive form, ready to be triggered. Plasma minus its clotting factors is called serum.

Formed elements — RBC, WBC, platelets

The 45 per cent cellular fraction of blood is made of three classes of formed elements: erythrocytes (RBC), leucocytes (WBC), and platelets (thrombocytes).

Erythrocytes (RBC)

Red blood cells are the most abundant cells in blood. A healthy adult man carries 5.0 to 5.5 million RBCs per cubic millimetre of blood. They are formed in the red bone marrow, lack a nucleus in mature mammalian form, and are biconcave in shape — features that maximise the haemoglobin packing inside each cell. Haemoglobin, the iron-containing pigment that gives blood its colour, is present at 12 to 16 g per 100 mL of blood and binds the respiratory gases. Each RBC lives roughly 120 days before being destroyed in the spleen — which is therefore called the graveyard of erythrocytes. NEET 2017 asked why mature RBCs are enucleate: the answer is that losing the nucleus frees up internal space for haemoglobin, maximising oxygen-carrying capacity.

Leucocytes (WBC)

White blood cells are nucleated, colourless cells of the immune defence. Their count averages 6,000 to 8,000 per mm³ of blood — far fewer than RBCs. WBCs split into two classes by the presence or absence of cytoplasmic granules: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). The five percentages are PYQ gold and must be memorised.

Platelets (thrombocytes)

Platelets are cell fragments derived from megakaryocytes in the bone marrow. Blood normally carries 1,50,000 to 4,50,000 platelets per mm³. They release the factors that trigger blood clotting; a sharp drop in their number — thrombocytopenia — causes uncontrolled bleeding.

Blood groups — ABO & Rh

Blood looks the same across humans, but the surface of every RBC carries protein-sugar markers — antigens — that the immune system uses to distinguish "self" from "non-self." Two such antigen systems matter clinically: the ABO system (Karl Landsteiner, 1900) and the Rh system.

ABO grouping

The ABO system rests on two surface antigens — A and B — and the antibodies that the plasma carries against them. The natural rule: an individual produces antibodies against whichever antigens are absent from their own RBCs. A group-A person carries anti-B antibodies; a group-B person carries anti-A; group-AB carries neither; group-O carries both.

Rh grouping

A separate antigen — the Rh antigen, first detected on the RBCs of the Rhesus monkey — sits on the RBC surface of roughly 80 per cent of humans (Rh⁺). The remaining 20 per cent are Rh⁻. The clinical worry is erythroblastosis foetalis: an Rh⁻ mother carrying an Rh⁺ foetus is exposed to foetal blood during delivery of her first child and produces anti-Rh antibodies. In subsequent pregnancies, those antibodies cross the placenta and destroy the foetal RBCs — causing severe anaemia, jaundice, and sometimes foetal death. The condition is prevented by administering anti-Rh immunoglobulin (RhoGAM) to the mother immediately after the first delivery.

Coagulation of blood

When a vessel is cut, three rapid events stop the bleeding: vasoconstriction narrows the vessel; platelets aggregate at the breach to form a platelet plug; and a fibrin meshwork stabilises that plug into a clot. The mature clot is a network of fibrin threads that traps RBCs, platelets, and damaged formed elements — visible as the reddish-brown scab.

Lymph — the second circulation

As blood flows through tissue capillaries, water and small solutes filter out of the capillary wall into the spaces between cells. This filtrate — interstitial fluid or tissue fluid — has the same mineral composition as plasma but is largely protein-free, because the proteins are too big to leave the capillary easily. All exchange of gases, nutrients, and wastes between blood and cells happens through this fluid. A separate network of vessels — the lymphatic system — collects the tissue fluid and drains it back into the great veins near the heart. The fluid inside the lymphatic vessels is the lymph.

Lymph is colourless and carries specialised lymphocytes — the cellular agents of the immune response. It also carries nutrients, hormones, and especially absorbed fats. Fats from the small intestine are absorbed not into blood capillaries but into specialised lymph capillaries called lacteals in the intestinal villi. The fats are packaged as chylomicrons, ferried by lymph, and only later released into the blood at the junction near the subclavian vein. NEET 2022 Q.175 tested this exact pathway.

Circulatory pathways — open vs closed, single vs double

Animal kingdoms have evolved two architectures for moving blood, and then layered a second classification on top of one of them.

All vertebrates have closed circulation, but the heart chamber count — and therefore the routing — differs. Fishes have a two-chambered heart (one atrium, one ventricle) and a single circulation: blood passes through the heart once per body circuit. Amphibians and most reptiles (crocodiles excepted) have a three-chambered heart (two atria, one ventricle) with incomplete double circulation — oxygenated and deoxygenated blood partially mix in the single ventricle. Crocodiles, birds, and mammals have a four-chambered heart and complete double circulation — no mixing.

Human heart — anatomy

The human heart is a mesodermally derived muscular organ the size of a clenched fist, lodged in the thoracic cavity between the two lungs and tilted slightly to the left. It is enclosed in a double-walled membranous bag — the pericardium — that contains the pericardial fluid, which lubricates the beating heart and dampens friction with surrounding tissue.

Four chambers, three septa

The heart has four chambers: two small upper atria (right and left) and two larger, thicker-walled lower ventricles (right and left). Three septa divide them: the thin inter-atrial septum separates the atria, the thick inter-ventricular septum separates the ventricles, and the fibrous atrio-ventricular septum separates each atrium from its own ventricle. The septa carry openings — each guarded by a one-way valve.

Four valves

Four valves enforce unidirectional flow through the heart. NEET 2018 Q.170 tested all four in a column-match — the configuration appears on the exam often enough that it must be locked in memory.

Anchoring the atrio-ventricular valve cusps to the ventricular wall are tough fibrous cords — the chordae tendineae — attached to muscular projections from the ventricular floor called the papillary muscles. When the ventricle contracts, the papillary muscles tighten the chordae and prevent the valve cusps from being blown back into the atrium ("valve prolapse"). The whole heart is built of cardiac muscle, and the ventricular walls are far thicker than the atrial walls because the ventricles must generate enough pressure to drive blood through the entire pulmonary and systemic circuits.

Cardiac cycle & cardiac output

The cardiac cycle is the sequence of events that constitutes one full heartbeat — atrial systole, ventricular systole, joint diastole, all repeating. At rest, a healthy heart completes 72 cardiac cycles per minute, which gives each cycle a duration of 0.8 seconds.

The three phases

1. Joint diastole. All four chambers relaxed. Tricuspid and bicuspid valves are open; semilunar valves are closed. Blood flowing in from the venae cavae and pulmonary veins fills the atria and passively drops through into the ventricles. This passive filling accounts for about 70 per cent of ventricular filling.
2. Atrial systole. The SA node fires; both atria contract simultaneously. This squeezes the remaining 30 per cent of blood into the ventricles — atrial systole "tops up" the ventricles.
3. Ventricular systole. The impulse reaches the ventricles via the AV node and the bundle of His; both ventricles contract together. Ventricular pressure rises, slamming the tricuspid and bicuspid valves shut (the first heart sound, lub). Pressure climbs higher until it forces the semilunar valves open, ejecting blood into the pulmonary artery and aorta. The ventricles then relax, the semilunar valves close as backflow is attempted (the second heart sound, dub), and joint diastole resumes.

Lub — bicuspid + tricuspid close. Dub — semilunar valves close.

The two heart sounds, in order

Stroke volume & cardiac output

Each ventricle ejects approximately 70 mL of blood per beat — the stroke volume. Multiplied by the heart rate, this gives the cardiac output: the volume of blood pumped out by each ventricle per minute. NEET 2019 Q.40 asked this calculation directly.

Double circulation — pulmonary & systemic loops

The blood flows by a fixed route through two great loops, joined only at the heart. Each artery and vein is built of three concentric layers: the inner tunica intima (squamous endothelium), the middle tunica media (smooth muscle and elastic fibres, thicker in arteries), and the outer tunica externa (fibrous connective tissue with collagen).

Pulmonary circulation

The right ventricle pumps deoxygenated blood into the pulmonary artery, which carries it to the lungs. There, CO₂ is exchanged for O₂ across the alveolar wall. Oxygenated blood returns to the heart through the pulmonary veins, emptying into the left atrium. The whole loop is short, low-pressure, and concerned solely with re-oxygenation.

Systemic circulation

The left ventricle pumps oxygenated blood into the aorta, which branches through arteries, arterioles, and capillaries to every tissue. At the capillaries, O₂ and nutrients diffuse out and CO₂ and wastes diffuse in. Deoxygenated blood is collected by venules, veins, and finally the vena cava, which empties into the right atrium. A unique sub-circuit, the hepatic portal system, carries nutrient-rich blood from the intestine to the liver before it enters general circulation — NEET 2017 Q.119 tested this. A coronary circulation serves the heart muscle itself.

Regulation, conduction & ECG

The heart is myogenic — its contraction signal originates inside the cardiac muscle itself, in a specialised network called the nodal tissue. A frog's heart isolated from the body keeps beating because of this property (NEET 2017 Q.122). Insect hearts, by contrast, are neurogenic — they need a nerve impulse to initiate each beat.

The conduction pathway

Four structures form the cardiac conduction system; the impulse runs through them in a fixed order.

Although the heart is myogenic, it is not unregulated. A neural centre in the medulla oblongata modulates rate and force through the autonomic nervous system: sympathetic nerves speed the heart up and strengthen contraction; parasympathetic nerves (the vagus) slow it down. Adrenaline from the adrenal medulla mimics the sympathetic effect.

Electrocardiogram (ECG)

The wave of depolarisation that travels through the heart can be detected from electrodes on the skin. An electrocardiograph records this as the ECG — a graph with three named deflections that map precisely onto the events of the cardiac cycle. Three leads suffice for a standard ECG: one on each wrist and one on the left ankle.

Disorders of the circulatory system

Four chronic conditions account for most NEET-asked pathology in this chapter.

Hypertension

Normal blood pressure is 120/80 mmHg — 120 mmHg systolic (pumping pressure during ventricular systole) over 80 mmHg diastolic (resting pressure during ventricular diastole). Repeated readings of 140/90 or higher indicate hypertension. Sustained high pressure damages the heart muscle, the kidneys, the retina, and the brain — accelerating heart disease and stroke.

Coronary Artery Disease (CAD)

Also called atherosclerosis. Deposits of cholesterol, calcium, fat, and fibrous tissue ("plaques") accumulate inside the coronary arteries — the arteries that feed the heart muscle itself — narrowing the lumen and reducing blood flow to the cardiac muscle.

Angina pectoris

Acute chest pain that flares when the heart muscle is not getting enough oxygen. Triggered by exertion, stress, or any condition that increases cardiac demand against a narrowed coronary supply. More common in middle-aged and elderly adults but can occur at any age.

Heart failure

The state in which the heart cannot pump blood effectively enough to meet the body's needs. Often called congestive heart failure because fluid backs up into the lungs (pulmonary congestion). Heart failure is distinct from cardiac arrest (heart stops beating altogether) and from a heart attack (myocardial infarction — sudden cardiac muscle damage from an arterial blockage).