Body Fluids and Circulation

2.1 Core concepts

• All living cells need a continuous supply of nutrients, O₂ and other essential substances, and continuous removal of wastes; simple organisms (sponges, coelenterates) circulate environmental water, while complex organisms use special body fluids — blood (the most common body fluid in higher organisms, including humans) and lymph — to transport materials (NCERT §15 intro, p. 193).
• Blood is a fluid connective tissue consisting of a fluid matrix (plasma) and formed elements (NCERT §15.1, p. 193).
• Plasma is a straw-coloured, viscous fluid making up nearly 55% of the blood; 90–92% is water and 6–8% is proteins — fibrinogen (clotting), globulins (defense) and albumins (osmotic balance). Plasma also carries small amounts of minerals (Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻), glucose, amino acids, lipids in transit, and inactive clotting factors; plasma minus the clotting factors is called serum (NCERT §15.1.1, pp. 193–194).
• Formed elements — erythrocytes, leucocytes and platelets — collectively form ~45% of blood (NCERT §15.1.2, p. 194).
• Erythrocytes / RBCs are the most abundant blood cells: a healthy adult man has on average 5 to 5.5 million RBCs mm⁻³. RBCs are formed in the red bone marrow in adults, are devoid of nucleus in most mammals, and are biconcave in shape. They carry the iron-containing protein haemoglobin (12–16 g of haemoglobin per 100 mL of blood in a healthy individual) and play a significant role in transport of respiratory gases. RBCs have an average life span of 120 days and are destroyed in the spleen ("graveyard of RBCs") (NCERT §15.1.2, p. 194).
• Leucocytes / WBCs are colourless (lack haemoglobin), nucleated and relatively fewer — 6,000–8,000 WBCs mm⁻³ — and generally short-lived. Two main groups: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). Neutrophils (60–65%) and monocytes (6–8%) are phagocytic cells that destroy foreign organisms; basophils (0.5–1%, the least) secrete histamine, serotonin and heparin — involved in inflammatory reactions; eosinophils (2–3%) resist infections and are associated with allergic reactions; lymphocytes (20–25%) are of two types — 'B' and 'T' — and are responsible for immune responses (NCERT §15.1.2, pp. 194–195).
• Platelets / thrombocytes are cell fragments produced from megakaryocytes in the bone marrow; blood normally contains 1,50,000–3,50,000 platelets mm⁻³. They release substances most of which are involved in coagulation; reduction in number leads to clotting disorders and excessive blood loss (NCERT §15.1.2, p. 195).
• ABO blood grouping is based on the presence or absence of two surface antigens (A and B) on RBCs; plasma carries the corresponding natural antibodies. Four groups exist — A (antigen A, anti-B antibody, can donate to A and AB, receive from A and O), B (antigen B, anti-A antibody), AB (both antigens, no antibodies — universal recipient, accepts blood from all four), O (no antigens, both anti-A and anti-B antibodies — universal donor, can give to all four). Cross-matching is essential before transfusion to avoid RBC clumping (NCERT §15.1.3.1, p. 195, Table 15.1).
• Rh grouping — the Rh antigen (similar to the one present in Rhesus monkeys) is found on RBCs of about 80% of humans (Rh-positive); those without are Rh-negative. Rh− persons exposed to Rh+ blood produce anti-Rh antibodies, so Rh status must also be matched. A special case is Rh incompatibility during pregnancy: an Rh− mother carrying an Rh+ foetus is normally protected by the placenta in the first pregnancy, but small amounts of foetal Rh+ blood can leak into maternal circulation at delivery, prompting antibody formation; in subsequent pregnancies these maternal anti-Rh antibodies cross the placenta and destroy foetal RBCs — erythroblastosis foetalis (severe anaemia and jaundice, potentially fatal). It is prevented by administering anti-Rh antibodies to the mother right after the first delivery (NCERT §15.1.3.2, p. 196).
• Coagulation of blood: within seconds of injury, a clot or coagulum forms — a network of threads called fibrins in which dead and damaged formed elements are trapped. Fibrins are formed by the enzyme thrombin from inactive fibrinogens; thrombin in turn is formed from inactive prothrombin by an enzyme complex called thrombokinase, which is built by a cascade of factors released by injured tissue and platelets. Calcium ions play a critical role in clotting (NCERT §15.1.4, p. 196).
• Lymph (tissue fluid) — as blood flows through capillaries, water and small solutes leave for the intercellular spaces; this fluid (mineral composition same as plasma) is the interstitial / tissue fluid, collected back to major veins via the lymphatic system. Lymph is colourless, contains specialised lymphocytes (immune responses) and is the route by which fats are absorbed through the lacteals in intestinal villi (NCERT §15.2, p. 197).
• Circulatory patterns — Open (arthropods, molluscs — blood pumped through large vessels into open sinuses) vs Closed (annelids, chordates — closed network of vessels; flow more precisely regulated) (NCERT §15.3, p. 197).
• Vertebrate hearts — fishes have a 2-chambered heart (single circulation: heart pumps deoxygenated blood, gills oxygenate, body receives, returns); amphibians and reptiles (except crocodiles) have a 3-chambered heart (two atria + single ventricle → incomplete double circulation); crocodiles, birds and mammals have a 4-chambered heart → complete double circulation with two separate pathways (NCERT §15.3, p. 197).
• The human heart is mesodermal, the size of a clenched fist, situated in the thoracic cavity between the two lungs and tilted to the left; it is enclosed in a double-walled membranous bag, the pericardium, with pericardial fluid inside. Four chambers: two atria separated by the inter-atrial septum, and two ventricles separated by the inter-ventricular septum; atrium and ventricle of the same side are separated by the atrio-ventricular septum with an opening. The right AV opening has the tricuspid valve (3 cusps), the left has the bicuspid / mitral valve. Openings of right ventricle into pulmonary artery and left ventricle into aorta are guarded by semilunar valves. Valves allow one-way flow only — atria → ventricles → arteries (NCERT §15.3.1, pp. 198–199).
• Nodal tissue — specialised cardiac musculature; the sino-atrial node (SAN) in the upper right corner of the right atrium can generate the maximum number of action potentials (70–75 min⁻¹), is autoexcitable, and is therefore the pacemaker that initiates and maintains the heart's rhythmic activity. The atrio-ventricular node (AVN) in the lower left of the right atrium near the AV septum picks up the signal; from it the AV bundle / Bundle of His passes into the inter-ventricular septum and divides into right and left bundles, giving rise to Purkinje fibres throughout ventricular musculature. The heart normally beats ~70–75 (average 72) times per minute (NCERT §15.3.1, p. 199).
• Cardiac cycle: starts in joint diastole with all four chambers relaxed, AV valves open, semilunar valves closed — blood flows from venae cavae and pulmonary veins into atria and on into ventricles. SAN fires → simultaneous atrial systole (boosts ventricular filling by ~30%). Signal reaches AVN → AV bundle → ventricular muscle → ventricular systole, raising pressure → AV valves close ("lub" — first heart sound) → semilunar valves open and blood is ejected. Then ventricular diastole — semilunar valves close ("dub" — second heart sound) — and the cycle restarts. One cardiac cycle lasts 0.8 s (72 cycles/min). Each ventricle pumps ~70 mL of blood (stroke volume) per beat; cardiac output = SV × HR ≈ 5000 mL (5 L)/min in a healthy individual, higher in athletes (NCERT §15.3.2, pp. 199–200).
• ECG is a graphical record of the heart's electrical activity. The P-wave = atrial depolarisation (leads to atrial contraction); the QRS complex = ventricular depolarisation (start of ventricular systole, just after Q); the T-wave = ventricular repolarisation (return to normal — end of systole). Counting QRS complexes gives heart rate; any deviation in shape from the standard ECG signals possible disease (NCERT §15.3.3, p. 201).
• Blood vessels — three layers: tunica intima (inner squamous endothelium), tunica media (middle smooth muscle + elastic fibres — thinner in veins), tunica externa (outer fibrous connective tissue with collagen). Two circulatory pathways exist: pulmonary circulation — right ventricle → pulmonary artery (deoxygenated) → lungs → pulmonary vein (oxygenated) → left atrium; systemic circulation — left ventricle → aorta → arteries → arterioles → capillaries → tissues → venules → veins → vena cava → right atrium. A unique hepatic portal vein carries blood from intestine to liver before delivery to systemic circulation; the coronary system supplies blood exclusively to the heart muscle (NCERT §15.4, pp. 201–202).
• Regulation — the heart is myogenic (auto-regulated by nodal tissue). The medulla oblongata moderates cardiac function via the autonomic nervous system: sympathetic signals ↑ heart rate, strength of ventricular contraction and cardiac output; parasympathetic signals ↓ them. Adrenal medullary hormones (adrenaline, noradrenaline) can also ↑ cardiac output (NCERT §15.5, p. 202).
• Disorders: Hypertension = BP higher than normal 120/80 (systolic/diastolic mm Hg); repeated readings of 140/90 or higher indicate hypertension, which damages brain and kidney. Coronary Artery Disease (CAD) / atherosclerosis — deposits of calcium, fat, cholesterol and fibrous tissues narrow the coronary lumen. Angina (pectoris) — acute chest pain from insufficient O₂ to the heart muscle. Heart failure — heart cannot pump enough to meet body needs (congestion of lungs is a major symptom — congestive heart failure); not the same as cardiac arrest (heart stops) or heart attack (heart muscle damaged by inadequate blood supply) (NCERT §15.6, pp. 202–203).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 15.1 (p. 194) — Formed elements: RBC, platelets, eosinophil, basophil, neutrophil, monocyte, T lymphocyte, B lymphocyte.
• Table 15.1 (p. 195) — Blood Groups and Donor Compatibility (antigens on RBCs, antibodies in plasma, donor's group for A/B/AB/O). Memorise: O donates to all, AB accepts from all.
• Figure 15.2 (p. 198) — Section of human heart: aorta, vena cava, pulmonary artery, pulmonary veins, SAN, AVN, bundle of His, chordae tendineae, atria, ventricles, interventricular septum, apex.
• Figure 15.3 (p. 201) — Standard ECG showing P, Q, R, S, T waves.
• Figure 15.4 (p. 202) — Schematic plan of blood circulation: heart (RA/LA/RV/LV), pulmonary artery → lungs → pulmonary vein, dorsal aorta → body parts → vena cava; vessel cross-section (vein vs artery).
• Coagulation cascade flow (p. 196) — Injury → platelet/tissue factors → thrombokinase + Ca²⁺ → prothrombin → thrombin → fibrinogen → fibrin clot.
• Cardiac cycle sequence (pp. 199–200) — Joint diastole → atrial systole → ventricular systole (lub) → ventricular diastole (dub) → joint diastole.

2.4 Common confusions / NTA trap points

• RBC count vs WBC count vs platelet count — NTA frequently swaps the numbers. Lock them: RBC 5–5.5 million mm⁻³, WBC 6000–8000 mm⁻³, platelets 1,50,000–3,50,000 mm⁻³.
• Tricuspid vs bicuspid valves — tricuspid = right AV (right atrium ↔ right ventricle); bicuspid/mitral = left AV. Don't reverse.
• "Lub" vs "dub" — lub = AV valves closing (start of ventricular systole); dub = semilunar valves closing (end of ventricular systole).
• Universal donor (O) vs universal recipient (AB) — distractors often flip this. O has no antigens (gives to all); AB has no antibodies (receives from all).
• Plasma vs serum — serum = plasma minus clotting factors. Plasma proteins are not all clotting proteins — globulins (defense) and albumins (osmotic balance) are not clotting factors; only fibrinogen is.
• P-wave vs T-wave — P = atrial depolarisation (atrial contraction); T = ventricular repolarisation (ventricles returning to normal). QRS = ventricular depolarisation, not ventricular relaxation.
• Erythroblastosis foetalis occurs in second / subsequent pregnancy, not the first — the first delivery sensitises the Rh⁻ mother.
• Atherosclerosis cause — Ca/fat/cholesterol/fibrous deposits in coronaries, NOT excess fibrinogen.
• Heart failure ≠ heart attack ≠ cardiac arrest — three distinct conditions defined separately in §15.6.
• Lymph carries fats via lacteals — distractors often say it carries O₂; lymph has no RBCs.

2.5 Key counts, percentages and structures (NCERT-cited)