Body Fluids and Circulation
This chapter explains how blood and lymph transport nutrients, gases and wastes, and how the human heart circulates blood through a closed, double-circulation system.
NCERT biology is a staple of the Prelims general-science segment, with direct questions on blood groups and donor compatibility, WBC functions, the clotting cascade, vertebrate heart chambers, the pacemaker and cardiac output. In Mains it feeds GS-III Science & Technology themes (medical technology like artificial pacemakers, blood and biotech) and public-health debates such as safe blood transfusion. Factual precision, not depth, is what UPSC rewards here.
Understand the chapter
Blood: The Fluid Connective Tissue
Blood is a special connective tissue made of a fluid matrix called plasma and the formed elements. Plasma is a straw-coloured, viscous fluid forming about 55% of blood, of which 90-92% is water and 6-8% is protein. Plasma without clotting factors is called serum.
- Plasma proteins: fibrinogen (clotting), globulins (defense/immunity), albumins (osmotic balance)
- Plasma ~55%, formed elements ~45% of blood
- Carries minerals (Na+, Ca++, Mg++, HCO3-, Cl-) plus glucose, amino acids and lipids in transit
- Serum = plasma minus the clotting factors
Formed Elements: RBC, WBC and Platelets
Erythrocytes (RBC), leucocytes (WBC) and platelets together form ~45% of blood. RBCs are the most abundant cells, biconcave and enucleate in most mammals, and carry the iron-rich pigment haemoglobin. WBCs are nucleated, colourless and defensive, while platelets are clotting fragments.
- RBC: 5-5.5 million/mm3, made in red bone marrow, lifespan 120 days, destroyed in spleen (graveyard of RBCs); Hb 12-16 g per 100 ml
- WBC: 6000-8000/mm3; granulocytes (neutrophil, eosinophil, basophil) and agranulocytes (lymphocyte, monocyte)
- Neutrophils most abundant (60-65%), basophils least (0.5-1%); neutrophils and monocytes are phagocytic; B and T lymphocytes drive immunity
- Platelets (thrombocytes): 1.5-3.5 lakh/mm3, from megakaryocytes; low count causes clotting disorders
Blood Groups: ABO and Rh
ABO grouping rests on the presence or absence of surface antigens A and B on RBCs, matched against natural antibodies in plasma. Group O has no antigens (universal donor) and AB has no antibodies (universal recipient). A separate Rh antigen, named after the Rhesus monkey, is present in about 80% of people.
- A: antigen A, anti-B; B: antigen B, anti-A; AB: A and B, no antibody; O: no antigen, anti-A and anti-B
- O = universal donor; AB = universal recipient
- Rh positive ~80%; Rh negative lacks the antigen and must also be matched before transfusion
- Erythroblastosis foetalis: Rh-ve mother and Rh+ve foetus; harms the second/subsequent pregnancy; prevented by anti-Rh antibodies given after the first delivery
Coagulation of Blood
Clotting prevents excessive blood loss after injury. The clot is a meshwork of fibrin threads that traps dead and damaged blood cells. It forms through an enzyme cascade in which inactive plasma factors are sequentially activated, and calcium ions are indispensable throughout.
- Cascade: prothrombin -> thrombin (via enzyme complex thrombokinase); fibrinogen -> fibrin (via thrombin)
- Injured platelets and tissue factors trigger the cascade
- Calcium (Ca++) ions are essential for clotting
- Coagulum = fibrin network trapping formed elements
Lymph (Tissue Fluid)
As blood flows through tissue capillaries, water and small solutes seep into intercellular spaces, leaving large proteins and most formed elements behind; this is interstitial or tissue fluid. The lymphatic system collects it and drains it back into the major veins. Lymph is colourless and rich in lymphocytes.
- Same mineral distribution as plasma but lacks large proteins and most formed elements
- All exchange of nutrients and gases between blood and cells occurs through this fluid
- Carries nutrients and hormones; fats are absorbed via lacteals in intestinal villi
- Specialised lymphocytes in lymph mediate immune responses
Circulatory Pathways: Open vs Closed, Hearts and Circulation
Open circulation (arthropods, molluscs) empties blood into body sinuses, while closed circulation (annelids, chordates) keeps blood within vessels and allows more precise regulation. Vertebrate hearts differ in chamber number, which determines how thoroughly oxygenated and deoxygenated blood are separated.
- Open: arthropods and molluscs (sinuses); Closed: annelids and chordates
- Fish: 2-chambered, single circulation
- Amphibians and reptiles (except crocodile): 3-chambered, incomplete double circulation (mixed blood)
- Crocodiles, birds and mammals: 4-chambered, complete double circulation
Human Heart and the Cardiac Cycle
The mesodermal heart sits in the thoracic cavity, tilted left, is the size of a clenched fist and is enclosed in a double-walled pericardium. Its four chambers, guarded by one-way valves, are coordinated by autoexcitable nodal tissue. The SA node, the pacemaker, sets the rhythm, driving a 0.8-second cardiac cycle at about 72 beats per minute.
- Valves: tricuspid (right AV), bicuspid/mitral (left AV), semilunar (at pulmonary artery and aorta); all prevent backflow
- Conduction: SAN -> AVN -> AV bundle (bundle of His) -> Purkinje fibres; SAN fires 70-75/min
- Cardiac cycle = 0.8 s; heart rate ~72/min; stroke volume ~70 mL per ventricle
- Cardiac output = stroke volume x heart rate ~5000 mL (5 L)/min; first sound (lub) = closure of tricuspid and bicuspid valves
Key terms
- Plasma
- Straw-coloured fluid matrix of blood (~55%), 90-92% water, carrying proteins, minerals and nutrients.
- Serum
- Plasma from which the clotting factors have been removed.
- Haemoglobin
- Iron-containing red pigment in RBCs that transports respiratory gases; 12-16 g per 100 ml of blood.
- Universal donor
- O blood group, lacking A and B antigens, donatable to all ABO groups.
- Universal recipient
- AB blood group, lacking plasma antibodies, able to receive all ABO groups.
- Erythroblastosis foetalis
- Destruction of Rh+ve foetal RBCs by an Rh-ve mother's anti-Rh antibodies in a later pregnancy.
- Pacemaker (SAN)
- Sino-atrial nodal tissue in the right atrium that initiates the heartbeat at 70-75 impulses/min.
- Stroke volume
- Volume of blood pumped out by each ventricle per beat (~70 mL).
- Cardiac output
- Volume of blood pumped by each ventricle per minute (~5 L); stroke volume x heart rate.
- Lymph
- Colourless tissue fluid in the lymphatic system, rich in lymphocytes, that aids immunity and fat absorption.
Must-know facts exam-ready
- Plasma is ~55% of blood and formed elements ~45%
- Plasma proteins: fibrinogen (clotting), globulin (defense), albumin (osmotic balance); serum = plasma minus clotting factors
- RBC count 5-5.5 million/mm3, lifespan 120 days, destroyed in the spleen; Hb 12-16 g per 100 ml
- WBC 6000-8000/mm3; neutrophils most abundant (60-65%), basophils least (0.5-1%)
- Platelets 1.5-3.5 lakh/mm3, produced from megakaryocytes
- O is universal donor, AB is universal recipient
- Rh antigen present in ~80% of humans and is named after the Rhesus monkey
- Clotting cascade: prothrombin -> thrombin (thrombokinase) and fibrinogen -> fibrin (thrombin); Ca++ is essential
- Heart chambers: fish 2, amphibians/reptiles 3, crocodiles/birds/mammals 4
- SAN is the pacemaker firing 70-75/min; cardiac cycle lasts 0.8 s at 72 beats/min
- Stroke volume ~70 mL and cardiac output ~5 L/min
- First heart sound (lub) marks closure of the tricuspid and bicuspid valves
Memory tricks remember it for good
Traps to avoid
- O is the universal donor and AB the universal recipient; aspirants routinely reverse these.
- Tricuspid valve guards the RIGHT AV opening and bicuspid/mitral the LEFT; they are frequently swapped.
- Serum is plasma minus clotting factors only, not plasma minus all proteins.
- Crocodile is a reptile but has a 4-chambered heart; other reptiles and amphibians have 3 chambers.
- Erythroblastosis foetalis endangers the SECOND/subsequent pregnancy, not the first; mother is Rh-ve and foetus Rh+ve.
- Neutrophils (not lymphocytes) are the most abundant WBC, and the SAN pacemaker lies in the RIGHT atrium.
Exam focus
🧠 Prelims angles
- Blood group antigen-antibody combinations and donor compatibility (universal donor/recipient)
- Functions of plasma proteins and the WBC types (phagocytic vs immune)
- Vertebrate heart chamber count and single vs incomplete vs complete double circulation
- Nodal-tissue conduction sequence, identity of the pacemaker, cardiac cycle duration and output
- Clotting cascade components (fibrinogen, thrombin, prothrombin, Ca++) and role of platelets
- Rh incompatibility and erythroblastosis foetalis
✍️ Mains angles GS-III
- Significance of blood group matching for safe transfusion and public health.Tie ABO/Rh compatibility to blood-bank governance, NACO and safe-transfusion policy under GS-II/III health.
- How does complete double circulation make birds and mammals metabolically efficient?Contrast single, incomplete double and complete double circulation in terms of oxygen delivery and endothermy.
- The heart's autoexcitable conduction system and its clinical/technological relevance.Link the SAN pacemaker to artificial pacemaker technology as an applied Science & Technology theme.
Last-minute revision tick as you recall
- Blood = plasma (55%) + formed elements (45%); serum = plasma minus clotting factors
- Plasma proteins: Fibrinogen (clot), Globulin (defense), Albumin (osmotic)
- RBC 5-5.5 M/mm3, 120 days, spleen graveyard; Hb 12-16 g/100 ml
- WBC 6000-8000; order N>L>M>E>B; neutrophil most, basophil least
- O donates to all, AB receives from all; Rh+ ~80%, named from Rhesus monkey
- Erythroblastosis foetalis: Rh-ve mother, Rh+ve foetus, second pregnancy
- Clot: prothrombin -> thrombin -> fibrinogen -> fibrin; Ca++ essential
- Hearts: fish 2, amphibian/reptile 3, crocodile/bird/mammal 4; double circulation in birds and mammals
- SAN pacemaker 70-75/min; cycle 0.8 s, 72 bpm, SV ~70 mL, CO ~5 L/min
Distilled from NCERT Class 11 · Biology (Class 11) for UPSC. Always cross-check facts with the original NCERT.