Body Fluids and Circulation (NEET Biology Class 11): Blood, Blood Groups, Cardiac Cycle & ECG
Blood: Plasma & the Formed Elements
📌 NEET priority: Important. 2–4 questions a year, and nearly all of them are exact numbers or table lookups — the WBC percentages, the ABO blood group table, the cardiac cycle values and the ECG waves. Learn the four tables below cold.
Blood — plasma and formed elements
Plasma is a straw-coloured viscous fluid making up ~55% of blood. It is 90–92% water and 6–8% proteins — the major ones being fibrinogen (clotting), globulins (body defence) and albumins (osmotic balance). Plasma WITHOUT clotting factors is called serum.
Formed elements (erythrocytes, leucocytes, platelets) make up ~45% of blood.
Formed element | Count / Value | Key facts |
Erythrocytes (RBC) | 5–5.5 million per mm³ | Most abundant blood cell. Formed in RED BONE MARROW in adults. Devoid of nucleus in most mammals, biconcave. Life span 120 days, destroyed in the SPLEEN — the 'graveyard of RBCs'. Haemoglobin: 12–16 g per 100 mL blood |
Leucocytes (WBC) | 6000–8000 per mm³ | Colourless (no haemoglobin), nucleated, generally short-lived. Two categories: GRANULOCYTES (neutrophils, eosinophils, basophils) and AGRANULOCYTES (lymphocytes, monocytes) |
Platelets (thrombocytes) | 1,50,000–3,50,000 per mm³ | Cell fragments from MEGAKARYOCYTES in bone marrow. Release factors for coagulation. Low count → clotting disorders and excessive blood loss |
The five WBC types — percentages NEET asks directly
WBC type | Percentage of total WBCs | Function |
Neutrophil (granulocyte) | 60–65% — MOST abundant | Phagocytic — destroys foreign organisms |
Lymphocyte (agranulocyte) | 20–25% | Two types, B and T — both responsible for immune responses |
Monocyte (agranulocyte) | 6–8% | Phagocytic |
Eosinophil (granulocyte) | 2–3% | Resists infections; associated with allergic reactions |
Basophil (granulocyte) | 0.5–1% — LEAST abundant | Secretes histamine, serotonin, heparin; involved in inflammatory reactions |
Blood Groups, Coagulation & Lymph
ABO blood grouping & donor compatibility
Blood group | Antigens on RBC | Antibodies in plasma | Can receive from |
A | A | anti-B | A, O |
B | B | anti-A | B, O |
AB | A and B | nil | AB, A, B, O — UNIVERSAL RECIPIENT |
O | nil | anti-A and anti-B | O only — UNIVERSAL DONOR |
Rh grouping and erythroblastosis foetalis
The Rh antigen (named after Rhesus monkeys) is present on RBCs of about 80% of humans — they are Rh positive; those lacking it are Rh negative.
Erythroblastosis foetalis: an Rh⁻ mother carrying an Rh⁺ foetus. In the FIRST pregnancy the placenta keeps the two bloods separate, so nothing happens. But during delivery of the first child, maternal blood may be exposed to foetal Rh⁺ blood, and the mother starts making anti-Rh antibodies. In subsequent pregnancies those antibodies leak into the foetus and destroy foetal RBCs — potentially fatal, or causing severe anaemia and jaundice. Prevented by giving the mother anti-Rh antibodies immediately after the first delivery.
Coagulation of blood — the cascade
A clot is a network of fibrin threads trapping dead and damaged formed elements. The cascade runs backwards from what you might expect:
Prothrombin (inactive, in plasma) —[thrombokinase enzyme complex]→ thrombin; then fibrinogen (inactive) —[thrombin]→ fibrin (the clot). Thrombokinase itself is formed by a cascade of linked enzymic reactions involving inactive plasma factors, triggered by platelets and by factors released by injured tissue. Calcium ions play a very important role in clotting.
Lymph (tissue fluid)
As blood passes through tissue capillaries, water and small solutes leak out — leaving larger proteins and formed elements behind — forming interstitial/tissue fluid, with the same mineral distribution as plasma. Collected by the lymphatic system and drained back into major veins, it is called lymph: a colourless fluid containing specialised lymphocytes. Fats are absorbed through lymph in the LACTEALS of the intestinal villi.
The Human Heart & the Cardiac Cycle
Circulatory patterns & hearts across animals
Open circulation — arthropods and molluscs; blood enters open sinuses. Closed circulation — annelids and chordates; blood stays in vessels, allowing more precise regulation.
Group | Heart chambers | Type of circulation |
Fishes | 2 (one atrium, one ventricle) | SINGLE circulation — heart pumps deoxygenated blood to gills |
Amphibians & reptiles (except crocodiles) | 3 (two atria, one ventricle) | INCOMPLETE double circulation — blood mixes in the single ventricle |
Crocodiles, birds, mammals | 4 (two atria, two ventricles) | COMPLETE double circulation — no mixing |
The human heart
Mesodermally derived, in the thoracic cavity between the lungs, tilted slightly left, the size of a clenched fist, protected by a double-walled pericardium containing pericardial fluid.
Valves: right atrium→right ventricle is guarded by the tricuspid valve (3 cusps); left atrium→left ventricle by the bicuspid (mitral) valve; both ventricular exits by semilunar valves. All valves allow flow in one direction only. Ventricle walls are much thicker than atrial walls.
Nodal tissue: the sino-atrial node (SAN) in the right upper corner of the right atrium; the atrio-ventricular node (AVN) in the lower left corner of the right atrium; the AV bundle (bundle of His) descends and branches into Purkinje fibres. Nodal tissue is autoexcitable — it generates action potentials without external stimuli.
The SAN generates the MAXIMUM action potentials, 70–75 per minute, so it initiates and maintains the heartbeat — hence it is called the pacemaker. The heart is therefore myogenic (auto-regulated by muscle, not nerve).
The cardiac cycle — the numbers
Quantity | Value | Note |
Heart rate | 70–75 per minute (average 72) | Set by the SAN |
Duration of one cardiac cycle | 0.8 seconds | Derived from 72 beats/minute |
Stroke volume | ~70 mL | Blood pumped by each ventricle per cardiac cycle |
Cardiac output | ~5000 mL (5 litres) per minute | Stroke volume × heart rate. Much higher in an athlete |
First heart sound (LUB) | — | Closure of the tricuspid and bicuspid valves |
Second heart sound (DUB) | — | Closure of the semilunar valves |
Atrial systole increases blood flow into the ventricles by about 30 per cent.
ECG, Double Circulation & Why This Matters for NEET
Electrocardiogram (ECG) — what each wave means
Wave | Represents | Note |
P-wave | Electrical excitation (DEPOLARISATION) of the ATRIA | Leads to contraction of both atria |
QRS complex | DEPOLARISATION of the VENTRICLES | Contraction starts shortly after Q — marks the BEGINNING of systole |
T-wave | REPOLARISATION of the ventricles (return to normal state) | End of the T-wave marks the END of systole |
Counting the QRS complexes in a given time gives the heart rate. Since ECGs have roughly the same shape across individuals, any deviation indicates possible abnormality — which is why ECG has great clinical significance.
Blood vessels & double circulation
Arteries and veins both have three layers: inner tunica intima (squamous endothelium), middle tunica media (smooth muscle + elastic fibres), outer tunica externa (fibrous connective tissue with collagen). The tunica media is comparatively THIN in veins.
Pulmonary circulation: right ventricle → pulmonary artery (deoxygenated) → lungs → pulmonary veins (oxygenated) → left atrium. Systemic circulation: left ventricle → aorta → arteries/arterioles/capillaries → tissues → venules/veins/vena cava → right atrium.
The hepatic portal system is a unique vascular connection: the hepatic portal vein carries blood from the intestine to the LIVER before it reaches systemic circulation. A separate coronary system serves the cardiac muscle itself.
Regulation of cardiac activity
The heart is myogenic — auto-regulated by nodal tissue. A neural centre in the medulla oblongata moderates it via the autonomic nervous system: sympathetic signals INCREASE heart rate, contraction strength and cardiac output; parasympathetic signals DECREASE them. Adrenal medullary hormones also increase cardiac output.
Why this matters for NEET
High-value one-liners: RBC = 5–5.5 million/mm³, life span 120 days; neutrophils 60–65% (most), basophils 0.5–1% (least); O = universal donor, AB = universal recipient; SAN = pacemaker; cardiac cycle = 0.8 s; stroke volume 70 mL; cardiac output 5 L/min; lub = AV valves closing, dub = semilunar valves closing.
Trap: the tricuspid valve is on the RIGHT and the bicuspid on the LEFT — remembering "tri = right" saves the mark. Also serum = plasma MINUS clotting factors, not plasma minus cells. And the QRS complex marks the BEGINNING of systole while the T-wave marks its END — the T-wave is repolarisation, not contraction.
Test Yourself: MCQs, PYQs & Active Recall
Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.
Practice Questions
Q1. Plasma constitutes approximately what percentage of blood?
(a) 70 per cent
(b) 90 per cent
(c) 55 per cent
(d) 45 per cent
Show answer
Answer: (c) — Plasma is about 55 per cent of blood; formed elements make up the remaining 45 per cent.
Q2. Plasma without clotting factors is called:
(a) Interstitial fluid
(b) Haemolymph
(c) Serum
(d) Lymph
Show answer
Answer: (c) — Serum is plasma from which the clotting factors have been removed.
Q3. The plasma protein responsible for osmotic balance is:
(a) Thrombin
(b) Fibrinogen
(c) Globulin
(d) Albumin
Show answer
Answer: (d) — Albumins help maintain osmotic balance; fibrinogen is for clotting and globulins for body defence.
Q4. A healthy adult man has how many RBCs per mm3 of blood?
(a) 12-16 million
(b) 5-5.5 million
(c) 5,000-8,000
(d) 1.5-3.5 lakh
Show answer
Answer: (b) — RBC count is 5 to 5.5 million per mm3; WBCs are 6000-8000 and platelets 1.5-3.5 lakh per mm3.
Q5. The average life span of an RBC and its site of destruction are:
(a) 365 days, spleen
(b) 120 days, spleen
(c) 20 days, liver
(d) 120 days, bone marrow
Show answer
Answer: (b) — RBCs live about 120 days and are destroyed in the spleen, called the graveyard of RBCs.
Q6. The most abundant type of WBC is:
(a) Monocyte
(b) Neutrophil
(c) Basophil
(d) Lymphocyte
Show answer
Answer: (b) — Neutrophils are 60-65 per cent of total WBCs — the most abundant; basophils at 0.5-1 per cent are least.
Q7. Which WBC secretes histamine, serotonin and heparin?
(a) Lymphocyte
(b) Eosinophil
(c) Basophil
(d) Neutrophil
Show answer
Answer: (c) — Basophils secrete histamine, serotonin and heparin, and are involved in inflammatory reactions.
Q8. Platelets are produced from:
(a) Lymph nodes
(b) Megakaryocytes in bone marrow
(c) Erythroblasts
(d) The spleen
Show answer
Answer: (b) — Platelets (thrombocytes) are cell fragments produced from megakaryocytes, special cells in the bone marrow.
Q9. A person with blood group AB has:
(a) Anti-A and anti-B antibodies
(b) Only anti-A
(c) No antigens on RBCs
(d) No antibodies in plasma
Show answer
Answer: (d) — Group AB has both A and B antigens on RBCs and NO antibodies in plasma — hence universal recipient.
Q10. Erythroblastosis foetalis occurs when:
(a) Both parents are Rh-
(b) The mother has blood group O
(c) An Rh- mother carries an Rh+ foetus in a subsequent pregnancy
(d) An Rh+ mother carries an Rh- foetus
Show answer
Answer: (c) — An Rh- mother sensitised during the first delivery produces anti-Rh antibodies that attack the RBCs of an Rh+ foetus in later pregnancies.
Q11. Fibrinogen is converted to fibrin by the enzyme:
(a) Carbonic anhydrase
(b) Thrombin
(c) Prothrombin
(d) Thrombokinase
Show answer
Answer: (b) — Thrombin converts inactive fibrinogen into fibrin; thrombin itself is formed from prothrombin by thrombokinase.
Q12. Fats are absorbed into the lymph through the:
(a) Alveoli
(b) Capillaries of the villi
(c) Hepatic portal vein
(d) Lacteals present in the intestinal villi
Show answer
Answer: (d) — Fats are absorbed through lymph in the lacteals of the intestinal villi.
Q13. Which group of animals has a 3-chambered heart with incomplete double circulation?
(a) Amphibians and most reptiles
(b) Birds
(c) Fishes
(d) Mammals
Show answer
Answer: (a) — Amphibians and reptiles (except crocodiles) have two atria and one ventricle, so blood mixes — incomplete double circulation.
Q14. The pacemaker of the human heart is the:
(a) Sino-atrial node
(b) Bundle of His
(c) Atrio-ventricular node
(d) Purkinje fibres
Show answer
Answer: (a) — The SAN generates the maximum action potentials (70-75/min) and initiates the heartbeat — hence the pacemaker.
Q15. The duration of one cardiac cycle is:
(a) 1.0 second
(b) 0.5 seconds
(c) 0.8 seconds
(d) 1.5 seconds
Show answer
Answer: (c) — At 72 beats per minute, each cardiac cycle lasts 0.8 seconds.
Q16. Cardiac output in a healthy individual averages:
(a) 70 mL per minute
(b) 500 mL per minute
(c) 5000 mL per minute
(d) 10,000 mL per minute
Show answer
Answer: (c) — Cardiac output = stroke volume (70 mL) x heart rate (72), averaging about 5000 mL or 5 litres per minute.
Q17. The first heart sound 'lub' is associated with:
(a) Closure of semilunar valves
(b) Closure of tricuspid and bicuspid valves
(c) Opening of the aorta
(d) Atrial depolarisation
Show answer
Answer: (b) — The first sound (lub) is the closure of the tricuspid and bicuspid valves; 'dub' is closure of the semilunar valves.
Q18. In an ECG, the QRS complex represents:
(a) Ventricular depolarisation
(b) Ventricular repolarisation
(c) Atrial repolarisation
(d) Atrial depolarisation
Show answer
Answer: (a) — The QRS complex is ventricular depolarisation; contraction starts shortly after Q, marking the beginning of systole.
NEET Previous Year Questions (PYQs)
Real NEET previous-year questions on this chapter, with explanations in our own words.
Q19. The T-wave of an ECG represents: (NEET PYQ)
(a) Depolarisation of atria
(b) Repolarisation of ventricles
(c) Depolarisation of ventricles
(d) Closure of semilunar valves
Show answer
Answer: (b) — The T-wave is the return of ventricles from an excited to a normal state (repolarisation); its end marks the end of systole.
Q20. Which blood group is known as the universal donor? (NEET PYQ)
(a) O
(b) AB
(c) A
(d) B
Show answer
Answer: (a) — Group O has no antigens on its RBCs, so it can be donated to any other group — the universal donor.
Q21. The middle layer of an artery wall, made of smooth muscle and elastic fibres, is the: (NEET PYQ)
(a) Endothelium
(b) Tunica media
(c) Tunica intima
(d) Tunica externa
Show answer
Answer: (b) — The tunica media is the middle layer; it is comparatively thin in veins compared to arteries.
Q22. The hepatic portal vein carries blood from the: (NEET PYQ)
(a) Heart to the intestine
(b) Intestine to the liver
(c) Liver to the heart
(d) Kidney to the liver
Show answer
Answer: (b) — The hepatic portal vein carries blood from the intestine to the liver before it is delivered to systemic circulation.
Q23. Which ion plays a very important role in blood clotting? (NEET PYQ)
(a) Sodium
(b) Potassium
(c) Magnesium
(d) Calcium
Show answer
Answer: (d) — Calcium ions play a very important role in the coagulation cascade.
Q24. Sympathetic neural signals to the heart cause: (NEET PYQ)
(a) A decrease in heart rate and cardiac output
(b) An increase in heart rate, contraction strength and cardiac output
(c) No change
(d) Only a change in conduction speed
Show answer
Answer: (b) — Sympathetic signals increase heart rate, strength of ventricular contraction and cardiac output; parasympathetic signals decrease them.
Q25. Approximately what percentage of humans are Rh positive? (NEET PYQ)
(a) 80 per cent
(b) 99 per cent
(c) 20 per cent
(d) 50 per cent
Show answer
Answer: (a) — The Rh antigen is present on the RBCs of nearly 80 per cent of humans.
Q26. The valve guarding the opening between the left atrium and left ventricle is the: (NEET PYQ)
(a) Tricuspid valve
(b) Semilunar valve
(c) Bicuspid (mitral) valve
(d) Pulmonary valve
Show answer
Answer: (c) — The bicuspid or mitral valve guards the left atrio-ventricular opening; the tricuspid guards the right.
Active Recall Prompt
Write everything you can recall about Body Fluids and Circulation, naming each part first: blood (plasma composition, the proteins, serum) and the formed elements (RBC, WBC and platelet counts, the five WBC types with their percentages and functions); ABO and Rh grouping (antigens, antibodies, donor compatibility, erythroblastosis foetalis); coagulation (the fibrinogen/thrombin/prothrombin cascade); lymph; circulatory patterns across animals; the human heart (chambers, valves, nodal tissue, SAN as pacemaker); the cardiac cycle with all its numbers; the ECG waves; double circulation and blood vessel layers; and regulation of cardiac activity. Begin each fact with its topic and end it with a full stop.