Body Fluids and Circulation — Blood, the Heart, the Cardiac Cycle and the ECG
This chapter has four blocks and each is examined in its own way. Blood composition is recall — the formed elements, their counts and jobs, and the ABO/Rh grouping. The double circulation turns on a single idea most students can state but few can defend: human blood passes through the heart twice per full circuit, and oxygenated and deoxygenated blood never mix. The cardiac cycle is a matter of getting the order of valve openings and closings right. The ECG is worth a guaranteed mark if you memorise what each of the P, QRS and T waves represents. Everywhere the trap is the same — confusing which side of the heart carries oxygenated blood, and which vessel does. Fix the plumbing first and the rest follows.
The Heart and Double Circulation
- The human heart is four-chambered — two atria, two ventricles — enclosed in the pericardium. The right side carries deoxygenated blood, the left carries oxygenated blood, and the muscular septum keeps them completely separate.
- Valves prevent backflow: the tricuspid (right AV), the bicuspid/mitral (left AV), and the semilunar valves at the roots of the pulmonary artery and aorta. The AV valves close at the start of ventricular contraction; the semilunar valves close at its end.
- The heartbeat is myogenic — it originates in the heart muscle itself, at the sino-atrial node (SA node), the pacemaker. The impulse spreads to the AV node, then along the bundle of His and Purkinje fibres to the ventricles.
⚠️ The pulmonary artery carries deoxygenated blood — the classic reversal
The rule "arteries carry oxygenated blood, veins carry deoxygenated" is true almost everywhere and false in exactly two places, and NEET lives in those two places. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the pulmonary veins carry oxygenated blood from the lungs back to the left atrium. The safe definition is by direction, not oxygen content: an artery carries blood away from the heart and a vein carries blood toward it, whatever the blood is carrying. This is also the whole logic of double circulation — blood makes two loops, a pulmonary circuit (heart → lungs → heart) and a systemic circuit (heart → body → heart), passing through the heart twice in one complete journey, and the two never mixing. A single-circuit fish heart, by contrast, sends blood through the heart only once per circuit.The Cardiac Cycle and the ECG
| Wave | Represents | Note |
|---|---|---|
| P wave | Atrial depolarisation (atria contract) | Small; precedes atrial systole |
| QRS complex | Ventricular depolarisation (ventricles contract) | Largest; atrial repolarisation is hidden within it |
| T wave | Ventricular repolarisation (ventricles relax) | End of systole; return to resting state |
ℹ️ The cardiac cycle in one pass, and where the two heart sounds come from
One cardiac cycle, lasting about 0.8 second at 72 beats per minute, runs in a fixed order. Blood fills the relaxed heart; then atrial systole (0.1 s) tops up the ventricles; then ventricular systole (0.3 s) begins — the AV valves snap shut, pressure builds, and when it exceeds the arterial pressure the semilunar valves open and blood is ejected; then ventricular diastole (0.4 s) — the semilunar valves close and the chambers refill. The stroke volume (~70 mL) times the heart rate (~72) gives the cardiac output of about 5 litres per minute. The two heart sounds you can hear are the valves closing: the first, "lubb", is the AV valves closing at the start of ventricular systole; the second, "dupp", is the semilunar valves closing at its end. Matching each sound to its valve is a favourite one-mark question.Key takeaways
- Plasma (~55%) + formed elements (~45%); serum is plasma minus clotting factors; albumin does osmotic balance, fibrinogen clots.
- Mammalian RBCs are biconcave and enucleate; neutrophils are the most numerous WBC; platelets enable clotting.
- O is universal donor (no antigen); AB is universal recipient (no antibody); Rh mismatch → erythroblastosis foetalis.
- Right heart = deoxygenated, left heart = oxygenated. Pulmonary ARTERY carries deoxygenated blood; pulmonary VEIN oxygenated.
- Heartbeat is myogenic; the SA node is the pacemaker; impulse: SA → AV → bundle of His → Purkinje fibres.
- ECG: P = atrial depolarisation, QRS = ventricular depolarisation, T = ventricular repolarisation.
- First sound "lubb" = AV valves closing; second sound "dupp" = semilunar valves closing.
- Cardiac output = stroke volume (~70 mL) × heart rate (~72) ≈ 5 L/min; one cycle ≈ 0.8 s.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Which blood vessel carries deoxygenated blood despite being an artery?
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Answer: A — Pulmonary artery
The pulmonary artery takes deoxygenated blood from the right ventricle to the lungs, one of the two exceptions to the usual rule; the pulmonary vein is the matching oxygenated exception. The aorta, carotid and renal arteries all carry oxygenated blood in the normal way.The QRS complex in an ECG corresponds to:
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Answer: A — Depolarisation of the ventricles
The QRS complex records ventricular depolarisation, when the ventricles contract; atrial repolarisation is masked within it. The P wave is atrial depolarisation and the T wave is ventricular repolarisation, so those options belong to the other two waves.Blood group O is called the universal donor because the red cells:
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Answer: A — Carry neither A nor B antigen
With no A or B antigen on its cells, group O blood offers nothing for a recipient's antibodies to attack, so it can be given to any ABO group. Having no plasma antibodies describes AB, the universal recipient — the complementary property this question deliberately sets against the right answer.The pacemaker of the human heart is the:
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Answer: A — Sino-atrial (SA) node
The SA node in the wall of the right atrium initiates each heartbeat and sets the rhythm, which is why it is called the pacemaker; the impulse then passes to the AV node and onward. The AV node, bundle of His and Purkinje fibres relay the impulse but do not set the pace.The first heart sound "lubb" is produced by the:
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Answer: A — Closure of the atrioventricular valves at the start of ventricular systole
The AV valves (tricuspid and bicuspid) snap shut as the ventricles begin to contract, making the "lubb"; the semilunar valves closing at the end of systole make the "dupp". Valve opening is silent, so the third option cannot be a heart sound.Which plasma protein is chiefly responsible for maintaining the osmotic balance of blood?
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Answer: A — Albumin
Albumin, the most abundant plasma protein, maintains the colloid osmotic (oncotic) pressure that keeps fluid in the vessels. Fibrinogen and prothrombin are clotting proteins and globulins act mainly in immunity, so each of those has a different primary role.Double circulation in mammals means that blood:
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Answer: A — Passes through the heart twice in one complete circuit, kept fully separated
Blood completes a pulmonary loop and a systemic loop, passing through the heart twice per full journey, and the four-chambered heart keeps oxygenated and deoxygenated blood entirely apart. A single pass per circuit describes the fish heart, and mixing in the ventricle describes an incompletely divided heart, not the mammalian one.The most numerous type of white blood cell, and the first to reach a site of infection, is the:
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Answer: A — Neutrophil
Neutrophils make up the largest fraction of WBCs (about 60–65%) and are the first phagocytes to reach infected tissue. Basophils are the rarest, lymphocytes drive the specific immune response, and monocytes are the largest WBC but not the most numerous.