Chapter Overview & Weightage
Body Fluids and Circulation is a high-yield NEET chapter — 2-3 questions per paper across topics like cardiac cycle, blood components, ECG interpretation, and disorders.
| Year | NEET Qs |
|---|---|
| 2024 | 3 |
| 2023 | 2 |
| 2022 | 3 |
| 2021 | 2 |
Most questions are direct recall. The cardiac cycle and ECG questions can trip you up if you haven’t drilled the timing and waveforms.
Key Concepts You Must Know
- Blood composition: plasma (55%) + formed elements (45%). Plasma = water + proteins (albumin, globulins, fibrinogen) + salts.
- RBC: 4.5-5.5 million/mm³, no nucleus (mammals), 120-day lifespan, contains haemoglobin.
- WBC: 6000-8000/mm³. Granulocytes (neutrophils, eosinophils, basophils) + agranulocytes (lymphocytes, monocytes).
- Platelets: 1.5-3.5 lakh/mm³, role in clotting.
- Blood groups: ABO and Rh systems. Universal donor: O−. Universal recipient: AB+.
- Lymph: tissue fluid + WBCs (especially lymphocytes), no RBCs, less protein than blood.
- Heart structure: 4 chambers (2 atria, 2 ventricles). Bicuspid (left), tricuspid (right). Aortic and pulmonary semilunar valves.
- Cardiac cycle phases: atrial systole, ventricular systole, joint diastole. Total duration ~0.8 s at 72 bpm.
- Cardiac output: stroke volume × heart rate ≈ 70 mL × 72 = 5040 mL/min ≈ 5 L/min.
- ECG components: P (atrial depolarisation), QRS (ventricular depolarisation, masks atrial repolarisation), T (ventricular repolarisation).
- Double circulation: pulmonary (heart-lungs-heart) + systemic (heart-body-heart).
- Disorders: hypertension (>140/90 mmHg), CAD, angina, heart failure.
Important Frameworks
- Atrial systole: 0.1 s (atria contract, push blood to ventricles)
- Ventricular systole: 0.3 s (ventricles contract, push blood to arteries; AV valves close = first heart sound “lubb”)
- Joint diastole: 0.4 s (all chambers relax, ventricles fill passively; semilunar valves close = second heart sound “dub”)
- Normal: 120/80 mmHg (systolic/diastolic)
- Hypertension: > 140/90 mmHg sustained
- Hypotension: < 90/60 mmHg
Solved Previous Year Questions
PYQ 1 (NEET 2024)
The first heart sound (lubb) is produced by: (a) Closure of AV valves (b) Opening of AV valves (c) Closure of semilunar valves (d) Opening of semilunar valves
Solution: (a) Closure of AV valves at the start of ventricular systole. The “dub” sound is the closure of semilunar valves at the start of ventricular diastole.
PYQ 2 (NEET 2023)
Which of the following has the lowest oxygen content? (a) Pulmonary vein (b) Pulmonary artery (c) Aorta (d) Hepatic vein
Solution: (b) Pulmonary artery. It carries deoxygenated blood from heart to lungs. (Pulmonary vein, in contrast, carries oxygenated blood — opposite of typical artery/vein convention.)
PYQ 3 (NEET 2022)
The QRS complex of an ECG represents:
Solution: Ventricular depolarisation. (Atrial repolarisation also occurs at this time but is masked by the larger QRS signal.)
Difficulty Distribution
- Easy (40%): Blood composition, definitions, blood group compatibility.
- Medium (45%): Cardiac cycle phases, ECG component meaning, valve mechanics.
- Hard (15%): Pressure-volume curves, multi-step physiology questions linking cycle to clinical scenarios.
Expert Strategy
For ECG questions, memorise: P = atria, QRS = ventricles depolarising, T = ventricles repolarising. Atrial repolarisation has no separate wave (hidden in QRS). NEET tests this every year.
For blood-vessel oxygen content questions, the rule of thumb is “arteries carry oxygenated, veins carry deoxygenated” — except pulmonary vessels and umbilical vessels, where it’s reversed. The exceptions are favourite NEET trick questions.
Common Traps
Trap 1: Counting heart chambers wrong. Mammals (including humans): 4 chambers. Amphibians: 3 (2 atria, 1 ventricle). Fish: 2 (1 atrium, 1 ventricle). Reptiles: 3 incomplete (or 4 in crocodiles). NEET tests this for comparative anatomy.
Trap 2: Universal donor/recipient confusion. O− is the universal donor (no antigens to attack). AB+ is the universal recipient (no antibodies to attack). Don’t reverse.
Trap 3: Lymph vs blood plasma. Lymph has WBCs but no RBCs and less protein than plasma. Tissue fluid is similar to plasma but lacks plasma proteins. Read the question carefully — “lymph” and “interstitial fluid” are sometimes interchangeable but technically distinct.
NEET 2021 asked the order of phases: atrial systole → ventricular systole → joint diastole. Memorise this sequence and the durations (0.1 s + 0.3 s + 0.4 s = 0.8 s).