Chapter Overview & Weightage
Body Fluids and Circulation covers blood composition, blood groups, coagulation, the heart, cardiac cycle, ECG, and disorders. The cardiac cycle and ECG interpretation are high-frequency NEET topics.
This chapter carries 3-4% weightage in NEET with 2-3 questions. Blood groups, cardiac cycle phases, and ECG wave components are tested most consistently.
| Year | NEET (Q count) | Key Topics Tested |
|---|---|---|
| 2024 | 3 | ABO blood groups, cardiac output, SA node |
| 2023 | 2 | ECG waves, blood coagulation pathway |
| 2022 | 2 | Double circulation, Rh factor |
Key Concepts You Must Know
Tier 1 (Core)
- Blood composition: plasma (55%) + formed elements (45% — RBCs, WBCs, platelets)
- ABO blood grouping: antigens (A, B) on RBC surface, antibodies (anti-A, anti-B) in plasma
- Rh factor: Rh+ has D antigen, Rh- lacks it. Erythroblastosis foetalis in Rh- mother with Rh+ baby
- Heart structure: 4 chambers, SA node (pacemaker), AV node, Bundle of His, Purkinje fibres
- Cardiac cycle: 0.8 sec total (atrial systole 0.1s, ventricular systole 0.3s, joint diastole 0.4s)
- Double circulation: pulmonary (heart ↔ lungs) + systemic (heart ↔ body)
Tier 2 (Frequently tested)
- ECG components: P wave (atrial depolarisation), QRS complex (ventricular depolarisation), T wave (ventricular repolarisation)
- Cardiac output = stroke volume heart rate (approx 5 L/min)
- Blood coagulation: platelet plug → fibrinogen → fibrin (needs thrombin, Ca)
- WBC types: neutrophils (60-65%), lymphocytes (20-25%), monocytes, eosinophils, basophils
Tier 3 (Occasionally tested)
- Portal system (hepatic portal vein)
- Disorders: hypertension, coronary artery disease, angina, heart failure
- Lymph: composition and function
Important Formulas
| Blood Group | Antigen on RBC | Antibody in Plasma | Can Donate To | Can Receive From |
|---|---|---|---|---|
| A | A | Anti-B | A, AB | A, O |
| B | B | Anti-A | B, AB | B, O |
| AB | A and B | None | AB only | A, B, AB, O (universal recipient) |
| O | None | Anti-A and Anti-B | A, B, AB, O (universal donor) | O only |
Total duration: 0.8 seconds (at 72 beats/min)
| Phase | Duration | Events |
|---|---|---|
| Atrial systole | 0.1 s | Atria contract, blood pushed into ventricles |
| Ventricular systole | 0.3 s | Ventricles contract, blood pushed into arteries |
| Joint diastole | 0.4 s | Both atria and ventricles relax, blood fills atria |
Cardiac output = Stroke volume Heart rate = 70 mL 72 = ~5040 mL/min ≈ 5 L/min
For ECG questions, remember: P = atria depolarise, QRS = ventricles depolarise (atria repolarise simultaneously but hidden), T = ventricles repolarise. The QRS complex is the tallest because ventricular muscle mass is the largest.
Solved Previous Year Questions
PYQ 1 — NEET 2024
Problem: The pacemaker of the heart is:
(A) AV node (B) SA node (C) Bundle of His (D) Purkinje fibres
Solution:
The SA node (sino-atrial node) is the pacemaker. It generates impulses at the highest rate (70-75 per minute), setting the rhythm for the entire heart. If SA node fails, AV node takes over at a slower rate (~40-60/min).
Answer: (B) SA node
PYQ 2 — NEET 2023
Problem: The
QRS complex in an ECG represents:
(A) Atrial depolarisation (B) Ventricular depolarisation (C) Atrial repolarisation (D) Ventricular repolarisation
Solution:
The QRS complex represents the spread of electrical impulse through the ventricles — ventricular depolarisation. This triggers ventricular contraction (systole). The T wave represents ventricular repolarisation.
Answer: (B) Ventricular depolarisation
PYQ 3 — NEET 2022
Problem: Erythroblastosis foetalis occurs when:
(A) Rh+ mother carries Rh- foetus (B) Rh- mother carries Rh+ foetus (first pregnancy) (C) Rh- mother carries Rh+ foetus (subsequent pregnancy) (D) Both parents are Rh+
Solution:
In the first pregnancy, an Rh- mother carrying an Rh+ baby gets sensitised (produces anti-Rh antibodies) during delivery when fetal blood enters maternal circulation. In subsequent pregnancies with an Rh+ baby, these antibodies cross the placenta and destroy fetal RBCs.
Answer: (C) Rh- mother carries Rh+ foetus (subsequent pregnancy)
The first pregnancy is usually safe — the mother only gets sensitised. Erythroblastosis foetalis occurs in the second or later pregnancy. This distinction is a NEET favourite. Also, Rh+ mother + Rh- baby = no problem at all.
Difficulty Distribution
| Difficulty | % of Questions | What to Expect |
|---|---|---|
| Easy | 45% | Blood group facts, SA node identification |
| Medium | 40% | ECG interpretation, cardiac cycle phases, Rh incompatibility |
| Hard | 15% | Coagulation cascade details, cardiac output calculation |
Expert Strategy
Day 1: Blood composition and blood groups. Make the ABO compatibility table from memory. Understand the Rh factor mechanism — why first pregnancy is safe, second is dangerous.
Day 2: Heart anatomy and cardiac cycle. Draw the heart, label all chambers, valves, and nodes. Trace the conduction pathway: SA node → AV node → Bundle of His → bundle branches → Purkinje fibres. Know the timing of each phase.
Day 3: ECG and disorders. Draw a normal ECG trace and label P, QRS, T. Know what each wave represents. For disorders, focus on hypertension and coronary artery disease.
NEET often gives an ECG trace and asks you to identify the abnormality. A widened QRS suggests bundle branch block. Absent P waves with irregular rhythm suggests atrial fibrillation. Know 2-3 common abnormal patterns.
Common Traps
Trap 1 — AB is the universal recipient, O is the universal donor. Students often swap these. AB has no antibodies in plasma (can receive from anyone). O has no antigens on RBCs (can donate to anyone).
Trap 2 — Veins carry deoxygenated blood EXCEPT pulmonary veins. Pulmonary veins carry oxygenated blood from lungs to left atrium. Similarly, pulmonary arteries carry deoxygenated blood. Define arteries and veins by direction (from/to heart), not by oxygen content.
Trap 3 — Joint diastole is the longest phase. At 0.4 seconds, joint diastole is longer than ventricular systole (0.3 s) or atrial systole (0.1 s). During this phase, blood passively fills the atria from veins — about 70% of ventricular filling happens passively, not by atrial contraction.
Trap 4 — Thrombin is not present in normal blood. Blood contains prothrombin (inactive). Thromboplastin (from damaged tissues/platelets) + Ca converts prothrombin → thrombin, which then converts fibrinogen → fibrin. NEET tests the sequence of this cascade.