Chapter 6 — Body Fluids and Circulation
Key Concepts Quick Recap
| Component | Normal values | Function |
|---|
| RBC (Erythrocyte) | 4.5–5.5 million/mm³ (male); 4–5 million/mm³ (female) | O₂ and CO₂ transport (haemoglobin) |
| WBC (Leukocyte) | 6,000–8,000/mm³ | Immunity, defence |
| Platelets (Thrombocytes) | 1.5–3.5 lakh/mm³ | Blood clotting |
| Plasma | 55% of blood | Transport of nutrients, hormones, gases, waste |
| Haemoglobin | 12–17 g/dL | O₂ carrier; contains iron |
| Blood pressure | 120/80 mmHg (normal) | Drives circulation |
I. Choose the Best Answer (MCQs)
1. The volume of blood in an average adult human is: (a) 2–3 litres (b) 4–5 litres (c) 6–7 litres (d) 8–10 litres
Answer(b) 4–5 litres An average adult (70 kg) has ~5 litres of blood (about 7–8% of body weight). Blood loss >20% causes shock; >40% is rapidly fatal.
2. The pacemaker of the heart is: (a) AV node (b) Bundle of His (c) SA node (Sinoatrial node) (d) Purkinje fibres
Answer(c) SA node (Sinoatrial node) The SA node (located in the right atrium near the opening of the superior vena cava) generates spontaneous electrical impulses → sets the heart rate (~72 beats/min). It is called the pacemaker of the heart. Impulse travels: SA node → atrial muscles → AV node → Bundle of His → Purkinje fibres → ventricular muscles.
3. Lub-Dub sounds of the heart are produced by: (a) Contracting heart muscles (b) Closing of heart valves (c) Blood rushing through aorta (d) Electrical impulses
Answer(b) Closing of heart valves 'Lub' (S1): Closing of AV valves (tricuspid and bicuspid/mitral) at the start of ventricular systole. 'Dub' (S2): Closing of semilunar valves (aortic and pulmonary) at the end of ventricular systole.
4. Universal blood donor is blood group: (a) A (b) B (c) AB (d) O
Answer(d) O Blood group O (genotype ii) has no antigens (A or B) on RBCs → cannot be attacked by anti-A or anti-B antibodies in recipient's blood → can donate to all groups. Blood group AB = universal recipient (has no antibodies; accepts any blood).
5. The bicuspid valve (mitral valve) is located between: (a) Right atrium and right ventricle (b) Left atrium and left ventricle (c) Right ventricle and pulmonary artery (d) Left ventricle and aorta
Answer(b) Left atrium and left ventricle Valves of the heart:
- Tricuspid (3 cusps): Right atrium → Right ventricle.
- Bicuspid/Mitral (2 cusps): Left atrium → Left ventricle.
- Pulmonary semilunar: Right ventricle → Pulmonary artery.
- Aortic semilunar: Left ventricle → Aorta.
6. The P wave in an ECG represents: (a) Ventricular depolarisation (b) Ventricular repolarisation (c) Atrial depolarisation (d) Atrial repolarisation
Answer(c) Atrial depolarisation ECG waves:
- P wave: Atrial depolarisation (contraction).
- QRS complex: Ventricular depolarisation (contraction). Atrial repolarisation is masked.
- T wave: Ventricular repolarisation (relaxation).
7. Stroke volume × Heart rate = : (a) Cardiac output (b) Blood pressure (c) Pulse pressure (d) Mean arterial pressure
Answer(a) Cardiac output Cardiac output = Stroke volume × Heart rate = 70 mL × 72 beats/min = ~5040 mL/min ≈ 5 litres/min. During exercise, cardiac output can increase to 20–25 litres/min.
II. Short Answer Questions
Q1. Describe the composition of blood.
Answer
Blood is a fluid connective tissue (~5 litres in adult).
Plasma (55%):
- Water (90–92%): Solvent for all dissolved substances.
- Plasma proteins (6–8%): Albumin (osmotic balance), globulins (immunity), fibrinogen (clotting).
- Inorganic salts: Na⁺, K⁺, Ca²⁺, Cl⁻ (maintain osmotic pressure, pH, nerve function).
- Nutrients: Glucose, amino acids, fatty acids.
- Gases: O₂, CO₂, N₂ (dissolved).
- Hormones, vitamins, waste: Urea, uric acid, creatinine.
Formed elements (45%):
| Cell | Count | Features | Function |
|---|
| RBC (Erythrocyte) | 4.5–5.5 million/mm³ | Biconcave disc; no nucleus; no mitochondria; ~120 day life | O₂ and CO₂ transport via Hb |
| WBC (Leukocyte) | 6,000–8,000/mm³ | Nucleated; various types | Immunity, defence |
| Platelets (Thrombocytes) | 1.5–3.5 lakh/mm³ | Cell fragments (from megakaryocytes); no nucleus | Blood clotting |
Q2. Describe the ABO blood group system and its significance in transfusion.
Answer
ABO system is based on antigens on RBC surface and antibodies in plasma.
| 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 |
Importance:
- Blood must be matched before transfusion — mismatched blood causes agglutination → haemolysis → kidney failure → death.
- Modern blood banks cross-match donor and recipient before transfusion.
Rh Factor:
- Rh positive (Rh+): Rh antigen present on RBC.
- Rh negative (Rh–): Rh antigen absent.
- Erythroblastosis foetalis: Rh– mother + Rh+ baby → mother produces anti-Rh antibodies → second Rh+ baby at risk.
Q3. Describe the cardiac cycle.
Answer
The cardiac cycle is the sequence of events in one complete heartbeat (~0.8 seconds at 75 beats/min).
Phases: 1. Atrial systole (0.1 s): Both atria contract → blood pushed into ventricles. AV valves open; semilunar valves closed.
2. Ventricular systole (0.3 s):
- Isovolumetric contraction: ventricular pressure rises, all valves closed (0.05 s).
- Ejection phase: ventricular pressure > arterial pressure → semilunar valves open → blood ejected into aorta/pulmonary artery.
3. Joint diastole (0.4 s): Both atria and ventricles relax. Blood refills atria from veins.
Stroke volume: Blood ejected per beat ~70 mL (at rest). Cardiac output: 70 mL × 75 = 5250 mL/min (~5.25 litres/min).
Common Mistakes
- Confusing tricuspid (right AV) and bicuspid/mitral (left AV) valves.
- Saying SA node is in the left atrium — it is in the RIGHT atrium.
- Confusing universal donor (O) and universal recipient (AB).
- Forgetting: P wave = atrial depolarisation; QRS = ventricular depolarisation; T = ventricular repolarisation.
Exam Checklist
- Blood components: plasma (55%) + formed elements (RBC, WBC, platelets).
- ABO blood group table with antigens, antibodies, donation rules.
- Heart valves: tricuspid (right AV), mitral (left AV), pulmonary semilunar, aortic semilunar.
- Pacemaker: SA node → AV node → Bundle of His → Purkinje fibres.
- ECG: P (atrial depol) → QRS (ventricular depol) → T (ventricular repol).
- Cardiac output = Stroke volume × Heart rate = 70 × 75 = 5250 mL/min.
