Quick Summary
Body fluids (blood, lymph) transport nutrients, gases, hormones, and waste. The heart pumps blood through two circuits — pulmonary and systemic. Blood groups, clotting, the cardiac cycle, and cardiovascular disorders are all tested in NEET. The 4-chambered human heart and double circulation are the central concepts.
Blood
Composition:
- Plasma (55%): Water (91%), proteins (fibrinogen, albumin, globulin), glucose, urea, hormones, electrolytes
- Formed elements (45%):
| Cell | Count | Features | Function |
|---|---|---|---|
| RBC (Erythrocytes) | 5 million/mm³ (M), 4.5 million (F) | No nucleus (mature), biconcave disc, 120-day lifespan | O₂ transport (haemoglobin) |
| WBC (Leucocytes) | 6000–8000/mm³ | Nucleated; granulocytes + agranulocytes | Immunity, defence |
| Platelets (Thrombocytes) | 1.5–3.5 lakh/mm³ | Fragments of megakaryocytes, no nucleus | Blood clotting |
WBC types:
- Granulocytes: Neutrophils (60–65%, phagocytosis), Eosinophils (2–3%, allergy), Basophils (<1%, heparin, histamine)
- Agranulocytes: Lymphocytes (20–25%, antibodies), Monocytes (6–8%, phagocytosis)
Blood Groups
ABO System
| 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 + B | None | AB only | All (universal recipient) |
| O | None | Anti-a + Anti-b | All (universal donor) | O only |
Rh Factor
- Rh positive (+): Has Rh antigen on RBCs (~80% of humans)
- Rh negative (−): No Rh antigen
- Erythroblastosis foetalis: Rh− mother + Rh+ foetus → anti-Rh antibodies in second pregnancy damage foetal RBCs
Blood Coagulation (Clotting)
1. Injury → platelet aggregation → release of thromboplastin 2. Thromboplastin + Ca²⁺ + clotting factors → Prothrombin → Thrombin (activated) 3. Thrombin → Fibrinogen → Fibrin (insoluble threads) 4. Fibrin traps RBCs → clot formed
Key: Vitamin K needed for synthesis of clotting factors; Ca²⁺ essential for clotting.
Heart — Structure
- 4 chambers: 2 atria (upper) + 2 ventricles (lower)
- Pericardium: Outer protective covering
- Valves: Bicuspid/mitral (left AV), Tricuspid (right AV), Semilunar valves (aorta and pulmonary artery)
- Wall thickness: Left ventricle > Right ventricle (pumps to body vs. lungs)
Conducting System
| Node/Pathway | Location | Role |
|---|---|---|
| SA node (sinoatrial) | Right atrium wall | Pacemaker — initiates heartbeat |
| AV node | AV junction | Delays impulse (0.1s) |
| Bundle of His | Interventricular septum | Transmits to ventricles |
| Purkinje fibres | Ventricular walls | Rapid ventricular contraction |
Cardiac Cycle
| Phase | Duration | Events |
|---|---|---|
| Atrial systole | 0.1 s | Atria contract, blood enters ventricles |
| Ventricular systole | 0.3 s | Ventricles contract, blood pumped to aorta/pulmonary artery |
| Joint diastole | 0.4 s | All chambers relax, heart fills |
| Total | 0.8 s | Heart rate = 72 beats/min |
Cardiac output = 72 × 70 mL = ~5040 mL/min ≈ 5 L/min
Heart sounds: "Lubb" (AV valves close at systole) + "Dubb" (semilunar valves close at diastole)
Lymph
- Fluid that leaks out of capillaries into tissue spaces → collected by lymph vessels
- Composition: similar to plasma but no large proteins, RBCs
- Functions: returns proteins and fluid to blood; immune response (lymphocytes)
Disorders
| Disorder | Description |
|---|---|
| Hypertension | BP > 120/80 mmHg (systolic/diastolic) |
| Coronary artery disease | Atherosclerosis of coronary arteries → angina, heart attack |
| Heart failure | Heart cannot pump enough blood |
| Angina pectoris | Chest pain due to ischaemia (reduced blood to heart) |
Important Questions
One Mark
- Name the pacemaker of the heart.
- What is the universal blood donor?
- How long does one cardiac cycle last?
- What is the role of Vitamin K in blood clotting?
Short Answer (3 marks)
- Describe the ABO blood group system with antigens and antibodies.
- Explain the mechanism of blood coagulation.
- What is double circulation? State its significance.
Long Answer (5 marks)
- Describe the conducting system of the heart. Trace the path of an electrical impulse from SA node to Purkinje fibres.
- Write an account of the composition of blood. Describe the types of WBCs with their functions.
Common Mistakes
- Blood group O is the universal DONOR (no antigens, so no reaction); AB is universal RECIPIENT (no antibodies).
- SA node (pacemaker) is in the RIGHT atrium, not left.
- Cardiac cycle: total = 0.8s (not 0.4 or 0.6); heart rate = 72 beats/min (60/0.8 × 60 = 72).
- Platelets are fragments of megakaryocytes, not complete cells — they have no nucleus.
Board Exam Tips
- ABO blood group table with antigens, antibodies, donor/recipient is a standard 5-mark question.
- Cardiac cycle with durations and events is a 3-mark or 5-mark question.
- Blood coagulation steps is a reliable 3-mark question.
- NEET: SA node, cardiac cycle, blood groups, and double circulation contribute 3–4 questions per paper.
Frequently Asked Questions
Blood = Plasma (55%) + Formed elements (45%). Plasma: water (91%), proteins (fibrinogen, albumin, globulin), salts, glucose, hormones. Formed elements: RBCs (erythrocytes), WBCs (leucocytes), Platelets (thrombocytes).