The Core of NEET Biology
Human Physiology is the highest-weightage unit in NEET Biology, contributing roughly 12-15 questions per paper. While the unit contains seven chapters, Breathing and Exchange of Gases and Body Fluids and Circulation are particularly rich in conceptual questions that test understanding over pure memory.
This guide breaks down the specific sub-topics within these two chapters that examiners love to test.
Breathing and Exchange of Gases
1. Respiratory Volumes and Capacities
This is tested almost every year, often as a direct matching question or a calculation.
Volumes (The Building Blocks):
- Tidal Volume (TV): Volume inspired/expired during normal respiration (~500 mL).
- Inspiratory Reserve Volume (IRV): Additional volume inspired by forceful inspiration (~2500-3000 mL).
- Expiratory Reserve Volume (ERV): Additional volume expired by forceful expiration (~1000-1100 mL).
- Residual Volume (RV): Volume remaining in lungs even after forceful expiration (~1100-1200 mL).
Capacities (Combinations of Volumes):
- Inspiratory Capacity (IC): TV + IRV
- Expiratory Capacity (EC): TV + ERV
- Functional Residual Capacity (FRC): ERV + RV (Volume remaining in lungs after normal expiration)
- Vital Capacity (VC): ERV + TV + IRV (Maximum air a person can breathe in after a forced expiration)
- Total Lung Capacity (TLC): RV + ERV + TV + IRV (or VC + RV)
NEET Trick: They will give you three values and ask you to calculate TLC or FRC. Know the formulas securely.
2. Transport of Gases
Oxygen Transport:
- ~97% transported by RBCs (as oxyhaemoglobin)
- ~3% dissolved in plasma
- 100 mL of oxygenated blood delivers ~5 mL of O₂ to tissues under normal conditions.
The Oxygen Dissociation Curve: A sigmoid (S-shaped) curve. NEET frequently asks what causes a "Right Shift" (decreased affinity of Hb for O₂, meaning O₂ is unloaded at tissues).
- Right Shift triggers (CADET faces right): ↑CO₂, ↑Acidity (↓pH), ↑DPG (2,3-Bohr effect), ↑Exercise, ↑Temperature.
Carbon Dioxide Transport (The more complex one):
- ~70% as bicarbonate ions (HCO₃⁻)
- ~20-25% bound to haemoglobin (as carbamino-haemoglobin)
- ~7% dissolved in plasma
- 100 mL of deoxygenated blood delivers ~4 mL of CO₂ to the alveoli.
Understand the Chloride shift (Hamburger phenomenon): HCO₃⁻ moves out of RBC into plasma, and Cl⁻ moves into RBC to maintain ionic balance.
3. Regulation of Respiration
- Respiratory Rhythm Centre: Located in the medulla (primary control).
- Pneumotaxic Centre: Located in the pons (moderates the medulla, can reduce duration of inspiration).
- Chemosensitive Area: Adjacent to the rhythm centre, highly sensitive to CO₂ and H⁺ (NOT primarily sensitive to O₂).
- NEET trap: The role of oxygen in the regulation of respiratory rhythm is quite insignificant. CO₂ and H⁺ levels are the primary drivers.
Body Fluids and Circulation
1. Blood Composition and Coagulation
- Erythrocytes (RBCs): 5-5.5 million/mm³, life span ~120 days.
- Leukocytes (WBCs): 6000-8000/mm³.
- Order of abundance (Never Let Monkeys Eat Bananas): Neutrophils (60-65%), Lymphocytes (20-25%), Monocytes (6-8%), Eosinophils (2-3%), Basophils (0.5-1%).
- Thrombocytes (Platelets): 1.5 - 3.5 lakh/mm³.
Coagulation cascade essentials: Injured tissue/platelets release Thromboplastin → forms Prothrombinase complex → converts Prothrombin to Thrombin → converts Fibrinogen (soluble) to Fibrin (insoluble network). Calcium ions (Ca²⁺) play a very important role in this cascade.
2. The Cardiac Cycle
The cardiac cycle lasts 0.8 seconds.
- Joint Diastole (0.4s): All chambers relaxed. AV valves open, semilunar valves closed.
- Atrial Systole (0.1s): Atria contract, pushing remaining 30% of blood into ventricles.
- Ventricular Systole (0.3s): Ventricles contract. AV valves close (First heart sound: LUB). Pressure rises, semilunar valves open, blood pumped into arteries. When ventricles relax, semilunar valves close (Second heart sound: DUB).
Cardiac Output: Heart Rate (72 beats/min) × Stroke Volume (70 mL/beat) ≈ 5000 mL or 5 L/min.
3. Electrocardiogram (ECG)
A standard NEET question format: Identifying the waves.
- P-wave: Electrical excitation (depolarisation) of the atria → leads to atrial contraction.
- QRS complex: Depolarisation of the ventricles → leads to ventricular contraction. (Contraction starts shortly after Q).
- T-wave: Repolarisation of the ventricles (return to normal state). The end of the T-wave marks the end of ventricular systole.
NOTE: Depolarisation of atria is hidden within the massive QRS complex.
4. Double Circulation and Regulation
- Systemic circulation: Left ventricle → Aorta → Tissues → Vena Cava → Right Atrium.
- Pulmonary circulation: Right ventricle → Pulmonary artery → Lungs → Pulmonary vein → Left Atrium.
Hepatic Portal System: A unique vascular connection between the digestive tract and the liver. The hepatic portal vein carries blood from intestine to liver before it is delivered to the systemic circulation.
Regulation: Medulla oblongata regulates cardiac function through the Autonomic Nervous System (ANS). Sympathetic nerves increase heart rate and ventricular contraction strength; parasympathetic (vagus nerve) decrease them.