Key Points
Key Points: Respiratory Volumes and Capacities
- Tidal Volume (TV) - Volume of air inhaled or exhaled during normal breathing - 500 mL.
- Inspiratory Reserve Volume (IRV) - Extra air inhaled by forceful inspiration - 2500 to 3000 mL.
- Expiratory Reserve Volume (ERV) - Extra air exhaled by forceful expiration - 1000 to 1100 mL.
- Residual Volume (RV) - Air remaining in lungs even after forceful expiration - 1100 to 1200 mL.
- Inspiratory Capacity (IC) = TV + IRV, Expiratory Capacity (EC) = TV + ERV, Functional Residual Capacity (FRC) = ERV + RV (AIPMT 2010)
- Vital Capacity (VC) = ERV + TV + IRV - Maximum air inhaled after forced expiration or exhaled after forced inspiration.
- Total Lung Capacity (TLC) = RV + ERV + TV + IRV = VC + RV - Total air in lungs after forced inspiration.
Key Points: Exchange of Gases
- Gas exchange occurs between alveoli, blood, and tissues by simple diffusion based on pressure and concentration gradients.
- Partial pressure is the pressure exerted by an individual gas in a mixture, represented as pO₂ for oxygen and pCO₂ for carbon dioxide.
- O₂ moves from alveoli (pO₂ = 104) → blood → tissues (pO₂ = 40); CO₂ moves in the opposite direction - from tissues (pCO₂ = 45) → blood → alveoli (pCO₂ = 40).
- CO₂ is 20-25 times more soluble than O₂, so it diffuses much more easily through the diffusion membrane.
- The diffusion membrane has 3 layers: thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and the basement membrane between them. Total thickness is less than 1 mm.
- Alveoli are the primary sites of gas exchange. Solubility of gases and the thickness of membranes also affect the rate of diffusion.
- Negative intrapleural pressure (pressure in the pleural cavity, lower than atmospheric pressure) is the key factor that prevents collapse of the lungs.
Key Points: Mechanism of Respiration > Inspiration
- Meaning - O₂ from blood is delivered to cells/tissues, and CO₂ from cells passes into the blood.
- O₂ Transport - 97% as oxyhaemoglobin (HbO₂) via RBCs, 3% dissolved in plasma. One Hb molecule has 4 Fe²⁺ ions, each binding one O₂: Hb + 4O₂ → Hb(O₂)₄
- Bohr Effect - Rise in CO₂ / lower pH / higher temperature → reduces Hb-O₂ affinity (curve shifts right) → O₂ released to tissues.
- Haldane Effect - Binding of O₂ with Hb displaces CO₂ from blood (curve shifts left, higher Hb-O₂ affinity).
- CO₂ Transport - 70% as bicarbonate ions (HCO₃⁻) in plasma | 23% as carbaminohaemoglobin | 7% dissolved in plasma.
- Chloride Shift (Hamburger's Phenomenon) - When CO₂ enters blood, Cl⁻ moves into RBCs (Na⁺ stays behind). When CO₂ leaves, Cl⁻ moves back out. This alternate Cl⁻ movement maintains electrical balance.
