💨 Gas Exchange & V/Q Mismatch
Type I vs Type II respiratory failure, V/Q ratios, right-to-left shunting, dead space, and the alveolar-arterial oxygen gradient — core respiratory physiology for MRCS.
Gas Exchange — Normal Physiology
Gas exchange occurs across the alveolar-capillary membrane by simple diffusion, driven by partial pressure gradients. Oxygen moves from alveolus (PaO₂ ~100 mmHg) into capillary blood (PvO₂ ~40 mmHg). CO₂ moves from capillary (PvCO₂ ~46 mmHg) into alveolus (PaCO₂ ~40 mmHg). Effective gas exchange requires matching of ventilation (V) and perfusion (Q) at the alveolar level.
Normal Partial Pressures
| Location | PO₂ (mmHg) | PCO₂ (mmHg) | Notes |
|---|---|---|---|
| Inspired air (sea level) | ~160 (21% × 760) | ~0.3 | Room air at sea level. FiO₂ = 0.21. |
| Alveolar gas (PAO₂) | ~100 | ~40 | Lower than inspired — diluted by water vapour + CO₂ exchange. Alveolar gas equation: PAO₂ = FiO₂(Patm − PH₂O) − PaCO₂/RQ |
| Arterial blood (PaO₂) | ~95–100 | ~35–45 | Slightly lower than PAO₂ due to normal physiological shunt (~2–5%). A-a gradient normally <15 mmHg (increases with age). |
| Mixed venous (PvO₂) | ~40 | ~46 | O₂ saturation ~75%. The difference between arterial and venous = O₂ extracted by tissues. |
| Pulmonary capillary (end-capillary) | ~100 | ~40 | Equilibrates with alveolar gas after ~0.25s of 0.75s capillary transit time. Reserve time for exercise. |
The Alveolar Gas Equation
At sea level, breathing room air:
PAO₂ = 0.21 × (760 − 47) − 40 / 0.8
PAO₂ = 0.21 × 713 − 50
PAO₂ = 149.7 − 50 = ~100 mmHg
PH₂O = 47 mmHg (water vapour pressure at body temperature — always subtracted)
RQ = 0.8 (Respiratory Quotient = CO₂ produced / O₂ consumed; varies: 0.7 for fat, 1.0 for carbohydrate, 0.8 for mixed diet)
Clinical use: calculates expected PAO₂ → compare to measured PaO₂ → derive A-a gradient. Simplified for clinical use: PAO₂ ≈ FiO₂ × 713 − PaCO₂/0.8
Fick’s Law of Diffusion
The rate of gas diffusion across the alveolar-capillary membrane is proportional to: (1) surface area × (2) solubility × (3) pressure gradient, and inversely proportional to (4) membrane thickness × (5) molecular weight.
- CO₂ diffuses 20× faster than O₂ (higher solubility despite similar MW) — explains why CO₂ elimination is preserved in diffusion impairment while O₂ transfer fails first
- DLCO (diffusing capacity for CO) — measures membrane diffusing capacity. Reduced in: emphysema (↓ surface area), pulmonary fibrosis (↑ membrane thickness), pulmonary hypertension (↓ capillary volume), anaemia (↓ haemoglobin to carry O₂). Increased in: polycythaemia, exercise (↑ perfusion).