Understanding Gas Exchange in the Lungs: Alveoli & Capillaries

Pulmonary gas exchange occurs across the alveolar-capillary membrane (0.2–0.5 µm thick).

Process

1. Ventilation: air reaches ~300M alveoli (total surface ~70 m²).
2. Diffusion: O₂ (PAO₂ ~100 mmHg) → capillary blood (PvO₂ ~40 mmHg); CO₂ (PvCO₂ ~45 mmHg) → alveoli (PACO₂ ~40 mmHg).
3. Perfusion: pulmonary capillary transit time ~0.75s; equilibration by ~0.25s.

Fick's Law Determinants

V̇gas ∝ (Area × ΔP × Solubility) / (Thickness × √MW)

Impairment Mechanisms

• V/Q mismatch: COPD, asthma, PE.
• Diffusion limitation: fibrosis, edema, altitude.
• Shunt: atelectasis, intracardiac right-to-left.

Clinical Assessment

• ABG: PaO₂, A-a gradient, PaCO₂.
• Pulse oximetry (SpO₂).
• DLCO (diffusing capacity).