Gas Exchange in the Lungs: Alveolar-Capillary Diffusion

Oxygen and carbon dioxide move between alveoli and pulmonary capillaries by passive diffusion down partial pressure gradients.

Step-by-Step Process

1. Ventilation: Air reaches alveoli; PO₂ ~100 mmHg, PCO₂ ~40 mmHg.
2. O₂ diffusion: Alveolar PO₂ (100) > capillary PO₂ (40) → O₂ enters blood, binds hemoglobin.
3. CO₂ diffusion: Capillary PCO₂ (45) > alveolar PCO₂ (40) → CO₂ enters alveoli, exhaled.
4. Transport: O₂ bound to Hb (98.5%) + dissolved (1.5%); CO₂ as bicarbonate (70%), carbamino-Hb (23%), dissolved (7%).

Key Principles

• Fick's Law: Diffusion rate ∝ (surface area × ΔP × solubility) / thickness.
• Alveolar-capillary barrier: Ultra-thin (~0.5 µm): type I pneumocyte + basement membrane + capillary endothelium.
• Perfusion-ventilation matching: Optimal V/Q ratio ~0.8 ensures efficient exchange.

Clinical Relevance

Impaired diffusion in fibrosis, edema, emphysema (reduced surface area), anemia (reduced Hb), or V/Q mismatch (PE, COPD) causes hypoxemia. CO₂ diffusion is 20x faster than O₂, so hypercapnia usually indicates ventilatory failure, not diffusion defect.