Respiratory Membrane: Structure & Gas Exchange Physiology

O₂ and CO₂ diffuse across the respiratory membrane (alveolar-capillary barrier) driven by partial pressure gradients.

Ultrastructure (Total Thickness ~0.5 μm)

1. Alveolar surfactant film
2. Type I pneumocyte (95% surface area; extremely thin cytoplasm)
3. Basement membrane (fused alveolar & capillary)
4. Capillary endothelial cell
5. RBC membrane (final barrier)

Diffusion Physics (Fick's Law)

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

• O₂: perfusion-limited normally; diffusion-limited in fibrosis, exercise, altitude
• CO₂: 20× more soluble → rarely diffusion-limited

Clinical Correlates

• ARDS: membrane thickening + edema → hypoxemia refractory to O₂
• Emphysema: surface area destruction → ↓ diffusion capacity (DLCO)
• Pulmonary edema: increased thickness → impaired O₂ transfer
• CO poisoning: competes for Hb, not membrane defect

Preserving membrane integrity is central to gas exchange efficiency.