Alveoli: Microscopic Structure Enabling Efficient Gas Exchange

Alveoli are the terminal, thin-walled air sacs where oxygen and carbon dioxide exchange occurs. Each lung contains ~300–500 million alveoli, providing ~70–100 m² surface area.

Structural Features for Efficient Diffusion

• Single-cell epithelium: Type I pneumocytes (95% surface) – 0.1–0.5 µm thick.
• Surfactant: Type II pneumocytes secrete dipalmitoylphosphatidylcholine, reducing surface tension, preventing collapse.
• Dense capillary network: Capillary endothelium fuses with alveolar epithelium → blood-gas barrier ~0.5 µm.
• Pores of Kohn: Inter-alveolar connections equalize pressure and allow collateral ventilation.

Gas Exchange Physics

O₂ diffuses down partial pressure gradient (PAO₂ ~100 mmHg → PaO₂ ~95 mmHg). CO₂ diffuses opposite (PvCO₂ ~45 mmHg → PACO₂ ~40 mmHg). Diffusion limited by barrier thickness, surface area, and transit time (~0.75 sec at rest).

Clinical Relevance

• ARDS: surfactant loss, edema, barrier thickening → shunt.
• Emphysema: alveolar wall destruction → ↓ surface area → diffusion impairment.
• Pulmonary fibrosis: thickened interstitium → diffusion limitation, especially on exercise.