Alveolar-Capillary Interface: Gas Exchange Microarchitecture

Each alveolus is surrounded by a dense capillary network forming the blood-gas barrier.

• Pulmonary arteries → arterioles → alveolar capillaries (deoxygenated blood).
• Barrier: alveolar epithelium (type I pneumocytes) + fused basement membrane + capillary endothelium = 0.2–0.5 µm thick.
• Diffusion: O₂ into blood (binds Hb); CO₂ out (dissolved, bicarbonate, carbamino).
• Pulmonary veins → left heart (oxygenated blood).

Total alveolar surface area ~70–100 m²; capillary blood volume ~100 mL. Transit time ~0.75 sec at rest – sufficient for equilibration.