Oxygen Diffusion in the Lungs: Alveolar-Capillary Gas Exchange

Oxygen movement from alveoli into pulmonary capillary blood is a classic example of passive diffusion down a partial pressure gradient.

Mechanism

• Gradient: Alveolar PₐO₂ ~100 mmHg; mixed venous PᵥO₂ ~40 mmHg.
• Path: Alveolar gas → alveolar epithelium → interstitial space → capillary endothelium → plasma → RBC membrane → hemoglobin binding.
• Fick's Law: Diffusion rate ∝ (Area × ΔP × Solubility) / (Thickness × √MW). Large surface area (~70 m²), thin barrier (~0.5 µm), high solubility optimize O₂ transfer.

Integration with Respiration

Ventilation maintains alveolar PₐO₂; perfusion brings deoxygenated blood. Ventilation-perfusion matching ensures efficient exchange. Diffusion limitation occurs in fibrosis, edema, or extreme exercise.

Clinical Relevance

Diffusing capacity (DLCO) measures this transfer; reduced in interstitial lung disease, emphysema, pulmonary vascular disease, anemia.