Oxygen Transfer in the Lungs: Why Diffusion Is So Efficient

Fick's Law of Diffusion

Rate ∝ (Surface Area × ΔP) / Thickness

Lung Adaptations Maximizing O₂ Uptake

1. Enormous Surface Area – ~70–100 m² (alveoli × 300M).
2. Ultra-Thin Barrier – Alveolar epithelium + capillary endothelium + fused basement membrane = 0.2–0.5 µm.
3. Steep Partial Pressure Gradient – Alveolar PₐO₂ ~100 mmHg; mixed venous PᵥO₂ ~40 mmHg → ΔP ~60 mmHg.
4. Capillary Transit Time – 0.75 sec at rest; equilibration in 0.25 sec (large reserve).
5. Ventilation-Perfusion Matching – Regional optimization.

Result

Blood leaves pulmonary capillaries ~98% saturated. Diffusion limitation only occurs in disease (fibrosis, edema) or extreme exercise at altitude.