Oxygen Diffusion Gradient: Alveoli to Pulmonary Capillaries

Partial Pressure and Concentration Gradients

Compartment	PO₂ (mmHg)	O₂ Concentration (mM)
Alveolar air	100	13.4
Pulmonary capillary blood (mixed venous)	40	5.3

Diffusion Gradient

• ΔPO₂ = 100 – 40 = 60 mmHg
• Δ[O₂] = 13.4 – 5.3 = 8.1 mM

Physiological Basis

• O₂ diffuses down its partial pressure gradient across the alveolar-capillary membrane (Type I pneumocytes, basement membrane, capillary endothelium).
• Equilibration occurs within ~0.25 sec (transit time ~0.75 sec at rest).
• Diffusion capacity (DLCO) depends on membrane surface area, thickness, hemoglobin, and capillary blood volume.

Factors Affecting the Gradient

• Altitude: ↓Alveolar PO₂ → ↓gradient.
• Hypoventilation: ↑Alveolar PCO₂ → ↓Alveolar PO₂ (alveolar gas equation).
• Shunt/V/Q mismatch: ↓Effective capillary PO₂.
• Fibrosis/edema: ↑Membrane thickness → ↓diffusion (not gradient per se, but flux).

Clinical Relevance

Explains hypoxemia in high altitude, interstitial lung disease, pulmonary edema. Supplemental O₂ ↑alveolar PO₂ → ↑gradient → ↑diffusion.