V̇x = DAΔP / Δx
Summary
This equation represents Fick's Law of Diffusion, where gas diffusion rate is proportional to area (A) and pressure gradient (ΔP), and inversely proportional to membrane thickness (Δx). Critical for understanding alveolar-capillary gas exchange.
Detail
V̇x is diffusion rate, D is diffusion coefficient (varies by gas solubility and molecular weight), A is surface area, ΔP is partial pressure gradient, and Δx is membrane thickness (~0.5 um). CO2 is ~20x more diffusible than O2 due to higher solubility, explaining why hypoxemia develops before hypercapnia in diffusion-limited disease. DLCO (diffusion capacity for CO) directly reflects D x A / Δx. Conditions like pulmonary fibrosis (increased Δx) and emphysema (reduced A) decrease diffusion capacity.
Sources
- First Aid for the USMLE Step 1
- West's Respiratory Physiology
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