Overview Diffusion of Gases (Fick's Law) gas exchange across cell membranes or capillary walls occurs by simple diffusion Vx = [(D)(A)(ΔP)] / Δx Vx = volume of gas transferred per unit time D = diffusion coefficient of gas depends on molecular weight (inversely proportional to square root) and solubility of gas A = surface area emphysema → ↓ A alveolar destruction decreases surface area for gas exchange ΔP = partial pressure difference of gas driving force for simple diffusion of gas Δx = thickness of membrane pulmonary fibrosis, pulmonary edema → ↑ Δx