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 login to view 1 more bullet A = surface area login to view 2 more bullets ΔP = partial pressure difference of gas login to view 1 more bullet Δx = thickness of membrane login to view 1 more bullet