: Gases diffuses from the alveoli to the blood in the pulmonary capillaries or vice versa across the thin alveolar capillary membrane made up of pulmonary epithelium, the capillary endotheliurn and their fused basement membrane. The diffusion capacity for oxygen is about 25 ml per min per mm Hg. Diffusing capacity of the lung is directly proportional to the surface area ofalveolo-capillary membrane and inversely proportional to its thickness.