Diffusion disorders are gas exchange disorders caused by reduced alveolar membrane area or abnormal thickening of the alveolar membrane. Diffusion disorders are mainly seen in: 1. Decreased diffusion area The diffusion area of normal adult alveolar membrane is about 80m2, at rest, the alveolar diffusion area is about 35m2~40m2, because of its large reserve, only when it is reduced by more than half will it cause gas exchange dysfunction. Decreased alveolar membrane area can be seen in solid lung, pulmonary atelectasis, emphysema and lobectomy, etc. 2, diffuse membrane thickening The alveolar membrane consists of capillary endothelial cells, basement membrane, reticular space between capillaries and alveolar epithelium, alveolar epithelium, liquid layer on the surface of alveolar epithelium and surface active substance layer and other structures. The thickness of the membrane is 0.35 to 1.0 μm, so the gas is easily diffused and exchanged quickly. When pulmonary edema, pulmonary hyaline membrane formation, pulmonary fibrosis, interstitial pneumonia, etc., can cause an increase in the thickness of the alveolar membrane, so that the alveolar membrane permeability is reduced or the diffusion distance is widened and the diffusion speed is slowed down, causing pulmonary diffusion dysfunction.