The amount of diffusion depends on the difference in partial pressure of gas between the two sides of the membrane, diffusion area, distance, time, molecular weight of the gas and its solubility in the diffusion medium. Decreased diffusion function can be caused by emphysema and other lung tissue lesions, diffuse interstitial lung fibrosis, and other diseases. When pulmonary pathology produces diffusion dysfunction clinically, it is often accompanied by a significant ventilation/blood flow ratio imbalance, the consequences of which all lead to hypoxia. 1. Decreased diffusion function can be seen in: ① Decreased diffusion area: e.g. emphysema, lobectomy, pulmonary infection, pulmonary edema, pulmonary hemorrhage, pneumothorax, scoliosis, etc. (ii) Thickening of alveolar capillary membrane: such as interstitial fibrosis, nodular disease, asbestosis, scleroderma, etc. ③Decreased oxygen-carrying capacity of hemoglobin: such as anemia, carboxyhemoglobinemia. 2. Increased diffusion function can be seen in: erythrocytosis, increased pulmonary artery pressure due to intracardiac left-to-right shunt, etc.