Diffusion function is a measure of gas exchange function. It is used to evaluate the efficiency of gas exchange by the alveolar-capillary membrane. The process of gas exchange between the alveoli and blood flow through the alveolar-capillary membrane (hereinafter referred to as the alveolar membrane) is a physical diffusion process. The amount of gas diffusion per unit time depends on the partial pressure difference between the two sides of the alveolar membrane, the area and thickness of the alveoli, and the diffusion constant of the gas. The diffusion constant is in turn related to the molecular weight and solubility of the gas. In addition, the total gas dispersion is determined by the time of blood contact with the alveoli. 1, Decreased diffusion area: e.g. emphysema, lobectomy, lung infection, pulmonary edema, pulmonary hemorrhage, pneumothorax, scoliosis, etc. 2, alveolar capillary membrane thickening: such as interstitial lung fibrosis, nodular disease, asbestosis, scleroderma, etc. 3, decreased oxygen-carrying capacity of hemoglobin: such as anemia, carboxyhemoglobinemia. 4, blood and alveolar contact time is too short: normal resting, blood flow through the alveolar capillaries for about 0.75sec, because the alveolar membrane is very thin, and the contact surface with the blood is wide, so only 0.25sec hemoglobin can be fully oxygenated. When the time of blood flow through alveolar capillaries is too short, the amount of gas diffusion will decrease. In patients with reduced alveolar membrane area and increased thickness, although the partial pressure of oxygen in the pulmonary capillary blood rises slowly, the gas exchange in the lungs can still generally reach equilibrium at rest, thus not resulting in hypoxemia, but often only when the physical load increases, because the blood flow is accelerated, the contact time between blood and alveoli is shortened and significant diffusion impairment occurs, thus causing hypoxemia. It is currently believed that respiratory failure occurs in the presence of alveolar membrane lesions, mainly because of the presence of an imbalance in the ratio of alveolar ventilation to blood flow.