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. Clinically, when pulmonary pathology produces diffusion dysfunction, there is often a concomitant significant ventilation/blood flow ratio imbalance, the consequences of which all lead to hypoxia. The diagnosis can be made by clinical examination of spirometry. It is (28.84plusmn;4.84)ml/(mmHg?min) in men and (22.13plusmn;3.09)ml/(mmHgmin) in women. Below this value there is a decrease in gas diffusion function. Decreased alveolar membrane area: The total alveolar area in normal adults is about 80m2, and the alveolar surface area involved in gas exchange is only about 35-40m2 during resting breathing, increasing during exercise. Because of the large reserve, alveolar membrane area only causes ventilatory dysfunction when it is extremely reduced. The reduction in alveolar membrane area can be seen when the lung is solid, when the lung is not distended, when the lung is lobectomized, etc. The alveolar membrane thickness increases: the thin part of the alveolar membrane is the site of gas exchange, which is composed of alveolar epithelium, capillary endothelium and the basement membrane common to both, and its thickness is less than 1 μm. Although the gas from the alveolar cavity to the red blood cells also needs to pass through the liquid layer on the alveolar surface, the intra-tubular plasma layer and the red blood cell membrane, but the total thickness is less than 5 μm. Therefore, normal gas exchange is rapid. When pulmonary edema, alveolar hyaline membrane formation, pulmonary fibrosis, alveolar capillary dilatation or dilution of blood leading to thickening of the plasma layer, etc., can affect gas diffusion due to reduced alveolar membrane permeability or widening of diffusion distance. The contact time between blood and alveoli is too short: at normal rest, the time for blood to flow through alveolar capillaries is about 0.75sec. Because the alveolar membrane is very thin and the contact surface with blood is wide, it only takes 0.25sec for hemoglobin to be fully oxygenated. When the time of blood flow through the 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 is generally balanced at rest and thus does not lead to hypoxemia, but often only when the physical load increases, because the blood flow is accelerated and the contact time between the blood and alveoli is shortened, significant diffusion impairment occurs, thus causing hypoxemia. It is now 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 Decreased alveolar membrane area: The total alveolar area in normal adults is about 80 m2, and the surface area of alveoli involved in gas exchange is only about 35-40 m2 during resting breathing, increasing during exercise. Because of the large reserve, alveolar membrane area only causes ventilatory dysfunction when it is extremely reduced. The reduction in alveolar membrane area can be seen when the lung is solid, when the lung is not distended, when the lung is lobectomized, etc. The alveolar membrane thickness increases: the thin part of the alveolar membrane is the site of gas exchange, which is composed of alveolar epithelium, capillary endothelium and the basement membrane common to both, and its thickness is less than 1 μm. Although the gas from the alveolar cavity to the red blood cells also needs to pass through the liquid layer on the alveolar surface, the intra-tubular plasma layer and the red blood cell membrane, but the total thickness is less than 5 μm. Therefore, normal gas exchange is rapid. When pulmonary edema, alveolar hyaline membrane formation, pulmonary fibrosis, alveolar capillary dilatation or dilution of blood leading to thickening of the plasma layer, etc., can affect gas diffusion due to reduced alveolar membrane permeability or widening of diffusion distance. The contact time between blood and alveoli is too short: at normal rest, the time for blood to flow through alveolar capillaries is about 0.75sec. Because the alveolar membrane is very thin and the contact surface with blood is wide, it only takes 0.25sec for hemoglobin to be fully oxygenated. When the blood flow time through the 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 is generally balanced at rest and thus does not lead to hypoxemia, but often only when the physical load increases, because the blood flow is accelerated and the contact time between blood and alveoli is shortened, 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.