Diffusion function is a measure of gas exchange function. It is used to evaluate the efficiency of gas exchange by the alveolar capillary membrane. It is important for early detection of lung and airway lesions, assessment of disease severity and prognosis, evaluation of the efficacy of drugs or other treatments, identification of the cause of dyspnea, diagnosis of the site of the lesion, assessment of the tolerance of lung function to surgery or labor intensity, and monitoring of critically ill patients. Pulmonary diffusion is the process of gas exchange between oxygen and carbon dioxide through the alveoli and pulmonary capillary walls in the lungs. The diffusion pathway includes alveolar gas, alveolar capillary walls, intracapillary plasma, red blood cells and hemoglobin. Gases are exchanged along this pathway, depending on which end is more concentrated, so the process can be bidirectional. Oxygen diffuses much more slowly than carbon dioxide, due to the fact that oxygen is not easily dissolved in body fluids. Therefore, when a patient has an abnormal diffusion function, oxygen exchange is more likely to be affected than carbon dioxide, and clinically impaired pulmonary diffusion can significantly affect arterial oxygen levels. How to check for decreased diffusion function? The diagnosis can be made by clinical examination of spirometry. It is (28.84±4.84) ml/(mmHg?min) in men and (22.13±3.09) ml/(mmHg?min) in women. Below this value, there is a decrease in gas diffusion function. It is important to go to the respiratory department of a regular hospital for examination and to pay attention to the standardization of the examination, etc. Diffusion is the process of gas exchange between the alveoli and capillaries for oxygen and carbon dioxide, through the alveolar-capillary membrane. Diffusion function is based on the partial pressure difference of gas on both sides of the alveolar-capillary membrane is 0.1333kPa (1mmHg); the amount of gas that may pass per minute as an indicator, expressed as diffusioncapacity, carbon dioxide has a strong diffusion capacity, 21 times greater than oxygen, and there is no carbon dioxide diffusion disorder clinically, so the diffusion disorder mainly refers to oxygen. The measurement method is to use carbon monoxide as the measurement gas. The advantages are (1) Except for heavy smokers, the CO entering capillary mixed venous blood is almost zero in general, and no calculation is needed. (2)The affinity of CO and hemoglobin is 210 times that of oxygen. After inhaling a small amount of CO through the capillary membrane to the plasma, it rapidly enters the red blood cells and combines with hemoglobin, and the partial pressure of CO in the plasma is equal to zero and can be disregarded. normal value of DLco: 206.2ml/kPa/m. pulmonary oxygen dispersion (Dlo2) = 1.23×DLco. The size of the dispersion depends on the gas partial pressure difference between the two sides of the membrane The size of the diffusion volume depends on the pressure difference, diffusion area, distance, time, molecular weight of the gas and its solubility in the diffusion medium. The diffusion function can be reduced in 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 dysfunction, the consequences of which all lead to hypoxia.