The air-blood barrier mainly includes the liquid layer containing lung surface active substances, the alveolar epithelial cell layer, the epithelial basement membrane, the gap between the alveolar epithelium and capillaries, the basement membrane of capillaries, and the capillary endothelial cell layer. 1, the liquid layer containing lung surface active substances: lipoproteins secreted by alveolar type II cells, covering the alveolar liquid surface. The liquid layer facilitates the uniform distribution of inhaled gases in the lung; 2. Alveolar epithelial cell layer: a complete layer of epithelium on the alveolar surface, mainly including type I alveolar cells and type II alveolar cells. Among them, type II alveolar cells can divide, proliferate, and differentiate into type I alveolar cells, which can repair the damaged epithelium; 3, epithelial basement membrane: is the basal tissue of type I alveolar cells, type I alveolar cells are closely connected to the basement membrane, which plays a certain role in stabilizing and nourishing the cells; 4, the gap between the alveolar epithelium and capillaries: that is, the stroma layer, which is the main place of gas exchange; 5, the basement membrane of capillaries: is the lung Capillary wall: one of the components of the capillary wall, usually filled between the epithelial cells, playing a role in nutrient cells; 6, capillary endothelial cell layer: one of the components of the capillary wall of the lung, containing a large number of epithelial cells, and a layer of connective tissue, playing a role in supporting the wall of the tube.