What are the characteristics of capillaries in alveoli suitable for gas exchange?

Hemispherical vesicle composed of monolayer epithelial cells. The bronchi of the lungs repeatedly branch into numerous bronchioles, and the ends expand into cysts. There are many prominent vesicles around the cyst, which are alveoli. The alveoli vary in size and shape, with an average diameter of 0.2 mm. There are about 300-400 million alveoli in adults, with a total area of nearly 100 square meter, which is several times larger than the surface area of human skin. Alveola is the main part of lung gas exchange and the functional unit of lung. When oxygen diffuses from alveoli to blood, it passes through four membranes: the liquid membrane on the inner surface of alveoli, the cell membrane of alveolar epithelium, the interstitial between alveolar epithelium and pulmonary capillary endothelium, and the cell membrane of capillary endothelium. These four membranes are collectively called respiratory membranes. The average thickness of breathing membrane is less than 1 micron, and it has high permeability, so the gas exchange is very rapid. After the gas inhaled into alveoli enters the blood, venous blood becomes arterial blood rich in oxygen, which is transported to all parts of the body with blood circulation. Carbon dioxide in blood in capillaries around alveoli can enter alveoli through capillary walls and alveolar walls and be excreted through exhalation. The surface liquid film in alveoli contains surfactant, which plays a role in reducing the surface tension of the surface liquid layer of alveoli, making cells not easy to contract and easy to expand when inhaling. When the lung tissue is deprived of oxygen, it will reduce the secretion of pulmonary surfactant. Edema or fibrinogen entering alveoli can reduce the activity of its surfactant, causing extensive atelectasis in the lungs, and blood cannot exchange gas when flowing through the capillaries of these collapsed alveoli. Clinically, neonatal atelectasis is caused by the lack of pulmonary surfactant. The tissue between two adjacent alveoli is alveolar septum, which contains abundant capillaries, elastic fibers and reticular fibers. Elastic fibers surround alveoli, making alveoli have good elasticity. When suffering from chronic bronchitis or bronchial asthma, alveoli are in a state of excessive expansion for a long time, which will make the elastic fibers of alveoli lose elasticity and be destroyed, forming emphysema and affecting respiratory function.