Capillaries have the function of connecting microarterioles and microveins to transport blood and have the function of exchanging substances between blood and tissues. Capillaries are blood vessels that connect microarterioles and microveins and have a very fine diameter, sometimes allowing only a single line of red blood cells to pass through. The tube wall consists of a layer of epithelial cells, the tube wall is thin, permeability is strong, widely distributed in all organs and tissues of the body. Capillaries are widely distributed, the blood flow rate is slow, and the walls of the blood vessels are highly permeable. Nutrients and oxygen transported by blood circulation are absorbed and utilized by tissues in capillaries. At the same time, the carbon dioxide and other wastes produced by the metabolism of the organization will pass through the capillaries and enter the blood circulation to complete the exchange of substances. Capillaries have the function of connecting arteries and veins, and are the place for material exchange between blood and tissues.