Capillaries are blood vessels whose walls are formed by only one layer of cells, the endothelium.
Capillaries form a dense network in all organs so that all cells are close to a capillary. Capillaries are the vessels where exchange between blood and the tissues occurs because the walls of other vessels are too thick to allow diffusion. The deformation may help the exchange of oxygen by keeping the membrane of the red cells in contact with the endothelium. It is not due to changes in the arterioles because the flow in one capillary may be increasing while the flow in another capillary supplied by the same arteriole is decreasing. As arteries are followed distally, the last piece of smooth muscle is usually found around the opening of the capillary, forming a precapillary sphincter.