Capillaries

The capillary wall is a one-layer endothelium so thin that gas and molecules such as oxygen, water,proteins, and lipids can pass through them driven by osmotic and hydrostatic gradients. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body. The physics of this exhange is explained by the Starling equation. The capillary bed usually carries no more than 25% of the amount of blood it could contain, although this amount can be increased through auto regulation by inducing relaxation of smooth muscle in the arterioles that lead to the capillary bed as well as constriction of the metarterioles. The capillaries do not possess this smooth muscle in their own wall, and so any change in their diameter is passive. Any signaling molecules they release act on the smooth muscle cells in the walls of nearby, larger vessels. Capillary permeability can be increased by the release of certain cytokines, such as in an immune response.