The network of capillaries in a particular area or organ of the body. Exchanges that occur in capillary beds include; transfer of oxygon and nutrients to cells, removal of carbon dioxide and wastes from cells, and the transfer of fluids with cells (which enables white blood cells to enter the interstitial tissues). The capillary bed usually carries no more than 25% of the amount of blood it could contain, although this amount can be increased through autoregulation by inducing relaxation of smooth muscle. The capillaries do not possess this smooth muscle in their own walls, and so any change in their diameter is passive. Any signalling molecules they release (such as endothelin for constriction and nitric oxide for dilation) act on the smooth muscle cells in the walls of nearby, larger vessels, e.g. arterioles.