Arterioles

Arterioles are small blood vessels that branch off arteries and carry blood into the capillary beds of organs and tissues.Compared with arteries, the wall of an arteriole has fewer elastic fibres and a much higher proportion of smooth muscle. By the time blood reaches the arterioles, the surges from the heart have largely been smoothed out, so less elastic tissue is needed. The smooth muscle lets arterioles control where blood goes.Vasoconstriction. When the smooth muscle contracts, the lumen narrows. Less blood flows through that arteriole into the capillaries beyond it.Vasodilation. When the smooth muscle relaxes, the lumen widens and more blood flows into the capillary bed.This allows the body to direct blood to the tissues that need it most. For example, during exercise the arterioles supplying the skeletal muscles dilate, so the working muscles receive more oxygen and glucose, while arterioles supplying the gut constrict. In hot conditions, arterioles supplying the skin dilate so more heat is lost from the body surface; in the cold they constrict to conserve heat.Because arterioles are narrow and numerous, they provide most of the resistance to blood flow in the circulation. Blood pressure therefore falls sharply as blood passes through them. This protects the delicate capillaries, which could be damaged by the high pressure found in the arteries. Widespread constriction of arterioles raises blood pressure, and widespread dilation lowers it.