The muscular walls of arterioles are under the influence of nerves, hormones, and chemicals produce by nearby tissues. They can therefore contract and relax in response to the changing needs of the tissues and organs they supply. The blood flow in capillaries is regulated by tiny rings of smooth muscle called precapillary sphincters that surround the junction between arterioles and capillaries.. hese sphincter opens and closes in response to the local changes that signal the needs of nearby tissues. For example, accumulation of carbon dioxide, lactic acid, or other cellular wastes signals the need for increased flow to the tissues. These signals cause the precapillary sphincter as well as the muscle in nearby arterioles to relax, thus increasing blood flow through capillaries.