After leaving the heart, blood first enters large vessels called arteries. These thick walls containing smooth muscle and elastic tissue. With each surge of blood from the ventricles, the arteries expand slightly, like thick-wall balloons. Between the heartbeats they recoil helping to pump the blood and maintain a steady flow through the smaller vessels. Arteries branch into vessels of even smaller diameter called arterioles, which p-lay a major role in determining how blood is distributed within the body.