Arteries are large-diameter vessels with a thick, elastic walls adapted for carrying blood at very high pressures. As arteries approach the capillaries, they begin to divide into thinner branches called arterioles. Capillaries are the smallest blood vessels that connect arterioles to venules. Their very thin walls enable the capillaries to be the site of exchange of nutrients and other substances between the blood and tissue cells of the body. Venules are tiny vessels that connect capillaries to veins, which are larger. These veins create pathways, roughly parallel to those used by the arteries, which carry blood back to the heart. Unlike the arteries, however, veins have thinner walls that are less muscular and contain less elastic tissue.