Veins are the blood vessels that supply blood to the heart. With the exception of the pulmonary vein which carries oxygenated blood to the heart from the lungs, veins only carry deoxygenated blood. The blood which is depleted of oxygen moves from the capillaries to the small veins known as venules. The venules then move in larger veins, which ultimately join to form the inferior and superior vena cavae, which are nothing but big vessels that take in the blood from the head, neck, arms, trunk and legs respectively. The vena cavae carries the blood back into the heart. Just like the walls of an artery, the walls of a vein are composed of three layers. However the layers of a vein are not as muscular and tend to collapse when emptied of blood.