Illustration comparing the anatomical structure of a vein (left) and artery (right). Veins are blood vessels that carry deoxygenated blood from the body to the heart to be reoxygenated. Veins have three layers (from outer to inner): tunica externa, tunica media and tunica intima. Unlike arteries, veins have valves to prevent the backflow of blood. Arteries are thick blood vessels carrying oxygenated blood around the body away from the heart-lung system. Arteries also have three layers (from outer to inner): tunica externa, tunica media and tunica intima. The tunica media in arteries is much thicker in arteries than veins. Red blood cells are shown in the lumen of both the artery and vein.