blood vessel

Cross sections of an artery and a vein. Arteries have thicker walls than veins as they have to withstand a higher blood pressure than do veins. Veins have valves to prevent blood from flowing backwards.

Tube that conducts blood either away from or towards the heart in multicellular animals. The principal types are arteries, which conduct blood away from the heart, veins, which conduct blood towards the heart, and capillaries, which conduct blood from arteries to veins. Arteries always carry oxygenated blood and veins deoxygenated blood, with the exception of the pulmonary artery which carries deoxygenated blood from the heart to the lungs, and the pulmonary vein, which carries oxygenated blood from the lungs to the heart.