Valves are flap-like structures that prevent the backward flow of blood in a vessel. Capillaries and arterioles do not have valves. Arteries carry oxygen-rich blood away from the heart and do not need valves to prevent backflow because the blood is under high pressure. Veins, on the other hand, carry oxygen-poor blood back to the heart and are under low pressure. To prevent the backflow of blood, veins have valves that keep the blood flowing in the right direction towards the heart. Therefore, the blood vessels that have valves are veins.