There are three coats, which include the tunic intima, tunica media, and tunica externa. Arteries are thicker walls close to the pumping action of heart, which must expand. The walls must be strong and stretchy. The aorta is the largest vein in the body. Veins are thinner walls further from heart, which contain valves to prevent backflow. Capillaries are one layer thick and allow blood and tissue exchange.
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