Arteries have walls composed of three layers called the tunica externa, tunica media and tunica intima, from outermost to innermost. Almost all arteries function to carry oxygenated blood from the heart to the rest of the body. These blood vessels must be able to withstand a very high amount of pressure that is exerted from the pumping of the heart. Arterial walls, therefore, are fairly dense, which allows for them to withstand this pressure.
Venous walls are made up of the same three layers as arteries. They function to carry deoxygenated blood from the body back to the heart. The three venous layers are thinner and less elastic than arterial walls, as the veins are not exposed to the same amount of pressure.
The walls of veins, unlike arteries, have valves attached. These valves function to prevent the backflow of blood as it is returned to the heart. In addition, a skeletal muscle pump function in the venous system helps propel blood black to the heart by creating a vacuum due to pressure differences between muscle contractions.
The pulmonary artery and vein are exceptions to these differences. The pulmonary artery, like other arteries, is thick walled, but instead carries deoxygenated blood from the heart to the lungs. Likewise, the pulmonary vein carries oxygenated blood back into the heart from the lungs and is also thick-walled. Both the pulmonary artery and vein contain valves to prevent backflow.