Artery walls are built with a smooth inner layer to reduce friction between the blood and the artery, a thick layer of elastic muscle, which allows them to expand and then relax as the pulse moves along the vessel, and a protective outer layer. Veins deal with blood that is being returned to the heart. By now the pressure, in the pulses of blood that the heart pumps through the body, has dropped so much that the blood can not return to the heart on its own. Instead of using a second pump to return the blood to the heart, normal movements of the muscles in our bodies gently massage the veins to push the blood along the vein. Valves in the veins make sure that blood doesn t flow backwards.
Breathing also helps move blood through the veins.
When we take a deep breath, this causes a drop of pressure in the chest that pulls blood up into the large veins in the chest area, which in turn draws blood into the smaller veins in the periphery of the body. The blood vessels that lead from the heart, and the veins that return to it, are the largest vessels in the body. As the blood vessels get further from the heart, they divide into millions of vessels that are tinier with every division. The smallest blood vessels in our bodies are called capillaries. They have thin walls that are only one cell thick.