They also help to push the rapid flow of blood when the ventricles are relaxed and the heart is refilling. As the arteries become smaller the tunica media consists almost entirely of smooth muscle. These cannot stretch as much as the larger arteries. They still pass a small amount of blood through them, directing it elsewhere in the circulation. These small arteries are known as arterioles. Arteries contain about 20% of blood at any one time. You can feel your pulse in an artery.

VEINS

Veins are the blood vessels that carry deoxygenated blood back to the heart. The walls of the veins are thinner than the arteries, but they do still have the same three layers in them. You will find less muscle and elastic tissue in the tunica media. Some veins have valves in them to ensure the flow of blood travels to the heart, and not backwards. The smallest of the veins (furthest away from the heart) are known as venules. Veins contain about 75% of blood at ant time. There is lower pressure in the vein, compared to that of the arteries, so no pulse can be felt.

CAPILLARY

Capillaries are the smallest blood vessels in the body. Their structure consists of just a single layer of endothelial cells. Water and other small-molecule substances can pass through this wall. Capillaries act as a link between arteries and veins. No valves can be found in the capillaries. The exchange of blood and tissue takes place at the capillary bed. Capillaries contain about 5% of blood at any time, and no pulse can be felt in one.