Arteries are the blood vessels which deliver oxygen-rich blood from the heart to the tissues of the body. The outer layer of an artery is made of collagen fibers. The middle layer has smooth muscle and elastic fibers. The inner layer is the lining called the endothelium which is smooth.Arteries also have thick, strong walls so that they are able to resist the high pressures that exist near the heart - when the heart pumps blood out to the body it travels at a fast speed and pressure so the strength of the artery is important so that the vessels don't burst.