The heart supplies blood to itself through two coronary arteries and to parts throughout the body through 20 major arteries. These arteries are pliable tubes with thick walls that enable them to withstand the high blood pressure they endure each time the heart beats. This structure helps even out the peaks and troughs of blood pressure caused by the heartbeat, so that blood flows at a relatively constant pressure by the time it reaches the smaller blood vessels. An artery's walls consist of three layers: a smooth inner lining that allows blood to flow easily; a muscular elastic middle layer; and a tough fibrous outer coating to protect the artery from damage.
When the heart is working hard — such as during exercise — the heart can pump blood out five times the amount of blood it pumps at rest. To accept this much extra blood, all the arteries in the body need to dilate or widen.
Arteries can be thought of as a tree in which the trunk splits into smaller and small branches and twigs. The main artery, the aorta, arches from the left ventricle with oxygen-rich blood, runs down through the chest and into the abdomen. Major arteries branch off from the aorta; they split into smaller and smaller arteries, then to still smaller vessels called arterioles, and finally into tiny capillaries.