- Circulatory system
As blood flows through the Heart, various valves ensure that blood flows through the heart in only one direction. The Heart supplies blood to itself through two coronary arteries and to the body through 20 major arteries.

The Arteries are pliable tubes with thick walls that enable them to withstand the high blood pressure they endure each time the heart beats. They have an elastic structure which helps to even out the fluctuations of blood pressure caused by the heartbeat. An artery's walls consist of three layers: a smooth inner lining; 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 coronary arteries dilate or widen to increase oxygen supply to the heart. Sometimes, arteries widen as much as six times their normal size. Arteries can be thought of as a pipe in which the pipe splits into smaller and smaller pipes and vessels.

The main artery to the body, the Aorta, arches from the left ventricle with oxygen-rich blood, runs down through the chest and into the abdomen. The Pulmonary Artery--leading from the right ventricle--divides into 2 branches that supply blood to the lungs. Other 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.

Veins -The seven major veins in the body bring blood back toward the heart. From the capillaries blood enters small veins, called venules, that merge into larger and larger veins, until they finally join the body's largest vein, the vena cava, returning the oxygen-poor blood to the right atrium of the heart. The vena cava actually has two branches: the lower branch (Inferior Vena cava) brings blood from the lower part of the body, while the upper branch (Superior Vena cava) carries blood from the upper part of the body and the brain. Oxygenated blood from the Lungs travels through the Pulmonary Vein and arrives at the Left Atrium of the Heart .

On its journey from the heart to the tissues, blood is forced through the arteries at high pressure. But on the return journey through the veins and back to the heart, the blood flows at low pressure. It 's kept moving by the muscles in the arms and legs that compress the walls of the veins, and by valves in the veins that prevent the blood from flowing backward. Because each type of blood vessel performs a different job under very different pressure, the structures of the arteries, veins, and capillaries are quite different.

Varicose veins (MCNBC Interactive, new window)

Capillaries - These are the tiny vessels--only slightly wider than a single blood cell--that carry blood between the smallest arteries and the smallest veins. Capillaries form a fine network throughout the body's organs and tissues. It's through the thin capillary walls that blood and its constituent cells pass oxygen and nutrients to tissues and receive carbon dioxide and other waste products for excretion. Capillaries are not open to blood flow all the time; they open and close according to each individual organ's requirements for oxygen and nutrients.

Blood