Arteries are the blood vessels that transport blood away from the heart. When the ventricles of the heart contract, blood is transported into the arteries. Arteries have thick muscular walls, which help them withstand high pressures caused by blood leaving the heart.

The largest artery is the aorta, which begins from the heart's left ventricle. The aorta branches above the heart into several arteries, which continue to branch into thousands of smaller arterioles throughout the body.

Arterioles connect with even smaller blood vessels called capillaries. You can see these capillaries, for example, in the white of your eyes. Oxygen and nutrients are exuded through the walls of the capillaries into the body's cells.

All harmful substances produced in the body are transported from the intercellular space back to the capillaries for further transport.

Veins are the blood vessels that transport blood to the heart. They carry oxygen-poor blood, and therefore appear as blue through the skin. Little pressure remains by the time blood leaves the capillaries and enters the venules. The flow of blood back to the heart is referred to as venous return. It is dependent on muscle action.

The muscle contractions near the veins squeeze the blood through the valves. The venous valves prevent blood from flowing backwards.