Found in most higher animals, the blood in a closed circulatory system is contained within the vessels. Blood vessels leading away from the heart are called arteries. These branch and enter various organs and tissues. They divide into smaller vessels called arterioles which branch again and again to form fine vessels called capillaries. Capillaries are found in between all body cells. Exchange of materials between the cells and the capillaries occur through the walls of the vessels and cell membranes by diffusion. Capillaries unite to form venules which join together to form veins, the blood vessels that transport blood back to the heart. The distribution of blood to the various body parts is well controlled in this system. The blood leaving the heart is a t high pressure, while that returning to the heart is at low pressure.
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