In order for the body to stay alive, each of its cells must receive a
continuous supply of food and oxygen. At
the same time, carbon dioxide and other materials produced by the cells must be
picked up for removal from the body.
This process is continually maintained by the body's circulatory
system. The primary circulatory system
consists of the heart and blood vessels, which together maintain a continuous
flow of blood through the body delivering oxygen and nutrients to and removing
carbon dioxide and waste products from peripheral tissues. A subsystem of the circulatory system, the
lymphatic system, collects interstiyial fluid and returns it to the blood. The heart pumps oxygen-rich blood from the
lungs to all parts of the body through a network of arteries, and smaller
branches called arterioles. Blood
returns to the heart via small venules, which lead to the larger veins. Arterioles and venules are linked even
smaller vessels called metarterioles.
Capillaries, blood vessels a single cell thick, branch off from the
metarterioles and then rejoin them. The
network of tiny capillaries is where the exchange of oxygen and carbon dioxide
between blood and body cells takes place.
The average adult has over 60,000 miles of blood vessels in their body.