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.
