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The circulation system is the transport system that supplies O2 and substances absorbed from the gastrointestinal tract to the tissues, returns CO2 to the lungs and other products of metabolism to the kidneys, functions in the regulation of body temperature, and distributes hormones and other agents that regulate cell function. The blood, the carries of these substances, is pumped through a closed system of blood vessels by the heart, which in mammals is really 2pumps in series with each other. From the left ventricle, blood is pumped through the arteries and arterioles to the capillaries, where the blood equilibrates with the interstitial fluid. The capillaries drain through venules into the veins and back to the right atrium. This is the major (system) circulation. From the right atrium, blood flows to the right ventricle, which pumps it through the vessels of the lungs —the lesser (pulmonary) circulation —and the left atrium to the left ventricles. In the pulmonary capillaries, the blood equilibrates with the O2 and CO2 in the alveolar air. Some tissue fluids enter
another system of closed vessels, the lymphatics, which drain lymph via the thoracic duct and the right lymphatic duct into the venous systems (the lymphatic circulation). The circulation is controlled by multiple regulatory systems that function is general to maintain adequate capillary blood flow —when possible, in all organs, but particularly in the heart and brain .
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