The circulatory system (also called the cardiovascular system) is made up of group of organs that transport blood and the substances it carries (oxygen, nutrients, etc) to and from all parts of the body. The organs of circulatory system consist of vessels that carry the blood and a muscular pump, the heart, that drives the blood.
Of the vessels, the arteries carry blood away from the heart; the main arterial vessel, the aorta, branches into smaller arteries, which in turn branch repeatedly into still smaller vessels and reach all parts of the body. Within the body tissues, the vessels are microscopic capillaries through which gas and nutrient exchange occurs (see respiration). Blood leaving the tissue capillaries enters converging vessels, the veins, to return to the heart and lungs. The human heart is a four-chambered organ with a dividing wall, or septum, that separates it into a right heart for pumping blood from the returning veins into the lungs and a left heart for pumping blood from the lungs to the body via the aorta.
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