The circulatory (cardiovascular) system has four main parts: the heart and three kinds of blood vessels — arteries, veins and capillaries. Together they move blood around the body, deliver oxygen and nutrients to organs, muscles and tissues, and carry away waste such as carbon dioxide, according to Cleveland Clinic.
The heart, the system’s only organ, is a fist-sized muscular pump with four chambers that circulates about 2,000 gallons (more than 7,500 liters) of blood a day. Arteries, led by the aorta, carry blood away from the heart; capillaries, the tiniest vessels, have thin walls that let oxygen, nutrients and wastes pass into and out of cells; and veins, ending in the superior and inferior venae cavae, return oxygen-poor blood to the heart, which sends it to the lungs to pick up fresh oxygen.