The circulatory SYSTEM is a network of organs and vessels that transport blood, oxygen, and other nutrients throughout the body. It is composed of the heart, arteries, veins, and capillaries. The heart pumps the blood, while the arteries and veins carry it to and from the body's tissues. The capillaries are the smallest vessels in the SYSTEM, connecting the arteries and veins and allowing for the exchange of oxygen and other nutrients between the blood and the body's cells.