Circulatory system: The system that moves blood throughout the body. The circulatory system is composed of the heart, arteries, capillaries, and veins. This remarkable system transports oxygenated blood from the lungs and heart throughout the body via the arteries. The blood goes through the capillaries which are situated between the arteries and veins. And the blood that has been depleted of oxygen by the body is then returned to the lungs and heart via the veins.