The Circulatory system distributes needed materials and removes unneeded ones. It is made up of the heart, blood vessels and blood. The blood is also part of the body's defense system. It has antibodies and white blood cells that protect the body against foreign invaders. When the heart muscle contracts, the blood is forced out into arteries and enters small capillaries.
Blood returns to the heart through veins.
Materials enter and leave the blood across the thin walls of capillaries, which are located near every cell of the body. In almost every case, blood leaving a group of capillaries travels to the heart and then to the lungs for more oxygen before it returns to the capillaries.