The primary function of our heart is pumping the blood throughout our body. The circulatory system plays a crucial role in providing the body with the oxygen needed by cells. Oxygen gets from the lungs to the body's cells by attaching to hemoglobin in the red blood cells, which are carried around the body in blood vessels along with blood. Oxygen diffuses across capillary cell membranes into cells, where it is taken into the mitochondria and used as a hydrogen acceptor during the production of ATP to produce water.