Air first enters in the lungs and then into the left part of the heart. it is driven by the heart into blood stream, all the way through the body. the heart pumps blood, which transports essential nutrients, oxygen, and other chemicals to every cell in the body. once it reaches the cells, oxygen processes the nutrients to release energy. carbon dioxide is given off during the process. the blood delivers carbon dioxide into the right portion of the heart, from which it is pumped to the lungs. CO2 leaves the lungs when we exhale.