Air first enters your lungs and then into the left part of your heart. It is then driven by your heart into the bloodstream, all the way through your body. The heart pumps blood, which transports essential nutrients, oxygen, and other chemicals to every cell in your body. Once it reaches the cells, oxygen processes the nutrients to release energy. Carbon dioxide isgiven off during this process. The blood delivers carbon dioxide into the right portion of your heart, from which it is pumped to the lungs. Carbon dioxide leaves your body through the lungs when you exhale.