Humans breathe in oxygen and breathe out carbon dioxide. Oxygen is necessary for the body to create energy efficiently, and when cells create energy, they produce carbon dioxide as a waste product. The job of the lungs is to provide the body with oxygen and to get rid of the waste gas, carbon dioxide. When we inhale, air enters our lungs, and oxygen from that air moves to our blood. At the same time, carbon dioxide, a waste gas, moves from the blood to the lungs and is breathed out. The diaphragm is the main breathing muscle, and it contracts when we breathe in, pulling the lungs down, stretching and expanding them. It then relaxes back into a dome position when we breathe out, reducing the amount of air in our lungs. When we exercise, our abdominal muscles are used to push air out of the lungs when we breathe out, which is called forced expiration.