Humans breathe out carbon dioxide, which is a waste gas produced by the bodys metabolism. When we inhale, air enters our lungs and oxygen from that air moves to our blood. At the same time, carbon dioxide is produced in our bodies as cells do their jobs. The lungs and respiratory system allow oxygen in the air to be taken into the body, while also sending carbon dioxide out. During exhalation, the diaphragm relaxes and the lungs naturally recoil, and air is gently pushed out. The process of breathing is essential for the human body to obtain the energy it needs to sustain itself and its activities.