Respiratory system
The first step is breathing in air, or inhaling.  Inhalation means bringing air rich in oxygen into the body.  Exhalation means giving out air rich in carbon dioxide from the body.  The second step is gas exchange in the lungs where oxygen is diffused into the blood and the carbon dioxide diffuses out of the blood.  The third step is cellular respiration, which makes the chemical energy that the cells in the body need, and carbon dioxide. Finally, the carbon dioxide from cellular respiration is breathed out of the body from the lungs.