Carbon dioxide is closely related to the life process in both animals and plants. Animal life, including people, produces carbon dioxide internally by combining carbon from their food with oxygen obtained from the air that they breathe. This process provides energy and heat for movement and the other life functions. The carbon dioxide that is thus produced is carried by the hemoglobin in the blood to the lungs where it is exchanged for oxygen and exhaled into the atmosphere.
On the other side of this cycle, plant life absorbs carbon dioxide from the atmosphere and with the help of energy from the sun, reduces the carbon dioxide back into carbon compounds and releases the oxygen back into the atmosphere. This process maintains a balance in the atmosphere and constantly renews the available oxygen.