Carbon dioxide (CO2) is a waste product made when the body breaks down food for energy (metabolism). The kidneys and lungs regulate the levels of carbon dioxide, bicarbonate, and carbonic acid in the blood.

The blood carries carbon dioxide to the lungs, where it is exhaled. More than 90% of carbon dioxide in the blood exists in the form of bicarbonate (HCO3). The remainder of the carbon dioxide is either dissolved carbon dioxide gas (CO2) or carbonic acid (H2CO3).