Carbon dioxide (CO2) is an invisible gas, devoid of color and odor, produced by many forms of combustion (factories, vehicles). Released into the atmosphere, this gas is one of the major components in the greenhouse effect, which causes global warming. It is also produced in small quantities in the body. Carbon dioxide is actually a gas exchanged between the lungs and the blood. The blood takes on oxygen in the pulmonary alveoli, then the arteries bring the oxygen to the cells that need it, and the product of the reactions using oxygen in the muscles is carbon dioxide. This is then brought back by the veins to the lungs and released into the air after a new gas exchange in the alveoli.