To sustain life, the body must produce sufficient energy. Energy is produced by burning molecules in food, which is done by the process of oxidation (whereby food molecules are combined with oxygen). Oxidation involves carbon and hydrogen being combined with oxygen to form carbon dioxide and water. The consumption of oxygen and the production of carbon dioxide are thus indispensable to life. It follows that the human body must have an organ system designed to exchange carbon dioxide and oxygen between the circulating blood and the atmosphere at a rate rapid enough for the body's needs, even during peak exercise. The respiratory system enables oxygen to enter the body and carbon dioxide to leave the body.
Last full review/revision August 2006 by Joseph D. Brain, ScD