Breathing is the process that moves air in and out of the lungs of terrestrial vertebrates. Aerobic organisms of these types--such as reptiles, birds and mammals--require oxygen to release energy via respiration, in the form of the metabolism of energy-rich molecules such as glucose. Breathing is only one process that delivers oxygen to where it is needed in the body and removes carbon dioxide. Another important process involves the movement of blood by the circulatory system. Gas exchange occurs in the pulmonary alveoli by passive diffusion of gases between the alveolar gas and the blood in lung capillaries. Once these dissolved gases are in the blood, the heart powers their flow around the body (via the circulatory system). The medical term for normal relaxed breathing is eupnea.

In addition to removing carbon dioxide, breathing results in loss of water from the body. Exhaled air has a relative humidity of 100% because of water diffusing across the moist surface of breathing passages and alveoli.