In physiology, respiration is defined as the movement of oxygen from the outside air to the cells within tissues, and thetransport of carbon dioxide in the opposite direction.

The physiological definition of respiration should not be confused with the biochemical definition of respiration, which refers to cellular respiration: the metabolic process by which an organism obtains energy by reacting oxygen with glucose to give water, carbon dioxide and 38ATP (energy). Although physiologic respiration is necessary to sustain cellular respiration and thus life in animals, the processes are distinct: cellular respiration takes place in individual cells of the organism, while physiologic respiration concerns the bulk flow and transport of metabolites between the organism and the external environment.

Gaseous exchange (which in organisms with lungs is called ventilation and includes inhalation and exhalation) is a part of physiologic respiration. Thus, in precise usage, the words breathing and ventilation are hyponyms, not synonyms, ofrespiration; but this prescription is not consistently followed, even by most health care providers, because the termrespiratory rate (RR) is a well-established term in health care, even though it would need to be consistently replaced withventilation rate if the precise usage were to be followed.