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

The physiological definition of respiration should not be confused with the biochemical definition, which refers to cellular respiration, a metabolic process by which an organism obtains energy (in the form of ATP) by oxidizing nutrients and releasing waste products. 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.

Physiological respiration involves ventilation of the lung aveoli with atmospheric air moved into and out of the lungs through inhalation and exhalation, otherwise known as breathing. Gas exchange then occurs between the alveolar air and the pulmonary capillary blood, which is then evenly distributed throughout the body by the circulatory system. The process of breathing does not fill the alveoli with atmospheric air during each inhalation (about 350 ml per breath), but the inhaled air is carefully diluted and thoroughly mixed with a large volume of gas (about 2.5 liters in adult humans) known as the functional residual capacity which remains in the lungs after each exhalation, and whose gaseous composition differs markedly from that of the ambient air. Physiological respiration involves the mechanisms that ensure that the composition of the functional residual capacity is kept constant, and equilibrates with the gases dissolved in the pulmonary capillary blood, and thus throughout the body. Thus, in precise usage, the words breathing and ventilation are hyponyms, not synonyms, of respiration; but this prescription is not consistently followed, even by most health care providers, because the term respiratory rate (RR) is a well-established term in health care, even though it would need to be consistently replaced with ventilation rate if the precise usage were to be followed.


Classifications of respirationEdit

See alsoEdit


  • Nelsons VCE Units 1-2 Physical Education. 2010 Cengage Copyright.

Further readingEdit