In physiology, respiration is the movement of oxygen from the outside environment to the cells within tissues, and the transport of carbon dioxide in the opposite direction. The physiological definition of respiration differs from the biochemical definition, which refers to a metabolic process by which an organism obtains energy by oxidising 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 diffusion and transport of metabolites between the organism and the external environment. In animals with lungs, physiological respiration involves respiratory cycles of inhaled and exhaledbreaths. Inhalation is usually an active movement. The contraction of the diaphragm muscle cause a pressure variation, which is equal to the pressures caused by elastic, resistive and inertial components of the respiratory system. In contrast, exhalation is usually a passive process. Breathing in brings air into the lungs where the process of gas exchange takes place between the air in the alveoli and the blood in the pulmonarycapillaries The process of breathing does not fill the alveoli with atmospheric air during each inhalation, but the inhaled air is carefully diluted and thoroughly mixed with a large volume of gas 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 wordsbreathing 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 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 respiration
There are several ways to classify the physiology of respiration:
