The physiological definition of respiration differs from the biological definition ofcellular respiration, which is a metabolic process by which an organism obtains energy (in the form ofATP andNADPH)[2] 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 thediffusion and transport ofmetabolites between the organism and the external environment.
Exchange of gases in the lung occurs byventilation (commonly called breathing) andperfusion.[1] Ventilation refers to the in-and-out movement of air of the lungs and perfusion is the circulation of blood in thepulmonarycapillaries.[1] In mammals, physiological respiration involves respiratory cycles ofinhaled andexhaledbreaths. Inhalation (breathing in) is usually an active movement that bringsair into thelungs where the process ofgas exchange takes place between the air in thealveoli and theblood in the pulmonary capillaries. Contraction of the diaphragm muscle causes a pressure variation, which is equal to the pressures caused by elastic, resistive and inertial components of therespiratory system. In contrast, exhalation (breathing out) is usually a passive process, though there are many exceptions: when generating functional overpressure (speaking, singing, humming, laughing, blowing, snorting, sneezing, coughing,powerlifting); when exhaling underwater (swimming, diving); at high levels of physiological exertion (running, climbing, throwing) where more rapid gas exchange is necessitated; or in some forms of breath-controlledmeditation. Speaking and singing in humans requires sustained breath control that many mammals are not capable of performing.
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 thefunctional residual capacity which remains in the lungs after each exhalation, and whosegaseous composition differs markedly from that of the ambient air. Physiological respiration involves the mechanisms that ensure that the composition of thefunctional residual capacity iskept constant, and equilibrates with the gases dissolved in the pulmonary capillary blood, and thusthroughout the body. Thus, in preciseusage, the wordsbreathing andventilation arehyponyms, notsynonyms, ofrespiration; but thisprescription is not consistently followed, even by mosthealth care providers, because the termrespiratory rate (RR) is a well-established term inhealth care, even though it would need to be consistently replaced withventilation rate if the precise usage were to be followed. During respiration the C-H bonds are broken by oxidation-reduction reaction and so carbon dioxide and water are also produced. The cellular energy-yielding process is called cellular respiration.
