Respiration (physiology)

In <a href="/facts/Physiology/fh25y2pQ">physiology</a>, respiration is the transport of <a href="/facts/Oxygen/acyn6o8r">oxygen</a> from the outside environment to the <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a> within <a href="/facts/Tissue_(biology)/H7CpML7A">tissues</a>, and the removal of <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a> in the opposite direction to the environment by a <a href="/facts/Respiratory_system/IG8v5JEr">respiratory system</a>.
The physiological definition of respiration differs from the <a href="/facts/Cellular_respiration/kb6gpsvL">biochemical definition</a>, which refers to a metabolic process by which an organism obtains energy (in the form of ATP and NADPH) 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 <a href="/facts/Diffusion/KbAxroBa">diffusion</a> and transport of <a href="/facts/Metabolite/nAyywVtA">metabolites</a> between the organism and the external environment.
Exchange of gases in the lung occurs by ventilation and perfusion. Ventilation refers to the in-and-out movement of air of the lungs and perfusion is the circulation of blood in the pulmonary capillaries. In mammals, physiological respiration involves respiratory cycles of <a href="/facts/Inhalation/eB5VlguF">inhaled</a> and <a href="/facts/Exhalation/5jS1RXA2">exhaled</a> <a href="/facts/Breathing/92Wov2gk">breaths</a>. Inhalation (breathing in) is usually an active movement that brings <a href="/facts/Air/hQ95Jznz">air</a> into the <a href="/facts/Lung/gTWXI6Fj">lungs</a> where the process of <a href="/facts/Gas_exchange/3vUewPNH">gas exchange</a> takes place between the air in the <a href="/facts/Pulmonary_alveolus/aEHP1ATT">alveoli</a> and the <a href="/facts/Blood/03zEulkj">blood</a> in the <a href="/facts/Pulmonary_circulation/mBM1zMzc">pulmonary</a> <a href="/facts/Capillary/4BIzKJAe">capillaries</a>. Contraction of the diaphragm muscle causes a pressure variation, which is equal to the pressures caused by elastic, resistive and inertial components of the <a href="/facts/Respiratory_system/IG8v5JEr">respiratory system</a>. 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, <a href="/facts/Powerlifting/veqDJJI4">powerlifting</a>); 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-controlled <a href="/facts/Meditation/8KjW0lHy">meditation</a>. 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 the <a href="/facts/Functional_residual_capacity/7Yqqe3Vf">functional residual capacity</a> which remains in the lungs after each exhalation, and whose <a href="/facts/Respiratory_system/IG8v5JEr">gaseous composition differs markedly from that of the ambient air</a>. Physiological respiration involves the mechanisms that ensure that the composition of the <a href="/facts/Functional_residual_capacity/7Yqqe3Vf">functional residual capacity</a> is <a href="/facts/Homeostasis/rloTGAHa">kept constant</a>, and equilibrates with the gases dissolved in the pulmonary capillary blood, and thus <a href="/facts/Extracellular_fluid/hzm4qatP">throughout the body</a>. Thus, in precise <a href="/facts/Usage/arBgNK1u">usage</a>, the words breathing and ventilation are <a href="/facts/Hyponymy/bjxUuoRb">hyponyms</a>, not <a href="/facts/Synonym/a3BgMcdu">synonyms</a>, of respiration; but this <a href="/facts/Linguistic_prescription/B8KvsTEf">prescription</a> is not consistently followed, even by most <a href="/facts/Health_care_provider/420AvB6L">health care providers</a>, because the term <a href="/facts/Respiratory_rate/Ja9lsl6E">respiratory rate</a> (RR) is a well-established term in <a href="/facts/Health_care/LPS1WKbI">health care</a>, even though it would need to be consistently replaced with ventilation 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.

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Respiration (physiology)