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"Breath" redirects here. For other uses, see Breath (disambiguation).
For other uses, see Breathing (disambiguation).
Breathing is the process that moves air in and out of the lungs, or the diffusion of oxygen and carbon dioxide to and from the external environment into and out of the blood through other respiratory organs such as gills. For organisms with lungs, breathing is also called pulmonary ventilation, which consists of inhalation (breathing in) and exhalation (breathing out). Breathing is one part of physiological respiration required to sustain life. Aerobic organisms such as birds, mammals, and reptiles require oxygen at cellular level to release energy by metabolizing energy-rich molecules such as fatty acids and glucose. This is often referred to as cellular respiration. Breathing is only one of the processes that delivers oxygen to where it is needed in the body and removes excess carbon dioxide. The next process in this chain of events is the transport of these gases throughout the body by the circulatory system, and then their uptake or release from the respiring cells. Breathing fulfills another vital function: that of regulating the pH of theextracellular fluids of the body. It is, in fact, this homeostatic function which determines the rate and depth of breathing. The medical term for normal relaxed breathing is eupnea.
At the end of each exhalation the adult human lungs still contain 2.5 - 3.0 liters of air, termed the functional residual capacity (FRC). Breathing replaces only about 15% of this volume of gas with each breath. This ensures that the composition of the FRC changes very little during the breathing cycle, and remains significantly different from the composition of the ambient air. The partial pressures of the gases in the blood flowing through thealveolar capillaries equilibrate with the partial pressures of the gases in the FRC, ensuring that the ,and of the arterial blood, and therefore its pH, remain constant. The equilibration of the gases in the alveolar blood with those in the alveolar air (i.e. the gas exchange between the two) occurs by passive diffusion.
Breathing is used for a number of other subsidiary functions, such as speech, expression of the emotions (e.g. laughing. yawning etc.), and, in animals that cannot sweat through the skin, panting.