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Inhalation (orinspiration) happens when air or other gases enter the lungs.
Inhalation of air, as part of the cycle ofbreathing, is a vital process for all human life. The process is autonomic (though there are exceptions in some disease states) and does not need consciouscontrol or effort. However, breathing can be consciously controlled or interrupted (within limits).
Breathing allows oxygen (which humans and a lot of other species need for survival) to enter the lungs, from where it can be absorbed into the bloodstream.
Examples of accidental inhalation includes inhalation of water (e.g. in drowning), smoke, food, vomitus and less common foreign substances[1] (e.g. tooth fragments, coins, batteries, small toy parts, needles).
Nitrous oxide ("laughing gas") has been used recreationally since 1899 for its ability to induceeuphoria,hallucinogenic states andrelaxation, and is legal in some countries.
Helium can be inhaled to give the voice a reedy, duck-like quality, but this can be dangerous as the gas is anasphyxiant and displaces the oxygen needed for normal respiration.[2]
Various illegal gaseous, vapourised or aerosolizedrecreational drugs exist, and are classed asinhalants.
Various specialized investigations use the inhalation of known substances for diagnostic purposes. Examples includepulmonary function testing (e.g.nitrogen washout test,diffusion capacity testing (carbon monoxide,helium,methane)) anddiagnostic radiology (e.g.radioactive xenon isotopes).
Gases and other drugs used in anaesthesia include oxygen, nitrous oxide, helium, xenon,volatile anaesthetic agents. Medication forasthma, croup, cystic fibrosis and some other conditions.
Inhalation begins with the contraction of the muscles attached to the rib cage; this causes an expansion in the chest cavity. Then takes place the onset of contraction of thethoracic diaphragm, which results in expansion of theintrapleural space and an increase in negative pressure according toBoyle's law. This negative pressure generates airflow because of the pressure difference between the atmosphere and alveolus.
The inflow of air into the lungs occurs via therespiratory airways. In health, these airwaysbegin with the nose.[3][4] It is possible to begin with the mouth, which is the backup breathing system. However, chronicmouth breathing leads to, or is a sign of, illness, and it does not have mucus in the mouth to trap the unwanted substance unlike the nostrils[5][6][7] They end in the microscopic dead-end sacs(alveoli) always opened, though the diameters of the various sections can be changed by thesympathetic andparasympathetic nervous systems. The alveolar air pressure is therefore always close to atmospheric air pressure (about 100 kPa at sea level) at rest, with the pressure gradients that cause air to move in and out of the lungs during breathing rarely exceeding 2–3 kPa.[8][9]
Other muscles that can be involved in inhalation include:[10]
Hyperinflation orhyperaeration is where the lung volume is abnormally increased, with increased filling of the alveoli. This results in an increasedradiolucency on X-ray, a reduction in lung markings and depression of the diaphragm. It may occur in partial obstruction of a large airway, as in e.g.congenital lobar emphysema, bronchialatresia and mucus plugs inasthma.[11]
Yogis such asB. K. S. Iyengar advocate bothinhaling and exhaling through the nose in the practice ofyoga, rather than inhaling through the nose andexhaling through the mouth.[12][13][14] They tell their students that the "nose is for breathing, the mouth is for eating."[13][15][16][12]
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