Inhalation (or Inspiration) happens when air or other gases enter the lungs.

Inhalation of air

Inhalation of air, as part of the cycle of

breathing Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen. All aerobic creatures need oxygen for cellular ...

, is a vital process for all human life. The process is autonomic (though there are exceptions in some disease states) and does not need conscious control 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.

Other substances – accidental

Examples of accidental inhalation includes inhalation of water (e.g. in drowning), smoke, food, vomitus and less common foreign substances (e.g. tooth fragments, coins, batteries, small toy parts, needles).

Other substances – deliberate

Recreational use

Legal – helium, nitrous oxide (" laughing gas") Illegal – various gaseous, vaporised or aerosolized recreational drugs

Medical use

Diagnostic

Various specialized investigations use the inhalation of known substances for diagnostic purposes. Examples include pulmonary function testing (e.g. nitrogen washout test,

diffusion capacity Diffusing capacity of the lung (DL) (also known as Transfer factor is another expression for the formerly used diffusing capacity.) measures the transfer of gas from air in the lung, to the red blood cells in lung blood vessels. It is part of a c ...

testing ( carbon monoxide, helium, methane)) and diagnostic radiology (e.g. radioactive xenon isotopes).

Therapeutic

Gases and other drugs used in anaesthesia include oxygen, nitrous oxide, helium, xenon, volatile anaesthetic agents. Medication for asthma, croup, cystic fibrosis and some other conditions.

Mechanism

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 the thoracic diaphragm, which results in expansion of the

intrapleural space The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication bet ...

and an increase in negative pressure according to Boyle'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 the

respiratory airways The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed in through the nose to th ...

. In health, these airways begin with the nose. It is possible to begin with the mouth, which is the backup breathing system. However, chronic mouth breathing leads to, or is a sign of, illness. They end in the microscopic dead-end sacs(

alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * ...

) always opened, though the diameters of the various sections can be changed by the sympathetic and

parasympathetic nervous system The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of ...

s. 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. Other muscles that can be involved in inhalation include: *

External intercostal muscles The external intercostal muscles, or external intercostals (Intercostales externi) are eleven in number on both sides. Structure The muscles extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they end ...

Scalene muscles The scalene muscles are a group of three pairs of muscles in the lateral neck, namely the anterior scalene, middle scalene, and posterior scalene. They are innervated by the third to the eight cervical spinal nerves (C3-C8). The anterior and m ...

* Sternocleidomastoid muscle * Trapezius muscle

Hyperinflation

''Hyperinflation'' or ''hyperaeration'' is where the lung volume is abnormally increased, with increased filling of the alveoli. This results in an increased

radiolucency Radiodensity (or radiopacity) is opacity to the radio wave and X-ray portion of the electromagnetic spectrum: that is, the relative inability of those kinds of electromagnetic radiation to pass through a particular material. Radiolucency or hyp ...

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 Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the alve ...

, bronchial

atresia Atresia is a condition in which an orifice or passage in the body is (usually abnormally) closed or absent. Examples of atresia include: *Aural atresia, a congenital deformity where the ear canal is underdeveloped. * Biliary atresia, a condition ...

and mucus plugs in asthma.

Yoga

Yogis such as

B. K. S. Iyengar Bellur Krishnamachar Sundararaja Iyengar (14 December 1918 – 20 August 2014) was an Indian teacher of yoga and author. He is founder of the style of yoga as exercise, known as " Iyengar Yoga", and was considered one of the foremost yoga guru ...

advocate both inhaling and exhaling through the nose in the practice of yoga, rather than inhaling through the nose and exhaling through the mouth. They tell their students that the "nose is for breathing, the mouth is for eating."

References

{{Respiratory physiology Respiration Respiratory physiology