Inhalation (also known as inspiration) happens when air or other gases enter the lungs.

Diagram showing inhalation.

Inhalation of air

Inhalation of air, as part of the cycle of breathing, 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[1] (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


Various specialized investigations use the inhalation of known substances for diagnostic purposes. Examples include pulmonary function testing (e.g. nitrogen washout test, diffusion capacity testing (carbon monoxide, helium, methane)) and diagnostic radiology (e.g. radioactive xenon isotopes).


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.


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 diaphragm, which results in expansion of the intrapleural space 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. Air enters, inflating the lung through either the nose or the mouth into the pharynx (throat) and trachea before entering the alveoli.

Other muscles that can be involved in inhalation include:[2]


Hyperinflation or hyperaeration is where the lung volume is abnormally increased, with increased filling of the alveoli. This results in an increased radiolucency 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, bronchial atresia and mucous plugs in asthma.[3]

See also


  1. Passàli, D; Lauriello, M; Bellussi, L; Passali, GC; Passali, FM; Gregori, D (2010). "Foreign body inhalation in children: an update". Acta Otorhinolaryngol Ital. 30: 27–32. PMC 2881610. PMID 20559470.
  2. Nosek, Thomas M. Essentials of Human Physiology. Section 4/4ch2/s4ch2_10
  3. "Hyperinflation". Medcyclopaedia. GE. Archived from the original on 2011-12-08.
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