Inhalation (also known as inspiration) is the flow of air into an organism. It is a vital process for all human life. In humans it is the movement of air from the external environment, through the airways, and into the alveoli. Inhalation (work of breathing) is also a term for a medical or recreational route of drug administration that involves inhaling a gaseous, vaporized, or aerosolized drug.
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:
Hyperaeration or hyperinflation 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.
It causes one form of overexpansion of the lung. Overexpansion, however, can also be caused by increase in lung mass itself.