Inhalation (also known as inspiration) is the flow of the respiratory current into an organism. In humans it is the movement of air from the external environment, through the airways, and into the alveoli.
Inhalation begins with 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: