Inhalation or inspiration is the process by which air is brought into the lungs during breathing in human beings.
It involves the following steps:
(i) The external intercostals muscles contract causing ribs to pull out and chest cavity to expand.
(ii) Diaphragm contracts and is brought down a little. This also expands the chest cavity.
(iii) There is a contraction of a abdominal muscles. The expansion of the chest cavity creates a partial vacuum and atmospheric air rushes the lung.