Air enters the body as a result of the same principle that ensures that gases are exchanged in the alveoli - diffusion; when there is an imbalance of pressure, the gas will move from an area of higher pressure to an area of lower pressure until the pressure is equalised.
Unlike the exchange of gas in the alveoli, however, it is the chest muscles that cause these differences in pressure and thus breathing to occur. The main muscle involved in this process is the diaphragm.
The diaphragm is a central sheet of tendon with muscular fibres at the edges positioned between the chest and the abdomen.
The diaphragm also aids in the expulsion of urine, defaecation and during child birth.
Situated between the ribs are the intercostal muscles. These contract during inspiration to pull the rib cage up and allow for the expansion in the chest cavity. Conversely, they relax when expiration occurs, allowing the rib cage to fall down and inwards.