Explain the process of inspiration and expiration under normal conditions.

Breathing involves two stages - inspiration and expiration.
Inspiration :
* In take of atmospheric air into the lungs.
* It is an active process, as the muscles contract.
* The diaphragm becomes flat due to contraction of its muscles.
* The contraction of external intercostal muscle lifts up the ribs and sternum.
* This increases the volume of the thoracic chamber.
* It causes increase in the pulmonary volume.
* It forces the air from the outside to move into the lungs.
 

Expiration :
* Release of alveolar air to the exterior.
* It is a passive process.
* Relaxation of the diaphragm and external inter costal muscles, returns the diaphragm and sternum to their normal position.
* It reduces the thoracic voloume and there by the pulmonary volume.
* This leads to an increase in the intrapulmonary pressure causing the expulsion of air from lungs.