Explain the mechanism of expiration.

(N/A) Expiration occurs when the diaphragm and the external intercostal muscles relax.
This relaxation causes the diaphragm and the sternum to return to their normal positions.
As a result,the thoracic volume decreases,which leads to a decrease in pulmonary volume.
This reduction in volume causes the intra-pulmonary pressure to increase to a level slightly above the atmospheric pressure.
Consequently,air is forced out of the lungs,a process known as expiration.