Explain the mechanism of expiration in the human body.

Expiration at rest in the human body is a passive process, due to elastic recoil in the muscles responsible to inspiration. The Diaphragmrelaxes and moves up, The intercostal muscles relaxes and the rib cage falls down and moves inwards. These movements cause a decrease in volume of the thoracic cavity. The pressure inside the lungs become higher inside the lungs compared to the pressureof the surrounding air, so the air will flow out of the lungs down a pressure gradient. At the end of expiration the lung will not empty completely, there will be residual volumeof air still in the lungs.