Q. How are inhalation and expiration carried out?
The diaphragm exclusive of mammals and the intercostals muscles can contract or relax varying the volume of the thorax the compartment where the lungs are located, the changing of the thorax volume forces inhalation or expiration.
When the thorax volume is increased an internal pressure lower than the atmospheric pressure external is created and gases naturally enter into the lungs when the thorax volume is lowered the internal pressure rises above the external pressure and the air is expelled from the lungs.