(i) The contraction of the external intercostal muscles and diaphragm increases the volume of the thoracic cavity and lowers the pressure in the lungs.
(ii) To fill up this gap, the fresh air rushes to the lungs resulting in the inspiration.
(iii) The relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions and decreases the volume of the thoracic cavity and subsequently, pressure in the lungs increases.
(iv) To equalise this pressure, the air from the lungs rushes out through the respiratory passage to bring out expiration.