Inspiration is the process of breathing in air from outside the body into the lungs. It is carried out by creating a air pressure gradient between the lungs and the atmosphere. When air enters the lungs, the diaphragm contracts toward the abdominal cavity, thereby increasing the space in the thoracic cavity for accommodating the inhaled air. Simultaneously. the volume of the thoracic chamber in the anteroposterior axis increases with the contraction of the external intercostal muscles. This causes the ribs and the sternum to move out, thereby increasing the volume of the thoracic chamber in the dorsoventral axis.

Thus, the overall increase in the thoracic volume leads to a similar increase in the pulmonary volume. Due to this increase, the intra-pulmonary pressure becomes lesser than the atmospheric pressure, and hence laeds to the movement of air from outside the body into the lungs.