What is the main reason for expiration to occur?

Step-by-step solution

Breathing involves two stages : inspiration during which atmospheric air is drawn in and expiration by which the alveolar air is released out.The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere. Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure. Similarly, expiration takes place when t