Explanation

The movement of the 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 the atmospheric pressure. Expiration takes place when the intra pulmonary pressure is higher than the atmospheric pressure, i.e., there is positive pressure in the lungs with respect to the atmospheric pressure