Inspiration is an active process while expiration is a passive process. Inspiration occurs when the muscles of diaphragm contract to increase the overall volume of the thoracic cavity. Thus the pressure within the lungs or intra-pulmonary pressure is less in comparison to the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to the atmospheric pressure. Inspiration is thus called an active process. As the muscles use energy for contraction. During expiration diaphragm muscles relax without the use of energy. Intra-pulmonary pressure becomes higher than the atmospheric pressure and air gushes out. Thus, it is a passive process.