Inspiration is an active process whereas expiration is a passive process. Inspiration occur when the muscles of diaphragm contract to increase the overall volume of thoracic cavity.
Thus the pressure within the lungs (intra-pulmonary pressure) is less in comparison to the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure. As the muscles use energy for contraction inspiration is called active process. Whereas, during the expiration diaphragm muscles relax without the use of energy intra-pulmonary pressure becomes higher than the atmospheric pressure and air noshes out. Thus, it is a passive process.

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