Diaphragm is the major muscle of inspiration and expiration.
It is located below the lungs which has a dome shape.
At the time of Inspiration it contracts which increases pulmonary volume and reduces the intrapulmonary pressure to less than the atmospheric pressure so that air moves into the lungs.
At the time of expiration , diaphragm relaxes which reduces the pulmonary volume increasing the intrapulmonary pressure more than the atmospheric pressure which leads to expulsion of air from the lungs.