CBSE Class 10 Answered

Diaphragm is a muscular partition separating the abdominal and thoracic cavities. It lines the lower part of the chest cavity, sealing it off from the rest of the body. During respiration, it contracts and relaxes to draw air into and expel air from the lungs.

(When you want to inhale, the dome-shaped diaphragm contracts, straightening itself out. This lowers the pressure in the chest cavity causing air outside the lungs to rush in to fill the space. The low pressure inside pulls in air to equalize the pressure. 

When the diaphragm relaxes, moving upwards, the chest cavity becomes less in volume, raising air pressure inside the lungs, forcing air out into the atmosphere. Muscles, such as the diaphragm, cannot push out, but only contract. When you inhale, different tissues in your chest cavity stretch. Relaxing the diaphragm allows them to return to normal size, which raises the pressure, thus forcing air out.)