CBSE Class 10 Answered

Respiration is the result of a vacuum formed within the lungs. 

The diaphragm lines the lower part of the chest cavity, sealing it off from the rest of your body. 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 lungs expand, the cartilage at the ends of the lungs stretch, allowing room for the lungs to hold air. With the ribs expanding outward and the diaphragm lowering downward, this increases the volume of the chest cavity by about half a liter, which is the average inhaling amount.

The exact opposite is the cause for expiration. 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