The diaphragm and other respiratory muscles contract. Because the diaphragm is curved, its contraction compresses the abdominal contents and decompresses the contents of the thorax, causing pleural pressure to fall. Thus, as pleural pressure falls, alveolar pressure falls by an equal amount, becoming subatmospheric.
Air flows into the lungs down the pressure gradient from the mouth to the alveoli .
The lungs and chest expand in volume, causing the recoil pressure of the lung to increase until a new equilibrium is reached.