When the air diffuses through the lung tissue, the adjoining air replaces the diffused air in the lungs.
The pressure of the air in the bronchus blows open the bronchioles causing air to move into the lungs.
When the lungs expand, the pressure inside the lungs becomes negative compared to the atmospheric pressure causing air to enter the lung
The muscles of the diaphragm contract causing a sympathetic constriction of the lungs and resultant movement of air.