Inhalation

Inhalation is the movement of air into the lungs. Inhalation is paired with exhalation, the movement of air out of the lungs; together they are ventilation. 

During inhalation the diaphragm contracts thus moving down and the external intercostal muscles contract moving the rib cage up and outwards. This increases the volume of the thoracic cavity therefore decreasing the pressure inside the lungs. This causes air to rush into the lungs.