Inspiration is the process in which the atmospheric air is drawn inside into the lungs. During inspiration, the dome shaped diaphragm becomes flat. As a result, the volume of the thoracic cavity increases. In accordance with Boyle's law, volume and pressure have inverse relation with each other. Thus, the intrapulmonary pressure becomes lesser than that of the atmospheric pressure. Air always flows from a region of high to low pressure. Thus, in this case, the air flows from outside into the lungs.
When the intrapulmonary pressure becomes greater than that of atmospheric pressure, it results in expulsion of air outside the lungs i.e. expiration.