Basic concept of Air flow is that, for air to flow through a tube or airway, pressure at one end must be higher than the pressure at the other end. Air always flows from the high pressure point to the low pressure point (pressure gradient).

 

The conductive airway begins at the mouth and nose and ends at the small airways near the alveoli. Therefore gas flows into the lungs when the pressure in the alveoli is lower than the pressure at the mouth and nose. Conversely, gas flows out of lungs when the pressure in the alveoli is greater than the pressure at the mouth and nose. When the pressure at the mouth and alveoli are same, as occurs at the end of inspiration or the end of expiration, no gas flow occurs as no pressure gradient exists.