Pulmonary ventilation (breathing) can be defined as the flow of air into the lungs during inhalation (i.e., inspiration) and the inverse process where the air out of the lungs during exhalation (i.e., expiration). The lungs are active during the process of inspiration, while they are passive during expiration. Inspiration happens when the intrapulmonary pressure is below the atmospheric pressure, thereby the lungs expand and air enters the lungs.
