Pressure changes in ventilation refers to the changes in the pressure during the mechanical process of respiration (breathing).
During inspiration the diaphragm contracts and the chest wall expands causing a decrease in the intrapulmonary pressure (the pressure of the gas within the lungs). This negative air pressure within the lungs causes air from outside the lungs (normal atmospheric pressure (generally 760 mmHg) to be sucked into the lungs. During expiration, the diaphragm rellaxes and the chest wall deflates leading to a smaller area within the lungs and therefore a greater inrapulmonary pressure, which causes air to be pushed out of the lungs and into the atmosphere. This is why the expiratory phase of respiration is normally passive, except in air flow limitations, such as asthma.