Changes in the intrapulmonary pressure (the pressure inside the lungs) is compared with changes in the interpleural pressure (the pressure between the parietal and visceral pleura). The interpleural pressure is ALWAYS negative (unless the lung is punctured) this is one factor that keeps the lungs inflated. Pressure inside the lungs (intrapulmonary) changes from negative to positive in order to inhale or exhale air. Find diagrams, notes, and practice questions at www.sciencewithsusanna.com