With pneumonia, if the lungs become edematous, the thickness of the exchange membrane may increase dramatically, restricting gas exchange, and body tissues begin to suffer from hypoxia.
Intrapulmonary pressure is the pressure within the alveoli of the lungs. Intrapleural pressure is the pressure within the intrapleural space. Intrapleural pressure is negative relative to the other two during normal inspiration/expiration.
Equalization of the intrapleural pressure with atmospheric pressure or intrapulmonary pressure immediately causes lung collapse.