The size of the lung is determined by the difference between the alveolar pressure and the pleural pressure, or the transpulmonary pressure. The bigger the difference, the bigger the lung. Since alveolar pressure is uniform throughout the lung, the top of the lung generally experiences a greater transpulmonary pressure and is therefore more expanded and less compliant than the bottom of the lung. During active expiration, the abdominal muscles are contracted to force up the diaphragm and the resulting pleural pressure can become positive. Positive pleural pressure may temporarily collapse the bronchi and cause limitation of air flow.
Pleural pressure is estimated in human subjects using an esophageal balloon.