Pleural pressure, or Ppl, is the pressure surrounding the lung, within the pleural space. During quiet breathing, the pleural pressure is negative; that is, it is below atmospheric pressure.
The pleura is a thin membrane which invests the lungs and lines the walls of the thoracic cavity. During development the lungs grow into the pleural sacs until they are completely surrounded by them. The side of the pleura that covers the lung is referred to as the visceral pleura and the side of the pleura which covers the chest wall is called the parietal pleura. These two sides are continuous and meet at the hilum of the lung. The two faces of the pleural membranes are directly opposed to one another, and the entire potential space within the pleura contains only a few milliliters of serous pleural fluid.
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. As a result of gravity, in an upright individual the pleural pressure at the base of the lung base is greater (less negative) than at its apex; when the individual lies on his back, the pleural pressure becomes greatest along his back. 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.