ABSTRACT
The pleural space is approximately 10 to 20 m wide and encompasses the area between the mesothelium of the parietal and visceral pleura (the two layers of the pleura) (1). The pleural space actually contains a tiny amount of fluid (0.3 mL/kg body mass) with a low concentration of protein (∼1 g/dL). The surface pressure of the visceral pleura together with alveolar pressure set the transpulmonary pressure, while the surface pressure of the parietal pleura sets the transthoracic pressure (2). These surface pressures usually differ from the pleural liquid pressure due to the presence of contacts between the lung and chest wall and the additional distortions that result (3,4). In upright humans, the vertical gradient of pleural surface pressure is approximately 0.25 cm H2O/cm height, whereas within the liquid column the gradient is somewhat greater (∼1 cm H2O/cm height) (5). In normal humans, the difference between surface and liquid pleural pressure is relatively small in the bottom of the lung and increases higher up, with liquid pressure becoming more negative than surface pressure.
