The SmaIIVoIume of Pleural Fluid Originates by Filtration from Capillaries in the Visceral and Parietal Pleura

The pleural space contains a small volume of fluid that pro­vides lubrication between the pleural surfaces. The protein content of pleural fluid is normally low (1.5 g∕dL), but the net Starling forces favor filtration of fluid into the pleural space.

The lymphatic vessels that remove pleural fluid communicate directly with the pleural space through holes (stomata) in the surface of the parietal pleura. Fluid accumulates in the pleural cavity when capillary pressures increase or when vascular per­meability is increased by inflammation of the pleura (pleur­itis). If fibrin accumulates in the pleural space, lymphatic vessels may be obstructed, and drainage of the pleural space may be impaired. As a result, large volumes of fluid can accu­mulate between the lungs and chest wall, impeding ventilation and necessitating drainage by the use of tubes.