PLEURAL DISEASE



The pleural spaces are defined by the visceral pleura of the lungs and the parietal pleura of the rib cage, diaphragm, and mediastinum. The spaces themselves are potential rather than real,since the visceral and parietal pleura are normally separated by only a thin film of fluid.

The lung’s elastic recoil pulls the visceral pleura inward, and the chest wall’s recoil pulls the par­ietal pleura outward. The net pressure in the pleural space at functional residual capacity is below atmospheric pressure. In the pleural space fluid flows from the parietal surface into the pleural space, with subsequent reabsorption by the capillaries of the visceral pleura . This system is remarkably well-balanced and or­dinarily prevents the collection of significant amounts of fluid despite the formation and ab­sorption of 5 to 10 liters of pleural fluid each day. In addition, fluid and leakage of protein are drained by lymphatics, which can increase their absorptive capacity several-fold.

Fluid accumulates with abnormalities in hy­drostatic and osmotic pressure, increased perme­ability of the capillaries, or lymphatic dysfunc­tion. Pleural inflammation, either infectious or noninfectious, increases permeability and results in the collection of a high-protein pleural fluid. Alterations in the systemic and pulmonary ven­ous pressures, as in heart failure, increase fluid transudation from the parietal capillaries and de­crease reabsorption on the visceral side. Decreas­ing the osmotic pressure (hypoalbuminemia) may also result in more rapid fluid transudation. Fi­nally, lymphatic dysfunction due to anatomical or functional obstruction also facilitates the ac­cumulation of pleural fluid.