Lee YCG., Clelland CA., Rahman NM.
The pleural cavity consists of a double-layered membrane lining the inside of the thoracic cavity (parietal pleura) and the outside of the lung surface (visceral pleura). Each pleural membrane consists of a layer of mesothelial cells lined with a brush border of microvilli, and several noncellular layers. The structure of the mesothelial cell and the noncellular layers are variable according to the underlying tissue and the amount of movement of the chest wall. Absorption of pleural fluid occurs via the parietal pleura through direct communications (stomata) between the pleural space and a complex underlying lymphatic network. These allow passage of large molecules, including cells, between the pleural space and the systemic circulation via the parietal lymphatics. A constant layer of pleural fluid is maintained between the visceral and parietal pleura, allowing smooth lung movement and elasticity within the thoracic cage. Fluid may accumulate within the pleural space in pathologic states via a variety of mechanisms which either increase production of pleural fluid or decrease its absorption, or both. Air may enter the pleural space (pneumothorax) from the lung via rupture of blebs through the visceral pleura; or from outside the chest cavity when the pleural space is penetrated accidentally or iatrogenically. The mesothelial cell is multipotent and is the predominant cell type in the pleural cavity. In the normal state, there are few leukocytes in the pleural cavity, but inflammatory cells can be recruited efficiently during pleural inflammation or infection. Cancer cells can invade the pleura and often lead to increased vascular permeability and accumulation of malignant effusions.