membrane lining the thoracic cavity (parietal pleura) and covering the lungs (visceral pleura). The parietal pleura folds back on itself at the root of the lung to become the visceral pleura. In health the two pleurae are in contact. When the lung collapses, however, or when air or liquid collects between the two membranes, the pleural cavity or sac becomes apparent (see pleurisy). There are actually two pleural cavities, the right and the left; each constitutes a closed unit not connected to the other. The glistening surface of the pleura is made up of a sheet of flat cells, the mesothelium, which covers an underlying layer of loose elastic tissue. The pleura exudes a thin fluid that keeps it moist and lubricated.

Major disorders of the pleura include pleurisy, the inflammation of the pleura; pleural effusion, the accumulation of excess fluid between the visceral and parietal pleurae; empyema, the collection of pus in the pleural space; mesothelioma and other tumours of the pleura; chylothorax, the rupture of the thoratic duct; hemothorax, the accumulation of blood in the pleural space; and fibrothorax, the encasement of the lung in fibrin following a severe pleural inflammatory process such as empyema.