The pleural cavity, pleural space, or interpleural space, is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication between the membranes, and also to create a pressure gradient.The serous membrane that covers the surface of the lung is the visceral pleura and is separated from the outer membrane the parietal pleura by just the film of pleural fluid in the pleural cavity. [Source: Wikipedia 
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