The pleural cavities are the two potential spaces on each side of the thorax which are lined by visceral pleura on the inside and parietal pleura on the outside. The cavity normally has a very small volume that is largely made up by serous fluid secreted by the pleura. The fluid acts as a lubricant between visceral and parietal layers. Also, it is normally at negative pressure and this acts as the intermediate by which changes in thoracic volume - mediated by excursions of the thoracic cage and diaphragm - force changes in lung volume during respiration; increased thoracic volume decreases the fluid pressure in the closed pleural cavity and this draws the visceral pleura outwards by virtue of increased surface tension.
During a deep inspiration, the lungs expand by the above mechanism to fill the entire pleural cavity. However, at rest there are certain areas within the cavities where the visceral pleura is relatively distant to parietal pleura; these are the pleural recesses.
As a potential space, the pleural cavity may be:
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