Final answer:
The pleural cavity is a fluid-filled space between the visceral and parietal pleurae that encloses the lungs, helps reduce friction during breathing, and maintains lung expansion.
Step-by-step explanation:
The pleural cavity is a fluid-filled space that exists between the visceral and parietal layers of the pleura surrounding each lung. Within the thoracic cavity, the pleural cavities are divided by the mediastinum, which contains the heart within its own pericardial cavity. The visceral pleura is the inner layer of the pleura that directly covers the lungs, and the parietal pleura is the outer layer attaching to the thoracic wall, mediastinum, and diaphragm. These layers secrete pleural fluid, which acts to lubricate the surfaces, reducing friction and allowing the lungs to move smoothly during breathing. In doing so, pleural fluid creates surface tension that helps keep the lungs expanded against the thoracic wall, facilitating breathing and also helping to prevent the spread of infections between organs.