Final answer:
Pleural fluid, secreted by mesothelial cells, lines the inside of the pleurae to reduce friction during breathing, and maintain proper lung function.
Step-by-step explanation:
The tissue that lines the inside of the pleurae and decreases friction during breathing is filled with pleural fluid. This fluid is secreted by mesothelial cells found in both the visceral and parietal pleurae. The visceral pleura is the innermost layer that is superficial to the lungs and extends into the lung fissures. On the other hand, the parietal pleura lines the interior of the thoracic cavity.
Their lubrication function is crucial because it not only reduces friction to prevent trauma during breathing but also creates surface tension that helps maintain the lungs' position against the thoracic wall. The pleural cavity, which is the space between the two layers of pleura, is critical in maintaining proper lung function and reducing the chances of pleurisy, a condition where these layers become inflamed.