What is contained within the pleural space?

The pleural space contains pleural fluid, which serves several important functions in the respiratory system. This fluid is produced by the pleural membranes and acts as a lubricant, reducing friction between the two layers of pleura (the visceral pleura that covers the lungs and the parietal pleura that lines the chest cavity) during respiration. Furthermore, this fluid helps maintain surface tension, which is critical for proper lung expansion and contraction.

Air, blood, and tissue are not typically found in the pleural space under normal physiological conditions. The presence of air can indicate a pneumothorax, blood can signify a hemothorax, and tissue outside of the pleura would indicate other pathological states, which do not represent the normal state of the pleural space.