What is the pleura?

The pleura is the protective double-layered membrane that encases the lungs, consisting of the visceral pleura (which directly covers the lung surface) and the parietal pleura (which lines the chest wall). This structure serves several key functions, including providing a smooth surface for the lungs to expand and contract during breathing, and housing pleural fluid, which lubricates the pleural space and minimizes friction between the lung surface and the chest wall. This fluid-filled space also helps maintain pressure differences essential for respiration.

The other options describe different components of the respiratory system, but they do not accurately define what the pleura is. The main airway leading to the lungs is the trachea, the muscle associated with breathing is the diaphragm, and the main branches of the bronchial tree refer to the bronchi, which lead air into the lungs. These terms, while related to respiratory anatomy and function, do not encompass the definition and characteristics of the pleura, making the understanding of its role critical in respiratory physiology.