Final answer:
The membrane that directly covers the outer surface of the lungs is the visceral pleura, which is the inner layer of the pleura located next to the actual lung tissue.
Step-by-step explanation:
The membrane covering the outer surface of the lungs is called the visceral pleura.
The pleura is a serous membrane that includes two layers surrounding the lungs: the visceral pleura and the parietal pleura. The visceral pleura is the inner layer that covers the lungs themselves. It is in direct contact with the lung tissue, extending into the lung fissures. On the other hand, the parietal pleura is the outer layer, lining the thoracic wall, the mediastinum, and the diaphragm. The space between these two layers is known as the pleural cavity, which contains a small amount of fluid. This fluid serves to reduce friction during the lungs' expansion and contraction, making respiration smooth and preventing wear on the lung surfaces. The roles of these membranes are critical as they support the lungs, help maintain optimal pressure in the pleural cavity, and limit the spread of infections. If these layers become inflamed, a condition known as pleurisy can occur, leading to pain and difficulty in breathing due to increased pressure within the thoracic cavity.