Each lung is surrounded by a double serous membrane, the pleura. The part of the pleura covering each lung is called the visceral pleura. The pleura lining the ribs, is the parietal pleura. With each breath, the lungs expand and contract. The whole inner lining that covers the lungs and the ribs secretes mucus which maintains a smooth surface. This smooth, well- lubricated surface helps in easy expansion and contraction of lungs and movement of ribs.