The main function of pleural membranes is to hold the two lungs within their pleural cavity. These membranes do this while ensuring that the lungs remain expanded during out-breaths and can move along each other smoothly during the process of breathing.
The two pleural membranes surround the lungs, which rest on the diaphragm. Each is a double membrane. The outer surface of the pleural membrane is called the visceral pleura, and the membrane that lines the body wall is called the parietal pleura. In between the two layers of the pleural membranes there is pleural liquid. This liquid creates surface tension in between two lungs, in a way similar to a thin layer of water holding together two pieces of glass. The surface tension acts as a cohesive force, which results in negative pressure that is crucial for keeping the lungs from collapsing after an exhale.
The importance of pleural membranes can be seen in instances when the membrane is pierced. This results in the air entering the space between the two pleural membranes. The lung is not held in their proper place, recoils and cannot expand anymore, thus breathing in this lung stops. This condition is called pneumothorax, and the treatment for this condition is to seal the hole in the pleural membrane.