The pleurae are a serous membrane that covers the lungs and mediastinum. It consists of two layers, the parietal pleura and the visceral pleura. The visceral pleura covers the lungs and mediastinal structures and the parietal pleura lines the thoracic cavity. Between these two layers is a potential space containing a small volume of serous fluid. This fluid has two main functions that are very important for breathing: firstly, to lubricate the layers so they can slide with minimal resistance as the lungs expand and relax; secondly, to provide a surface tension between the two layers, keeping the space between them closed and the underlying lung fully expanded. The parietal pleura neurovascular supply comes from the intercostal nerves and vessels, while the visceral pleural only has autonomic innervation and its blood supply comes from the bronchial vessels.