The surface of the lung is covered by a serous membrane, the visceral pleura, which extends into the fissures. At the hilum, the visceral pleura turns back on itself and forms the parietal pleura, which adheres to the mediastinum, superior surface of the diaphragm, and inner surface of the rib cage An extension of the parietal pleura, the pulmonary ligament, extends from the base of each lung to the diaphragm. The space between the parietal and visceral pleurae is called the pleural cavity. The two membranes are normally separated only by a film of slippery pleural fluid; thus, the pleural cavity is only a potential space, meaning there is normally no room between the membranes, but under pathological conditions this space can fill with air or liquid.The pleurae and pleural fluid have three functions:
1. Reduction of friction. Pleural fluid acts as a lubricant that enables the lungs to expand and contract with minimal friction. In some forms of pleurisy, the pleurae are dry and inflamed and each breath gives painful testimony to the function that the fluid should be serving.
2. Creation of pressure gradient. Pressure in the pleural cavity is lower than atmospheric pressure; as explained later, this assists in inflation of the lungs.
3. Compartmentalization. The pleurae, mediastinum, and pericardium compartmentalize the thoracic organs and prevent infections of one organ from spreading easily to neighboring organs.
