Wednesday, April 11, 2012
The pleural sac is a double-layered sac-like structure which completely surrounds each lung. It is made up of two layers: 1) an inner layer called the visceral pleura, which is firmly attached to the lung by connective tissue; 2) an outer layer, called the parietal pleura, is attached to and lines the interior thoracic wall and diaphragm. The two layers of pleura in each sac are so close to each other that normally they are always in virtual contact, but they are not attached to each other. Rather, they are separated by an extremely thin layer of intrapleural fluid, the total volume of which is only a few milliliters. The intrapleural fluid totally surrounds the lungs and lubricates the pleural surfaces so that they can slide over each other during breathing.