Anatomy and physiology of the pleural space.

The pleural cavity is created between the 4th and 7th week of embryologic development and is lined by the splanchnopleurae and somatopleurae. These embryonic components of visceral and parietal pleurae develop different anatomic characteristics with regard to vascular, lymphatic, and nervous supply. Both pleurae have two layers: a superficial mesothelial cell layer facing the pleural space and an underlying connective tissue layer. Various ultrastructures of the pleura show a close relationship to the basic functions of the pleural membranes, such as local inflammatory response and maintenance of the pleural fluid. The latter function is especially important in the mechanical coupling of the lung and chest wall. The fluid in the pleural space transmits transpleural forces involved in normal respiration, and the maintenance of the optimal volume and thickness is regulated closely. Fluid is filtered into the pleural space according to the net hydrostaticoncotic pressure gradient. It flows downward along a vertical pressure gradient, presumably determined by hydrostatic pressure and resistance to viscous flow. There also may be a net movement of fluid from the costal pleura to the mediastinal and interlobar regions. In these areas, pleural fluid is resorbed primarily through lymphatic stomata on the parietal pleural surface.