Anatomy and physiology of the pleural space.

The early development of a coelom, a primitive body cavity, with stretchable mesothelial cells endows the subsequently developed internal organs a great flexibility to expand, retract, and deform. In the pleural cavity, in addition, the lung is maintained in an inflated state by the mechanical coupling between the chest wall and the lung; the mesothelial cells with bush-like elongated microvilli enmesh hyaluronic acid-rich lubricants; both minimize the work of breathing. Normal mesothelial cells are fragile in the air, and their intercellular junctions are narrow but labile. The activated mesothelial cells, on the other hand, are resilient and rich in organelles and enzymes to partake in functions such as fibrinolysis, in order to maintain the patency of the pleural cavity. Fluid and electrolytes permeate freely between normal mesothelial cells; the fluid moves in and out of the pleural cavity following Starling's law, with the endothelium as the main barrier. Transudates in the pleural cavity are formed from alterations of the hydrostatic-osmotic pressure relationship, which normally keeps the pleural cavity dry. More than one factor is usually involved in the formation of exudates. Proteins, particles, and cells in the pleural exudates are removed mainly from the preformed stomas and the lymphatic lacuna present in the lower mediastinum and also in the subcostal region and portions of the diaphragm. This removal of the pleural fluid and particles by the lymphatic route is enhanced by the respiratory movements and is partly responsible for the topographic differences of the pleural pressure. Kampmeier's foci found in the location of the stomas are conglomerates of activated mesothelial and lymphoreticular cells with central capillary and lymphatic vessels. They act like tonsils to impede direct entry of undesirable materials from the pleural cavity into the chest wall and mediastinum.