When the lungs expand and contract during normal breathing, they slide back and forth within the pleural cavity. To facilitate this movement, a thin layer of mucoid fluid lies between the parietal and visceral pleurae.
Figure 1 shows the dynamics of fluid exchange in the pleural space. The pleural membrane is a porous, mesenchymal, serous membrane through which small amounts of interstitial fluid transude continually into the pleural space. These fluids carry with them tissue proteins, giving the pleural fluid a mucoid characteristic, which is what allows extremely easy slippage of the moving lungs.
Fig1. Dynamics of fluid exchange in the intrapleural space.
The total amount of fluid in each pleural cavity is normally slight—only a few milliliters. Whenever the quantity becomes more than barely enough to begin flowing in the pleural cavity, the excess fluid is pumped away by lymphatic vessels opening directly from the pleural cavity into (1) the mediastinum, (2) the superior surface of the diaphragm, and (3) the lateral surfaces of the parietal pleura. Therefore, the pleural space—the space between the parietal and visceral pleurae—is called a potential space because it normally is so narrow that it is not obviously a physical space.
“Negative Pressure” in Pleural Fluid. A negative force is always required on the outside of the lungs to keep the lungs expanded. This force is provided by negative pressure in the normal pleural space. The basic cause of this negative pressure is pumping of fluid from the space by the lymphatics (which is also the basis of the negative pressure found in most tissue spaces of the body). Because the normal collapse tendency of the lungs is about −4 mm Hg, the pleural fluid pressure must always be at least as negative as −4 mm Hg to keep the lungs expanded. Actual measurements have shown that the pressure is usually about −7 mm Hg, which is a few millimeters of mercury more negative than the collapse pressure of the lungs. Thus, the negativity of the pleural fluid pressure keeps the normal lungs pulled against the parietal pleura of the chest cavity, except for an extremely thin layer of mucoid fluid that acts as a lubricant.
Pleural Effusion—Collection of Large Amounts of Free Fluid in the Pleural Space. Pleural effusion is analogous to edema fluid in the tissues and can be called “edema of the pleural cavity.” The causes of the effusion are the same as the causes of edema in other tissues, including (1) blockage of lymphatic drainage from the pleural cavity; (2) cardiac failure, which causes excessively high peripheral and pulmonary capillary pressures, leading to excessive transudation of fluid into the pleural cavity; (3) greatly reduced plasma colloid osmotic pressure, thus allowing excessive transudation of fluid; and (4) infection or any other cause of inflammation of the surfaces of the pleural cavity, which increases permeability of the capillary membranes and allows rapid dumping of both plasma proteins and fluid into the cavity.
