The Pleural Space
A shiny, thin, transparent membrane called the serous coat, or pleura, covers each lung. The inner (visceral) layer of the pleura is attached to the lungs and the outer (parietal) layer is attached to the chest wall.
Both layers are covered with mesothelial cells, which secrete a small amount of fluid (i.e., less than 2 tablespoons) that provides lubrication between the chest wall and the lung. Both layers are held in place by a film of pleural fluid, like two glass microscope slides that are wetted and stuck together.
The pleural space is called a potential space because it is virtually nonexistent. The pleural membranes prevent the lung from making direct contact with the chest wall and the diaphragm. Cells in the pleural space are primarily mesothelial cells that line the surfaces of the pleural membranes and some white blood cells.
The pleural membranes are semipermeable. A small amount of fluid continuously seeps out of the blood vessels through the parietal pleura. The visceral pleura absorbs fluid, which then drains into the lymphatic system and returns to the blood. Protein in the circulation and balanced pressures keep excessive amounts of fluid from seeping out of the blood vessels into the pleural space.
Mechanisms of Pleural Fluid Accumulation
An excessive amount of pleural fluid probably results from a combination of fluid draining into the tissues from the blood vessels and the overproduction of fluid by the mesothelial cells. Fluid accumulates in the pleural space by three mechanisms: increased drainage of fluid into the space, increased production of fluid by cells in the space, and decreased drainage of fluid from the space.
Increased amounts of fluid drain from the circulation when there is hypertension in the venous system (creating pressure imbalance) or when there is too little protein in the blood. Ascites (fluid in the peritoneal space, or abdominal cavity) can drain through small perforations in the diaphragm. A large amount of fluid can drain directly into the pleural space this way.
Abnormal mesothelial cells (as in asbestosis) can produce large amounts of fluid. White blood cells can accumulate in response to infection and inflammation in the pleural space (empyema). These cells produce fluid that is difficult to drain or that are in such large quantities that normal drainage through the lymphatic system simply cannot keep up.
Malignant tumor cells can migrate (or metastasize) to the pleural space from essentially any type of tumor in the body. These cells can attach to either the visceral or parietal pleural surfaces or float freely in the pleural space and produce large amounts of fluid.
When tumor cells block lymphatic drainage, fluid accumulates. If the blockage is located in the central lymphatic drainage system that drains chyle (milky fluid consisting of lymph and fat) to the thoracic duct, fluid rich in chyle accumulates in the pleural space.