There are two types of pleural effusion, transudative and exudative. Transudates contain few protein cells and little solid material content and have a clear, pale yellow color. Cardiac failure and, less commonly, liver and kidney disease cause transudative pleural effusion. Exudative effusions are caused by inflammation, infection, and cancer. Exudates have large amounts of protein cells, white blood cells, and immune cells that have migrated into the pleural fluid and deposited in tissues or on tissue surfaces.
Exudates also are pale yellow in color but have a cloudy appearance.
A shiny, thin, transparent membrane called the serous coat, or pleura, covers each 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.