The pleura is a thin membrane that lines the surface of the lungs and the inside of the chest wall outside the lungs. In pleural effusions, fluid accumulates in the space between the layers of pleura. Normally, only teaspoons of watery fluid are present in the pleural space, allowing the lungs to move smoothly within the chest cavity during breathing.

A pleural effusion is a buildup of fluid in the pleural space, an area between the layers of tissue that line the lungs and the chest cavity. It may also be referred to as effusion or pulmonary effusion. The type of fluid that forms a pleural effusion may be categorized as either transudate orexudate. Transudate is usually composed of ultrafiltrates of plasma due to an imbalance in vascular hydrostatic and oncotic forces in the chest (heart failure, cirrhosis), while exudate is typically produced by inflammatory conditions (lung infection, malignancy). Exudative pleural effusions are usually more serious and difficult to treat.