Pleural effusion is the accumulation of fluid in between the parietal and visceral pleura, called pleural cavity. It can occur by itself or can be the result of surrounding parenchymal disease like infection, malignancy or inflammatory conditions. Pleural effusion is one of the major causes of pulmonary mortality and morbidity.
All healthy humans have a small amount of pleural fluid that lubricates the space and facilitates normal lung movements during respiration. This delicate balance of fluid is maintained by the oncotic and hydrostatic pressure and the lymphatic drainage; disturbances in any one of these systems can lead to a build-up of pleural fluid.