Disturbances to any of these can lead to the formation of excess pleural fluid. Pleural fluid volume is maintained by the balance of pulmonary capillary hydrostatic and oncotic pressure, lymphatic drainage and the integrity of the pleural and capillary membranes. The visceral pleura and its capillaries and lymphatic vessels, plays an insignificant part in pleural fluid production and turnover. These vessels ultimately drain into mediastinal lymph nodes. Pleural fluid is absorbed and drained via parietal lymphatic vessels predominantly in the region of the mediastinum and diaphragm where lymphatic vessel numbers have been shown to be greatest. The fluid allows for smooth movement of the lungs during normal ventilation. In health, approximately 0.13 mls/kg bodyweight of pleural fluid lubricates the space between the parietal and visceral pleura. The fluid enters the pleural space down a small pressure gradient. Pleural fluid is produced by filtering from systemic capillaries within the parietal interstitium and production is greatest at the apex of the lung. The pleural space is walled by the parietal pleura which lines the inside of the chest wall, and the visceral pleura which lines the lung. transudate in heart failure exudate in pneumonia or malignancy). This is because it reflects the underlying cause of the effusion (e.g. Mortality and morbidity is also related to the biochemical make-up of the pleural fluid (e.g. Patients with pneumonia have a poorer outcome if it is associated with a para-pneumonic effusion and patients with a malignant pleural effusion (particularly those associated with carcinoma of the lung or gut) have a mean survival time of only a few months. However, pleural effusions are not entirely innocuous. Mortality and morbidity for patients with pleural effusion is related more to the underlying cause than the size of the effusion. heart failure, pneumonia) or a chronic condition already known to the patient (e.g. Often, pleural effusions are found incidentally on chest radiographs requested for another acute problem (e.g. Little data relating specifically to the frequency of pleural effusions presenting to ED exists. There are no reliable figures for the UK, however there are estimated to be 50,000 new cases of pleural effusions due to malignancy alone each year.

In the USA approximately 1.5 million people are diagnosed with a pleural effusion each year. The annual incidence of pleural effusion in the developed world has been estimated at 320 per 100,000 population per year. A pleural effusion is an abnormal collection of fluid within the pleural space.