ABSTRACT
INTRODUCTION The pleura is a serous membrane that covers the lung, the mediastinum, the diaphragm, and the rib cage. It is comprised of a visceral layer that covers the lung parenchyma and a parietal layer that lines the inside of the thoracic cavity. A thin film of fluid, pleural fluid, acts as a lubricant to enable normal lung movement during respiration. The area occupied by this thin layer of fluid is the pleural space. Pleural fluid normally originates from within the capillaries in the parietal pleura and is cleared by lymphatics in the same way. The normal volume of pleural fluid in a healthy individual is 1-5 ml although the turnover of pleural fluid is thought to be between 1 and 2 l/day. The hydrostatic gradient in the capillaries in the parietal pleura favours an efflux of fluid into the pleural space. Pressure in the capillaries in the visceral pleura is lower in keeping with that of the pulmonary capillaries. This lower pressure favours resorption of fluid from the visceral surface.
