ABSTRACT
The fluid collects in the pleural space, the outer border of which (the chest wall) is lined by a membrane comprising a single layer of cells (the parietal pleura). The inner lining covering the outer layer of the lung is termed the visceral pleura. In health, this ‘space’ has zero volume, but is moistened by pleural fluid, which is secreted by the lungs and re-absorbed through the veins and lymphatics. Following inflammation, infection, invasion by cancer or blockage of local lymphatics, there is an exudation of fluid with a high protein content – protein > 30 g/litre. If fluid accumulates in the pleural space faster than the normal rate of re-absorption, a pleural effusion develops. In these circumstances, the volume of fluid can be very large. In order for fluid to be seen at all on a chest X-ray, there needs to be around 500 ml. In extreme cases, there can be a complete obscuration of one lung on the chest X-ray (a ‘white out’). In these extreme cases, 3 litres of fluid or more may have collected.
