The pleural cavity is a potential space which normally contains little fluid.

A pleural effusion is an accumulation of fluid within the pleural space.

The fluid may be either transudative or exudative:

• a transudate results from an alteration in the hydrostatic forces operating across the pleural membrane
• an exudate results from a change in the permeability of the membrane due to inflammation

A pleural effusion will only be detected:

• on a chest radiograph when the volume of the effusion exceeds 300 ml
• clinically when the volume exceeds 500 ml