Pleural effusion. Coloured frontal X-ray of the lungs of a 60-year-old patient with a pleural effusion of the left lung (right). The lungs are the red spaces either side of the chest. The left lung is smaller than the right lung due to a pleural effusion, an accumulation of fluid (lower right) in the chest cavity. The liquid compresses the lung, which leads to breathing difficulties. An effusion is treated by aspiring (draining) the fluid, which relieves the pressure.