Pleural effusion, X-ray. This is a condition in which liquid accumulates in the lining of the chest cavity. It is seen here as the large white area in lower frame, which would appear black in a healthy person. The liquid compresses the lung, which leads to breathing difficulties. In this case, the effusion was caused by a broken rib (not clearly seen). It is treated by aspiring (draining) the fluid, which relieves the pressure.