Pleural effusion is a condition in which fluid builds up in the pleural cavity around the lungs. The lungs are surrounded by a thin membrane called the pleura or pleural lining. Small amounts of pleural fluid normally protects the lungs from rubbing against this membrane. When there is too much of this fluid it is called pleural effusion.

Figure 1: Normal lungs with pleural membrane and small amount of pleural fluid.

Figure 2: Lungs with pleural membrane and a large amount of pleural fluid called pleural effusion.