A pleural effusion is a buildup of fluid between the layers of tissue that line the lungs and chest cavity.
Your body produces pleural fluid in small amounts to lubricate the surfaces of the pleura, the thin tissue that lines the chest cavity and surrounds the lungs. A pleural effusion is an abnormal, excessive collection of this fluid.
There are two different types:
Transudative pleural effusions are caused by fluid leaking into the pleural space. This is caused by increased pressure in the blood vessels or a low blood protein count. Congestive heart failure is the most common cause. Exudative effusions are caused by blocked blood vessels or lymph vessels, inflammation, lung injury, and tumors.