A pleural effusion is a buildup of fluid between the layers of tissue that line the lungs and chest cavity.
Fluid in the chest; Fluid on the lung; Pleural fluid
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.
Two different types of effusions can develop: