Pleural effusion is a condition of the pleura involving fluid build-up. The pleura is a large, thin sheet of tissue that wraps around the outside of your lungs and lines the inside of your chest cavity. Between the layers of the pleura is a very thin space that is normally filled with a small amount of fluid. The fluid helps the two layers of the pleura glide smoothly past each other as your lungs breathe air in and out. Pleural effusion is a condition in which there is excess fluid in the pleural space. The most common cause of pleural effusion is congestive heart failure. Treatment involves the removal of excess fluid to relieve discomfort and treating the underlying condition.