Pleural effusion occurs when extra fluid builds up in the space between the chest wall and the lung.
This area is called the pleural space, and it normally has a small amount of fluid (approx. 1 tablespoonful) to help lubricate the lungs as they expand and deflate with breathing. However, when excess fluid accumulates in the pleural space it can cause breathing problems and chest discomfort.
Pleural effusion is usually divided into two broad categories: transudate and exudate. Transudate effusions have a low protein level and are mostly water, they are commonly caused by congestive heart failure or liver and kidney issues. Exudate effusions have a high protein level and are related to inflammatory conditions, infection or malignancy/cancer.