Pleural effusion is a condition in which fluids begin to build up inside the lungs. This condition is also often referred to as water on the lungs because of the fluid buildup between the tissues of the lungs and the chest cavity. This can severely compromise the respiratory capabilities of the lungs, since the fluid buildup occurs between the chest cavity and the pleura or the outer layers of the lungs and it is this cavity, which lubricates the lungs and helps improve breathing. Though there is a small amount of fluid present in the pleura, an increase in the fluid can create an imbalance.
There are two different types of pleural effusion. The first of them is transudative pleural effusion, which is caused when external fluids leak into the pleural space. An increased pressure outside the lungs may cause this leaking of fluids inside the lungs. Low protein content in the blood vessels may also cause the fluids to leak into the lungs. Another type of pleural effusion is exudative effusion; this is caused by blocking of the blood vessels. Drug reactions and lung injuries may also cause such effusions into the lungs.