A pleural effusion is caused by fluid leaking into the pleural space.  This is the space between two membranes or layers around the lungs within the chest cavity.   To function normally, this space contains a small amount of fluid which prevents friction and help to ensure smooth movement and expansion of the lungs within the chest.  Unfortunately, under certain circumstances, abnormal amounts of fluid can find themselves in the pleural space and cause expansion of the space, crowding the lungs and causing the symptoms we mentioned earlier.