The pleura are membranes which cover the lung and fold back to attach to the ribs. There’s a tiny space within this fold, where fluid can enter due to a variety of conditions. This can occur without symptoms, but often causes shortness of breath, or chest pain with each breath. On exam, we can’t hear breath sounds in the bottom of the affected lung. Diagnosis of fluid is easily made by chest x-ray.
The harder part is identifying what caused the fluid. If there are effusions in both lungs, and a patient is known to have Heart Failure, that’s the most likely cause. In other cases, it’s usually necessary to remove the fluid for analysis. This is easily done by numbing the skin, then inserting a thin needle.