Pleural fluid prevents friction between the lungs and the chest cavity during respiration, and is found in a thin space between two layers of tissue. The tissues surrounding the lungs produce the fluid that rhythmically ebbs and flows in the small pleural space.
When the lining tissues become inflamed due to viral infections, the pleural fluid is restricted, and breathing becomes difficult and painful. However, pleural effusion -- the presence of excess fluid -- is indicative of common medical conditions, such as pneumonia, congestive heart failure and autoimmune diseases, such as lupus. An acute lung trauma may cause the formation of a pneumothorax, which is the presence of air or gas in the pleural space, a condition that causes pain, shortness of breath and can lead to lung collapse.