Pleural fluid is the fluid that is found between the layers of the pleura, the membranes that line the thoracic cavity and surround the lungs. The space containing the fluid is referred to as the pleural cavity. Normal pleural fluid consists of a small amount of a thin (serous) fluid that functions as a lubricant during breathing.
An excess amount of pleural fluid can be caused by many conditions and is known as a pleural effusion. Some common causes of pleural effusions include heart failure, pneumonia, and autoimmune diseases such as rheumatoid arthritis.
If your doctor finds that you have too much pleural fluid, she may recommend that a sample of the fluid be removed by a procedure called thoracentesis and sent for analysis to help determine the cause (pleural fluid cytology).
With lung cancer, an excess amount of pleural fluid (pleural effusion) is quite common, and can be either benign (non-cancerous) or due to the spread of lung cancer cells into the pleural cavity (malignant pleural effusion).