Pleural effusion is inflammation of the pleura. The pleura are a large, thin sheet of tissue that wraps around the outside of your lungs and lines the inside of your chest cavity. Between the layer of the pleura that wraps around your lungs and the layer that lines your chest cavity is a very thin space. This is called the pleural space. Normally it's filled with a small amount of fluid-about 4 teaspoons full. The fluid helps the two layers of the pleura glide smoothly past each other as your lungs breathe air in and out.
Pleural effusion occurs when the two layers of the pleura become red and inflamed. Then they rub against each other every time your lungs expand to breathe in air. This can cause sharp pain with breathing.
Sometimes there is excess fluid in the pleural space. The buildup of fluid usually forces the two layers of the pleura apart so they don't rub against each other when you breathe. This can relieve your pain. However, a large amount of extra fluid can push the pleura against your lung until the lung, or a part of it, collapses. This can make it hard for you to breathe. In some cases of pleural effusion, the extra fluid gets infected and turns into an abscess. This is called an empyema.
Infections like pneumonia are the most common cause of swelling, or inflammation, of the pleura and pleurisy. Heart failure, cancer, or a pulmonary embolism can also lead to a pleural effusion.
This information has been adapted from the National Heart Lung and Blood Institute.