The pleura is a thin, transparent, two-layered membrane that covers the lungs and also lines the inside of the chest wall. The layer that covers the lungs lies in close contact with the layer that lines the chest wall. Between the two thin flexible layers is a small amount of fluid that lubricates them as they slide smoothly over one another with each breath.
In abnormal circumstances, air or excess fluid can get between the pleural surfaces, creating a space. If excess fluid accumulates (called pleural effusion) or if air accumulates (called pneumothorax), one or both lungs may not be able to expand normally with breathing, resulting in the collapse of lung tissue.
Last full review/revision February 2008 by Richard W. Light, MD