Introduction

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.

See the figure Two Views of the Pleura.

In abnormal circumstances, air, blood, plasma (the liquid component of blood without cells), or lymph (a fluid containing white blood cells and fat) can get between the pleural surfaces, creating a space. If too much fluid accumulates (called pleural effusion), one or both lungs may not be able to expand normally with breathing, resulting in the collapse of lung tissue. Air in the pleural space (pneumothorax) can do the same.

See the sidebar Major Causes of Pleurisy.