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 (pleural fluid) that lubricates them as they slide smoothly over one another with each breath. The area containing the fluid is called the pleural space.
In abnormal circumstances, air or excess fluid can get between the pleural surfaces, enlarging the pleural 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. The pleura can become infected (see Viral Pleuritis).
The mediastinum (chest cavity) refers to an area that is bordered by the breastbone (sternum) in front, the spinal column in back, the neck on top, and the diaphragm below. It contains the heart, the thymus gland, some lymph nodes, and parts of the windpipe (trachea), esophagus, aorta, thyroid gland, and parathyroid glands. It does not include the lungs. The mediastinum is divided into three parts:
Mediastinal masses, abnormal masses such as cysts and tumors, can form in the mediastinum. Mediastinitis may occur when contents from the esophagus leak into the mediastinum, causing irritation and infection. Mediastinitis also may occur after chest surgery (such as those that open the breastbone, for example, median sternotomy). Pneumomediastinum occurs when air enters the mediastinum.