pleura (plŏrˈə) [key], membranous lining of the upper body cavity and covering for the lungs. The pleura is a two-layered structure: the parietal pleura lines the walls of the chest cage and covers the upper surface of the diaphragm, and the pulmonary pleura, or visceral layer, tightly covers the surface of the lungs. The two layers, which are in fact one continuous sheet of tissue, are generally connected to each other. In humans, the pleural cavity is further separated into left and right sides by the heart and pericardial cavity. There is normally a slight amount of watery fluid within the pleural cavity that lubricates the pleural surfaces and allows the lungs to slide freely over the inner surface of the thoracic wall during breathing. When a lung collapses or develops an infection, a condition known as pleurisy can develop. The pleura becomes inflamed, and the pleural cavity becomes noticeably larger. Pleurisy can be extremely painful, but can be medically eradicated in many cases. Mesothelioma is a tumor of the pleura seen most frequently in asbestos workers.