Pleurisy is an inflammation of the pleura. The pleura is a two layered membrane that both encloses the lung and lines the chest cavity.
People have two pleurae, one around each lung. The pleurae act as a protective wrapping, fitting snugly over your lungs. Pleurae are made up of two layers. Normally, there is no space between the inner and outer layer. The layers are joined at the edges, so that the pleura might be compared to a balloon with no air, completely empty of air and wrapped tightly around the outside of each of the lungs.
Normally, there is nothing but a thin layer of Iubricating layer of fluid between the inner pleural lining and the outer pleural lining. The smooth pleura linings and lubricating fluid allow your lungs to move freely in your chest, as they do in normal breathing.
In people with pleurisy, the two layers of pleura get inflamed (red and swollen). This can create a space between the layers called the pleural cavity (cavity means space). In wet pleurisy, this space can fill up with fluid that can get infected.