The pleural cavity, also known as the pleural sac or pleural space, is a body cavity that surrounds the lung in humans and other mammals. It is separated from the rest of the perivisceral cavity by the diaphragm. There is a pair of pleural cavities separated from each other by the mediastinum and the pericardial cavity. The actual space of the pleural cavity is normally filled with a thin layer of fluid between the pleura of the lungs and the body wall, which are almost in contact.