In anatomy, the pleural cavity is the potential space between the lungs and the chest wall . It has virtually nothing in it in the normal non-diseased state, except a small amount of pleural fluid. The cavity is lined by specialized epithelium called pleura. The pleura that is connected to the chest wall is called the parietal pleura and is highly sensitive to pain. The pleura that is connected to the lung and other visceral tissues is called the visceral pleura, is not sensitive to pain and has a dual blood supply, from the bronchial and pulmonary arteries. In humans, there is no anatomical connection between the left and right pleural cavities.

During quiet breathing, the cavity normally experiences a negative pressure (compared to the atmosphere) which helps adhere the lungs to the chest wall, so that movements of the chest wall during breathing are coupled closely to move the lungs.

The pleura and pleural fluid function to reduce friction between the lungs and the inside of the chest wall during breathing. This function of the pleural cavity can be disrupted in several ways: -

• Pneumothorax (collapsed lung): air enters the pleural cavity, either from the outside or from the lungs. This can be the result of a penetrating chest wound, or of an internal injury. A tension pneumothorax is where the punctured chest wall forms a one way valve; such that air may enter (through the puncture) on inspiration, but cannot exit on expiration. With each breath air builds up in the chest cavity, compressing the lung still further, thus reducing the surface area available for gas exchange. It is a medical emergency.
• Pleural effusion: fluid accumulates in the pleural space, compressing the lungs. This can result for example from lung cancer, infection or heart failure.
• Mesothelioma: this is a type of cancer caused by asbestos exposure.

An older term, pluracy, is sometimes encountered: it indicated an inflammation of the pleura, especially one causing painful respiration, and could be provoked by a variety of infectious and non-infectious causes.