The lungs are located within individual serous cavities, called the pleural cavities. The lungs with their pleural cavities constitute the major contents of the thoracic cavity. The pleural cavities help to provide lubrication.
Each lung is intimately covered with a serous membrane, the visceral pleura. The outer wall of the pleural cavity is lined with another serous membrane known as the parietal pleura. Areas of the parietal pleura are variously named according to their location.
The mediastinal pleura forms the lateral wall of the mediastinum.
The diaphragmatic pleura covers the superior surface of the diaphragm.
The costal pleura lines the inner surface of the rib cage.
The cupolar pleura is a dome-like extension into the root of the neck. It contains the apex of the lung.
When each lung is in its smaller volume, its corresponding diaphragmatic pleura lies close to the lower costal pleura. The slit-like cavity between them is called the costophrenic sinus.
Fluids of each pleural cavity tend to collect in this sinus, since it is the lowest area for each. When the diaphragm contracts and flattens out, each costophrenic sinus opens up and the inferior portion of the expanding lung occupies this space.