Pleura

The pleurae (sg.: pleura) are the two flattened pleural sacs filled with pleural fluid that surround each lung, and lines their surrounding tissues. They are formed of two opposing layers of serous membrane that separate the lungs from the mediastinum, the inside surfaces of the surrounding chest walls and the diaphragm. Although wrapped onto itself resulting in a double layer, each lung is surrounded by a single, continuous pleural membrane.

The pleura that covers the surface of each lung is the visceral pleura. The pleura typically dips between the lobes of the lung as fissures, and is formed by the invagination of lung buds into each thoracic sac during embryonic development. The pleura that lines the chest wall, is the parietal pleura.

The parietal pleura can be subdivided by its region. The cervical pleura goes into the root of the neck; the costal pleura lies next to the ribs; the mediastinal pleura covers the mediastinum, and the diaphragmatic pleura covers the upper surface of the diaphragm.

The pulmonary pleura covers each entire lung, enclosing its parenchyma. At the root of the lung the visceral and parietal pleurae meet and form the hilum. A bell sleeve-like extension of the pulmonary pleura hanging under the hilum is known as the pulmonary ligament.

Between the two layers of the pleura is a space of about 15 μm, called the pleural cavity (also pleural space). It contains a tiny amount of serous fluid (pleural fluid) secreted by the pleurae, at an average pressure that is below the atmospheric pressure under healthy conditions. The two lungs, each bounded by a two-layered pleural sac, almost fill the thoracic cavity.