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.