Pleura

The pleurae (: 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.

thin fluid-filled space between the two pulmonary pleurae (visceral and parietal) of each lung