: pleura)[1] are the two flattened closed sacs filled with pleural fluid, each ensheathing each lung and lining their surrounding tissues, locally appearing as two opposing layers of serous membrane separating the lungs from the mediastinum, the inside surfaces of the surrounding chest walls and the diaphragm.

Although wrapped onto itself resulting in an apparent double layer, each lung is surrounded by a single, continuous pleural membrane.

The portion of pleura that covers the mediastinum is called mediastinal (fibrous pericardium, oesophagus, thoracic aorta and its main branches).

It meets the mediastinal pleura at the root of the lung ("hilum") through a smooth fold known as pleural reflection.

Each pleura comprises a superficial serosa made of a simple monolayer of flat (squamous) or cuboidal mesothelial cells with microvilli up to 6 μm (0.00024 in) long.

The mesothelium is without basement membrane, and supported by a well-vascularized underlying loose connective tissue containing two poorly defined layers of elastin-rich laminae.

Both pleurae are quite firmly attached to their underlying structures, and are usually covered by surface glycocalyces that limit fluid loss and reduce friction.

'organ') covers the lung surfaces and the hilar structures and extends caudally from the hilum as a mesentery-like band called the pulmonary ligament.

The fissures are double folds of pleura that section the lungs and help in their expansion,[6] allowing the lung to ventilate more effectively even if parts of it (usually the basal segments) fail to expand properly due to congestion or consolidation.The function of the visceral pleura is to produce and reabsorb fluid.

[citation needed] The cranial end of the intraembryonic coeloms fuse early to form a single cavity, which rotates anteriorly and apparently descends inverted in front of the thorax, and is later encroached by the growing primordial heart as the pericardial cavity.

The two cavities communicate via a slim pair of remnant coeloms adjacent to the upper foregut called the pericardioperitoneal canal.

The adhesive property of the pleural fluid to various cellular surfaces, coupled with its oncotic pressure and the negative fluid pressure, also holds the two opposing pleurae in close sliding contact and keeps the pleural space collapsed, maximizing the total lung capacity while maintaining a functional vacuum.

The condition can be treated by mechanically removing the fluid via thoracocentesis (also known as a "pleural tap") with a pigtail catheter, a chest tube, or a thoracoscopic procedure.