Pleura

The pleurae (sg.: pleura) 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 the pleura that covers the surface of each lung is often called the visceral pleura. This can lead to some confusion, as the lung is not the only visceral organ covered by the 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 portion of the pleura seen as the outer layer covers the chest wall, the diaphragm and the mediastinum and is often also misleadingly called the parietal pleura.

A correct anatomical nomenclature refrains from using the ambiguous terms visceral and parietal in favour of a 4-portion system based on the structures the pleura covers: pulmonary (of the lung proper), costal, diaphragmatic and mediastinal pleura.

Using the verb to line leads to additional confusion, as this is connected to the concept of concavity, which might not necessarily apply in all cases (the mediastinal surface is concave in some regions and convex in others).

The portion of pleura that covers the mediastinum (fibrous pericardium, oesophagus, thoracic aorta and its main branches) is called mediastinal pleura. The diaphragmatic pleura is the portion that covers the upper surface of the diaphragm. The costal pleura portion covers the inside of the rib cage. Some authors also designate a cervical pleura covering the underside of the suprapleural membrane.

The pulmonary pleura covers the entire lung parenchyma. It meets the mediastinal pleura at the root of the lung ("hilum") through a smooth fold known as pleural reflection. 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 what historically has been referred to as a potential space, which in reality is an actual space of about 15 μm. This is 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.

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. The costal parietal pleurae also have adipocytes in the subserosa, which present as subpleural/extrapleural fats and are histologically considered belonging to the endothoracic fascia that separates the subserosa from the inner periosteum of the ribs. Both pleurae are quite firmly attached to their underlying structures, and are usually covered by surface glycocalyces that limit fluid loss and reduce friction.