| Pleura | |
|---|---|
 Lung detail showing the pleurae. Thepleural cavity is exaggerated since normally there is no space between the pulmonary pleurae. | |
| Details | |
| Pronunciation | /ˈplʊərə/ |
| System | Respiratory system |
| Nerve | Intercostal nerves,phrenic nerves,vagus nerve |
| Identifiers | |
| Latin | pleurae pulmonarius |
| MeSH | D010994 |
| TA98 | A07.1.02.001 |
| TA2 | 3322 |
| TH | H3.05.03.0.00001 |
| FMA | 9583 |
| Anatomical terminology | |
Thepleurae (sg.:pleura)[1] are the two flattened closed sacs filled withpleural fluid, each ensheathing eachlung and lining their surrounding tissues, locally appearing as two opposing layers ofserous membrane separating the lungs from themediastinum, the inside surfaces of the surroundingchest walls and thediaphragm. 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 thevisceral 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 thelobes of the lung asfissures, and is formed by theinvagination oflung buds into eachthoracic sac duringembryonic development.[2] 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 theparietal pleura.
A correct anatomical nomenclature refrains from using the ambiguous termsvisceral andparietal in favour of a 4-portion system based on the structures the pleura covers:pulmonary (of the lung proper),costal,diaphragmatic andmediastinal pleura.
Using the verbto 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 calledmediastinal pleura. Thediaphragmatic pleura is the portion that covers the upper surface of thediaphragm. Thecostal pleura portion covers the inside of therib cage. Some authors also designate acervical pleura covering the underside of thesuprapleural membrane.
The pulmonary pleura covers the entire lung parenchyma. It meets the mediastinal pleura at theroot of the lung ("hilum") through a smooth fold known aspleural reflection. Abell sleeve-like extension of the pulmonary pleura hanging under the hilum is known as thepulmonary ligament.
Between the two layers of the pleura is what historically has been referred to as apotential space, which in reality is an actual space of about 15 μm. This is called thepleural cavity (also pleural space).[2] It contains a tiny amount ofserous 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 thethoracic cavity.
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Each pleura comprises a superficialserosa made of a simplemonolayer of flat (squamous) or cuboidalmesothelial cells withmicrovilli up to 6 μm (0.00024 in) long. The mesothelium is withoutbasement membrane, and supported by a well-vascularized underlyingloose connective tissue containing two poorly defined layers ofelastin-rich laminae. The costal parietal pleurae also haveadipocytes in thesubserosa, which present assubpleural/extrapleural fats and arehistologically considered belonging to theendothoracic fascia that separates the subserosa from the innerperiosteum of theribs. Both pleurae are quite firmly attached to their underlying structures, and are usually covered by surfaceglycocalyces that limit fluid loss and reduce friction.
The enclosed space between the parietal and visceral pleurae, known as thepleural space, is normally filled only by a tiny amount (less than 10 mL or 0.34 US fl oz) ofserous fluid secreted from the apical region of the parietal pleura. The combination ofsurface tension,oncotic pressure, and thefluid pressure drop caused by the inwardelastic recoil of thelung parenchyma and the rigidity of thechest wall, results in a normally negative pressure of -5 cmH2O (approximately −3.68 mmHg or −0.491 kPa) within the pleural space, causing it to mostly stay collapsed as apotential space that acts as a functionallyvacuumousinterface between the parietal and visceral pleurae. Contracting therespiratory muscles expands thechest cavity, causing the attached parietal pleura to also expand outwards. If the pleural functional vacuum stays intact, the pleural space will remain as collapsed as possible and cause the visceral pleura to be pulled along outwards, which in turn draws the underlying lung also into expansion. This transmits the pressure negativity into thealveoli andbronchioli, thus facilitatinginhalation.[4][5]
The visceral pleura (fromLatin:viscera,lit. 'organ') covers thelung surfaces and thehilar structures and extendscaudally from the hilum as amesentery-like band called thepulmonary ligament. Each lung is divided intolobes by the infoldings of the pleura as fissures. The fissures are double folds of pleura that section the lungs and help in their expansion,[6] allowing the lung toventilate more effectively even if parts of it (usually the basalsegments) fail to expand properly due tocongestion orconsolidation.The function of the visceral pleura is to produce and reabsorb fluid.[7] It is an area that is insensitive to pain due to its association with the lung and innervation by visceral sensory neurons.[8]
Visceral pleura also forms interlobular septa (that separates secondary pulmonary lobules).[9] Interlobular septa contains connective tissue, pulmonary veins, and lymphatics.[10]
The parietal pleura (fromLatin:paries,lit. 'wall') lines the inside of thethoracic cavity which is set apart from thethoracic wall by the endothoracic fascia. The Parietal includes the inner surface of therib cage and the upper surface of thediaphragm, as well as the side surfaces of themediastinum, from which it separates the pleural cavity. It joins the visceral pleura at thepericardial base of thepulmonary hilum and pulmonary ligament as a smooth butacutely angled circumferential junction known as thehilar reflection.[11]
The parietal pleura is subdivided according to the surface it covers.
As arule of thumb, theblood andnerve supply of a pleura comes from the structures under it. The visceral pleura is supplied by thecapillaries that supply the lung surface (from both thepulmonary circulation and thebronchial vessels), and innervated by thenerve endings from thepulmonary plexus.
The parietal pleura is supplied by blood from the cavity wall under it, which can come from theaorta (intercostal,superior phrenic andinferior phrenic arteries), theinternal thoracic arteries (pericardiacophrenic,anterior intercostal andmusculophrenic branches), or theiranastomoses. Similarly, its nerve supply is from its underlying structures — the costal pleura is innervated by theintercostal nerves; the diaphragmatic pleura is innervated by thephrenic nerve in its central portion around thecentral tendon, and by the intercostal nerves in its periphery near thecostal margin; the mediastinal pleura is innervated by branches of the phrenic nerve over thefibrous pericardium.[12]
The visceral and parietal pleurae, like allmesothelia, both derive from thelateral plate mesoderms. During the third week ofembryogenesis, each lateral mesoderm splits into two layers. The dorsal layer joins overlyingsomites andectoderm to form thesomatopleure; and the ventral layer joins the underlyingendoderm to form thesplanchnopleure.[13] The dehiscence of these two layers creates a fluid-filled cavity on each side, and with the ventral infolding and the subsequent midline fusion of thetrilaminar disc, forms a pair ofintraembryonic coeloms anterolaterally around thegut tube during the fourth week, with the splanchnopleure on the inner cavity wall and the somatopleure on the outer cavity wall.[citation needed]
Thecranial end of the intraembryonic coeloms fuse early to form a single cavity, which rotates anteriorly and apparentlydescends inverted in front of the thorax, and is later encroached by the growingprimordial heart as thepericardial cavity. Thecaudal portions of the coeloms fuse later below theumbilical vein to become the largerperitoneal cavity, separated from the pericardial cavity by thetransverse septum. The two cavities communicate via a slim pair of remnant coeloms adjacent to the upperforegut called thepericardioperitoneal canal. During the fifth week, the developinglung buds begin to invaginate into these canals, creating a pair of enlarging cavities that encroach into the surrounding somites and further displace the transverse septum caudally — namely the pleural cavities. The mesothelia pushed out by the developing lungs arise from the splanchnopleure, and become the visceral pleurae; while the other mesothelial surfaces of the pleural cavities arise from the somatopleure, and become the parietal pleurae.[citation needed]
As aserous membrane, the pleura secretes aserous fluid (pleural fluid) that contains various lubricatingmacromolecules such assialomucin,hyaluronan andphospholipids. These, coupled with the smoothness of the glycocalyces andhydrodynamic lubrication of the pleural fluid itself, reduces thefrictional coefficient when the opposing pleural surfaces have toslide against each other duringventilation, thus help improving thepulmonary compliance.
Theadhesive property of the pleural fluid to various cellular surfaces, coupled with itsoncotic pressure and the negativefluid pressure, also holds the two opposing pleurae in close sliding contact and keeps thepleural space collapsed, maximizing thetotal lung capacity while maintaining a functionalvacuum. Wheninhalation occurs, the contraction of thediaphragm and theexternal intercostal muscles (along with thebucket/pump handle movements of theribs andsternum) increases thevolume of the pleural cavity, further increasing the negativepressure within the pleural space. As long as the functionalvacuum remains intact, the lung will be drawn to expand along with the chest wall, relaying a negativeairway pressure that causes anairflow into the lung, resulting ininhalation.Exhalation is however usually passive, caused byelastic recoil of thealveolar walls and relaxation ofrespiratory muscles. In forced exhalation, the pleural fluid provides some hydrostatic cushioning for the lungs against the rapid change of pressure within the pleural cavity.[medical citation needed]
Pleuritis orpleurisy is aninflammatory condition of pleurae. Due to thesomatic innervation of the parietal pleura, pleural irritations, especially if from acute causes, often produce a sharpchest pain that is worse by breathing, known aspleuritic pain.[14]
Pleural disease orlymphatic blockages can lead to a build-up of serous fluid within the pleural space, known as apleural effusion. Pleural effusion obliterates the pleural vacuum and can collapse the lung (due tohydrostatic pressure), impairing ventilation and leading totype 2 respiratory failure. The condition can be treated by mechanically removing the fluid viathoracocentesis (also known as a "pleural tap") with apigtail catheter, achest tube, or athoracoscopic procedure. Infected pleural effusion can lead topleural empyema, which can create significantadhesion andfibrosis that require division anddecortication. For recurrent pleural effusions,pleurodesis can be performed to establish permanent obliteration of the pleural space.[15]
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