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The pulmonary pleurae (''sing.'' pleura) are the two opposing layers of serous membrane overlying the
lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of ...
s and the inside of the surrounding
chest wall The thoracic wall or chest wall is the boundary of the thoracic cavity. Structure The bony skeletal part of the thoracic wall is the rib cage, and the rest is made up of muscle, skin, and fasciae. The chest wall has 10 layers, namely (from sup ...
s. The inner pleura, called the visceral pleura, covers the surface of each lung and 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 An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
. The outer layer, called the parietal pleura, lines the inner surfaces of the thoracic cavity on each side of the
mediastinum The mediastinum (from ) is the central compartment of the thoracic cavity. Surrounded by loose connective tissue, it is an undelineated region that contains a group of structures within the thorax, namely the heart and its vessels, the esopha ...
, and can be subdivided into ''mediastinal'' (covering the side surfaces of the fibrous pericardium,
oesophagus The esophagus ( American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to ...
and thoracic aorta), ''diaphragmatic'' (covering the upper surface of the
diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...
), ''costal'' (covering the inside of rib cage) and cervical (covering the underside of the suprapleural membrane) pleurae. The visceral and the mediastinal parietal pleurae are connected at the root of the lung ("hilum") through a smooth fold known as ''pleural reflections'', and a
bell sleeve A bell sleeve can be either long or short and is usually set smoothly into the armscye (no pleating or shirring) and flares toward the bottom. Bell sleeves end anywhere from the elbow to the wrist. Flared sleeves ending at the upper bicep are s ...
-like extension of visceral pleura hanging under to the hilum is known as the pulmonary ligament. Between two pleurae is a potential space called the pleural cavity (also pleural space), which is normally collapsed and filled with only a tiny amount of serous fluid (''pleural fluid'') secreted by the pleurae, and is clinically considered
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
ous under healthy conditions. The two lungs bounded by parietal pleura, almost fill the thoracic cavity.


Anatomy

Each pleura comprises a superficial serosa made of a simple monolayer of flat (squamous) or cuboidal mesothelial cells with
microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, ...
up to long. The mesothelium is without
basement membrane The basement membrane is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between epithelial tissues including mesothelium an ...
, and supported by a well-vascularized underlying loose connective tissue containing two poorly defined layers of
elastin Elastin is a protein that in humans is encoded by the ''ELN'' gene. Elastin is a key component of the extracellular matrix in gnathostomes (jawed vertebrates). It is highly elastic and present in connective tissue allowing many tissues in the bo ...
-rich laminae. The costal parietal pleurae also have
adipocyte Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. I ...
s in the
subserosa The subserosa or tela subserosa, is a thin layer of tissue in the walls of various organs. It is a layer of connective tissue (usually of the areolar type) between the muscular layer (muscularis externa) and the serosa ( serous membrane). The ...
, 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
rib In vertebrate anatomy, ribs ( la, costae) are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the ches ...
s. Both pleurae are quite firmly attached to their underlying structures, and are usually covered by surface glycocalyces that limit fluid loss and reduce friction. The enclosed space between the parietal and visceral pleurae, known as the pleural space, is normally filled only by a tiny amount (less than ) of serous fluid secreted from the apical region of the parietal pleura. The combination of
surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) t ...
, oncotic pressure, and the fluid pressure drop caused by the inward elastic recoil of the lung parenchyma and the rigidity of the
chest wall The thoracic wall or chest wall is the boundary of the thoracic cavity. Structure The bony skeletal part of the thoracic wall is the rib cage, and the rest is made up of muscle, skin, and fasciae. The chest wall has 10 layers, namely (from sup ...
, results in a normally negative pressure of -5 cm (approximately ) within the pleural space, causing it to mostly stay collapsed as a potential space that acts as a functionally
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
ous interface between the parietal and visceral pleurae. Contracting the respiratory muscles expands the
chest cavity The thoracic cavity (or chest cavity) is the chamber of the body of vertebrates that is protected by the thoracic wall (rib cage and associated skin, muscle, and fascia). The central compartment of the thoracic cavity is the mediastinum. There ...
, 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 the
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * M ...
and bronchioli, thus facilitating
inhalation Inhalation (or Inspiration) happens when air or other gases enter the lungs. Inhalation of air Inhalation of air, as part of the cycle of breathing, is a vital process for all human life. The process is autonomic (though there are exceptions ...
.


Visceral pleura

The visceral pleura (from la, viscera, lit=organ) covers the
lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of ...
surfaces and the hilar structures and extends caudally from the hilum as a
mesentery The mesentery is an organ that attaches the intestines to the posterior abdominal wall in humans and is formed by the double fold of peritoneum. It helps in storing fat and allowing blood vessels, lymphatics, and nerves to supply the intesti ...
-like band called the pulmonary ligament. Each lung is divided into lobes by the infoldings of the pleura as fissures. The fissures are double folds of pleura that section the lungs and help in their expansion, 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. It is an area that is insensitive to pain due to its association with the lung and innervation by visceral sensory neurons. Visceral pleura also forms interlobular septa (that separates secondary pulmonary lobules). Interlobular septa contains connective tissue, pulmonary veins, and lymphatics.


Parietal pleura

The parietal pleura (from la, paries, lit=wall) lines the inside of the thoracic cavity which is set apart from the thoracic wall by the endothoracic fascia. The Parietal includes the inner surface of the rib cage and the upper surface of the
diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...
, as well as the side surfaces of the
mediastinum The mediastinum (from ) is the central compartment of the thoracic cavity. Surrounded by loose connective tissue, it is an undelineated region that contains a group of structures within the thorax, namely the heart and its vessels, the esopha ...
, from which it separates the pleural cavity. It joins the visceral pleura at the pericardial base of the pulmonary hilum and pulmonary ligament as a smooth but acutely angled circumferential junction known as the ''hilar reflection''. The parietal pleura is subdivided according to the surface it covers. * The ''costal pleura'' is the pleural portion covering the inner surfaces of the rib cage, and is separated from the
rib In vertebrate anatomy, ribs ( la, costae) are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the ches ...
s/
cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck ...
s and intercostal muscles by the endothoracic fascia. ** The apical part of the costal pleura, sometimes referred to as the ''cervical pleura'' or ''cupula of pleura'', bulges beyond the thoracic inlet into the posterior triangle of the
neck The neck is the part of the body on many vertebrates that connects the head with the torso. The neck supports the weight of the head and protects the nerves that carry sensory and motor information from the brain down to the rest of the body. In ...
, where it is covered by an extension of the endothoracic fascia known as the suprapleural membrane. This is the most superficial (and thus most vulnerable) part of the pleura and can be punctured by subclavian catheterization or a penetrating neck injury. * The ''diaphragmatic pleura'' is the portion covering the convex upper surface of the
diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...
. Its junction with the costal pleura at the diaphragmatic margin is a sharp gutter known as the ''
costodiaphragmatic recess The costodiaphragmatic recess, also called the costophrenic recess or phrenicocostal sinus, costodiaphragmatic-recess Retrieved May 2011 Imaging In anatomy, the costophrenic angles are the places where the diaphragm (''-phrenic'') meets the ribs ...
'', which has diagnostic significance on plain radiography. * The ''mediastinal pleura'' is the portion covering the lateral surfaces of the
mediastinum The mediastinum (from ) is the central compartment of the thoracic cavity. Surrounded by loose connective tissue, it is an undelineated region that contains a group of structures within the thorax, namely the heart and its vessels, the esopha ...
, predominantly the fibrous pericardium, thoracic aorta, superior vena cava/ azygos vein,
esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
and (very rarely) an enlarged
thymus The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or '' T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders ...
. Its anterosuperior part (especially of the left side) not infrequently can bulge into the
anterior mediastinum The mediastinum (from ) is the central compartment of the thoracic cavity. Surrounded by loose connective tissue, it is an undelineated region that contains a group of structures within the thorax, namely the heart and its vessels, the esophagu ...
behind the upper sternal body and even touch its contralateral counterpart in forced
inhalation Inhalation (or Inspiration) happens when air or other gases enter the lungs. Inhalation of air Inhalation of air, as part of the cycle of breathing, is a vital process for all human life. The process is autonomic (though there are exceptions ...
, but the left and right pleurae ''do not'' communicate unless there is a significant
injury An injury is any physiological damage to living tissue caused by immediate physical stress. An injury can occur intentionally or unintentionally and may be caused by blunt trauma, penetrating trauma, burning, toxic exposure, asphyxiation, o ...
( traumatic or iatrogenic) or
disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
process (e.g. malignancy).


Neurovascular supply

As a rule of thumb, the
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
and nerve supply of a pleura comes from the structures under it. The visceral pleura is supplied by the capillaries that supply the lung surface (from both the pulmonary circulation and the bronchial vessels), and innervated by the
nerve ending A free nerve ending (FNE) or bare nerve ending, is an unspecialized, afferent nerve fiber sending its signal to a sensory neuron. ''Afferent'' in this case means bringing information from the body's periphery toward the brain. They function as cut ...
s from the pulmonary plexus. The parietal pleura is supplied by blood from the cavity wall under it, which can come from the aorta ( intercostal, superior phrenic and inferior phrenic arteries), the internal thoracic arteries (
pericardiacophrenic The pericardiacophrenic artery is a long slender branch of the internal thoracic artery. It anastomoses with the musculophrenic and superior phrenic arteries. Location The pericardiacophrenic artery branches from the internal thoracic artery. I ...
, anterior intercostal and musculophrenic branches), or their
anastomoses An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf veins, or streams. Such a connection may be norm ...
. Similarly, its nerve supply is from its underlying structures — the costal pleura is innervated by the intercostal nerves; the diaphragmatic pleura is innervated by the
phrenic nerve The phrenic nerve is a mixed motor/sensory nerve which originates from the C3-C5 spinal nerves in the neck. The nerve is important for breathing because it provides exclusive motor control of the diaphragm, the primary muscle of respiration. ...
in its central portion around the central tendon, and by the intercostal nerves in its periphery near the costal margin; the mediastinal pleura is innervated by branches of the phrenic nerve over the fibrous pericardium.


Development

The visceral and parietal pleurae, like all
mesothelia The mesothelium is a membrane composed of simple squamous epithelial cells of mesodermal origin, which forms the lining of several body cavities: the pleura (pleural cavity around the lungs), peritoneum ( abdominopelvic cavity including the mesen ...
, both derive from the lateral plate mesoderms. During the third week of
embryogenesis An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
, each lateral mesoderm splits into two layers. The dorsal layer joins overlying somites and
ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...
to form the somatopleure; and the ventral layer joins the underlying
endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
to form the splanchnopleure. 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 the
trilaminar disc A trilaminar embryonic disc (or trilaminary blastoderm, or trilaminar germ disk) is an early stage in the development of triploblastic organisms, which include humans and many other animals. It is the next stage from the earlier bilaminar embryoni ...
, forms a pair of intraembryonic coeloms anterolaterally around the gut tube during the fourth week, with the splanchnopleure on the inner cavity wall and the somatopleure on the outer cavity wall. The
cranial Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
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 caudal portions of the coeloms fuse later below the umbilical vein to become the larger peritoneal cavity, separated from the pericardial cavity by the transverse septum. The two cavities communicate via a slim pair of remnant coeloms adjacent to the upper foregut called the ''pericardioperitoneal canal''. During the fifth week, the developing
lung bud The lung bud sometimes referred to as the respiratory bud forms from the respiratory diverticulum, an embryological endodermal structure that develops into the respiratory tract organs such as the larynx, trachea, bronchi and lungs. It arises from ...
s 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.


Function

As a serous membrane, the pleura secretes a serous fluid (pleural fluid) that contains various lubricating macromolecules such as sialomucin,
hyaluronan Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglyca ...
and
phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
s. These, coupled with the smoothness of the glycocalyces and hydrodynamic lubrication of the pleural fluid itself, reduces the frictional coefficient when the opposing pleural surfaces have to slide against each other during ventilation, thus help improving the
pulmonary compliance The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...
. The
adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...
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 Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle. The average total lung capacity of an adult human male is about 6 litres of air. Tidal breathing is normal, resting breath ...
while maintaining a functional
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
. When
inhalation Inhalation (or Inspiration) happens when air or other gases enter the lungs. Inhalation of air Inhalation of air, as part of the cycle of breathing, is a vital process for all human life. The process is autonomic (though there are exceptions ...
occurs, the contraction of the
diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...
and the external intercostal muscles (along with the bucket/ pump handle movements of the
rib In vertebrate anatomy, ribs ( la, costae) are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the ches ...
s and sternum) increases the
volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). ...
of the pleural cavity, further increasing the negative
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
within the pleural space. As long as the functional
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
remains intact, the lung will be drawn to expand along with the chest wall, relaying a negative
airway The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed in through the nose t ...
pressure that causes an airflow into the lung, resulting in
inhalation Inhalation (or Inspiration) happens when air or other gases enter the lungs. Inhalation of air Inhalation of air, as part of the cycle of breathing, is a vital process for all human life. The process is autonomic (though there are exceptions ...
. Exhalation is however usually passive, caused by elastic recoil of the alveolar walls and relaxation of respiratory muscles. In forced exhalation, the pleural fluid provides some hydrostatic cushioning for the lungs against the rapid change of pressure within the pleural cavity.


Clinical significance

Pleuritis or pleurisy is a inflammatory condition of pleurae. Due to the somatic innervation of the parietal pleura, pleural irritations, especially if from acute causes, often produce a sharp chest pain that is worse by breathing, known as ''pleuritic pain''. Pleural disease or lymphatic blockages can lead to a build-up of serous fluid within the pleural space, known as a pleural effusion. Pleural effusion obliterates the pleural vacuum and can collapse the lung (due to
hydrostatic pressure Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body " fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an i ...
), impairing ventilation and leading to type 2 respiratory failure. 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 A chest tube (also chest drain, thoracic catheter, tube thoracostomy or intercostal drain) is a surgical drain that is inserted through the chest wall and into the pleural space or the mediastinum in order to remove clinically undesired substance ...
, or a thoracoscopic procedure. Infected pleural effusion can lead to pleural empyema, which can create significant
adhesion Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can ...
and fibrosis that require division and decortication. For recurrent pleural effusions, pleurodesis can be performed to establish permanent obliteration of the pleural space.


See also

* Pleural friction rub


References


Sources

*


External links

* * - "X-ray, chest, posteroanterior view" * - "Transverse section through lung" *
Diagram at kent.edu
{{Authority control Pleura