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The pulmonary pleurae (''sing.'' pleura) are the two opposing layers of
serous membrane The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The 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 t ...
s and the inside of the surrounding chest walls. The inner pleura, called the visceral pleura, covers the surface of each lung and dips between the
lobes of 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 th ...
as ''fissures'', and is formed by the
invagination Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination is a mechanism that takes place during gastrulation. This mechanism or cell movement happens mostly in the vegetal ...
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 c ...
. The outer layer, called the parietal pleura, lines the inner surfaces of the
thoracic 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 ...
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 esophagu ...
, and can be subdivided into ''mediastinal'' (covering the side surfaces of the
fibrous pericardium The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made of ...
,
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 the ...
and
thoracic aorta The descending thoracic aorta is a part of the aorta located in the thorax. It is a continuation of the aortic arch. It is located within the posterior mediastinal cavity, but frequently bulges into the left pleural cavity. The descending thoracic ...
), ''diaphragmatic'' (covering the upper surface of the diaphragm), ''costal'' (covering the inside of
rib cage The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a se ...
) and cervical (covering the underside of the
suprapleural membrane The suprapleural membrane, eponymously known as Sibson's fascia, is a structure described in human anatomy. It is named for Francis Sibson. Anatomy It refers to a thickening of connective tissue that covers the apex of each human lung. It is an ...
) pleurae. The visceral and the mediastinal parietal pleurae are connected at the
root of the lung The root of the lung is a group of structures that emerge at the hilum of each lung, just above the middle of the mediastinal surface and behind the cardiac impression of the lung. It is nearer to the back (posterior border) than the front (an ...
("hilum") through a smooth fold known as ''pleural reflections'', and a bell sleeve-like extension of visceral pleura hanging under to the hilum is known as the
pulmonary ligament The root of the lung is a group of structures that emerge at the hilum of each lung, just above the middle of the mediastinal surface and behind the cardiac impression of the lung. It is nearer to the back (posterior border) than the front (an ...
. Between two pleurae is a
potential space In anatomy, a potential space is a space between two adjacent structures that are normally pressed together (directly apposed). Many anatomic spaces are potential spaces, which means that they are potential rather than realized (with their realiz ...
called the
pleural cavity The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication bet ...
(also pleural space), which is normally collapsed and filled with only a tiny amount of
serous fluid In physiology, serous fluid or serosal fluid (originating from the Medieval Latin word ''serosus'', from Latin ''serum'') is any of various body fluids resembling serum, that are typically pale yellow or transparent and of a benign nature. The fl ...
(''pleural fluid'')
secreted 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical ...
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 dis ...
ous under healthy conditions. The two lungs bounded by parietal pleura, almost fill the
thoracic 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 ...
.


Anatomy

Each pleura comprises a superficial
serosa The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane ...
made of a simple
monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials. C ...
of flat (squamous) or cuboidal
mesothelial cell 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 m ...
s 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, ce ...
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 and ...
, and supported by a well-vascularized underlying
loose connective tissue Loose connective tissue, sometimes called areolar tissue, is a cellular connective tissue with thin and relatively sparse collagen fibers. Its ground substance occupies more volume than the fibers do. It has a viscous to gel-like consisten ...
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. ...
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 s ...
, which present as ''subpleural/extrapleural fats'' and are
histologic Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...
ally considered belonging to the
endothoracic fascia The endothoracic fascia is the layer of loose connective tissue deep to the intercostal spaces and ribs, separating these structures from the underlying pleura. This fascial layer is the outermost membrane of the thoracic cavity. The endothora ...
that separates the subserosa from the inner
periosteum The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones. Endosteum lines the inner surface of the medullary cavity of all long bones. Structu ...
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 ch ...
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 The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication be ...
, is normally filled only by a tiny amount (less than ) of
serous fluid In physiology, serous fluid or serosal fluid (originating from the Medieval Latin word ''serosus'', from Latin ''serum'') is any of various body fluids resembling serum, that are typically pale yellow or transparent and of a benign nature. The fl ...
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) to ...
,
oncotic pressure Oncotic pressure, or colloid osmotic-pressure, is a form of osmotic pressure induced by the proteins, notably albumin, in a blood vessel's plasma (blood/liquid) that causes a pull on fluid back into the capillary. Participating colloids displace ...
, and the
fluid 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 and e ...
drop caused by the inward
elastic recoil Elastic recoil means the rebound of the lungs after having been stretched by inhalation,Sherwood, L. (2007). ''Human Physiology: From Cells to Systems'', 6th ed, Thomson Brooks/Cole, Belmont, CA. or rather, the ease with which the lung rebounds. Wi ...
of the
lung parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word � ...
and the rigidity of the chest wall, results in a normally negative pressure of -5 cm (approximately ) within the pleural space, causing it to mostly stay collapsed as a
potential space In anatomy, a potential space is a space between two adjacent structures that are normally pressed together (directly apposed). Many anatomic spaces are potential spaces, which means that they are potential rather than realized (with their realiz ...
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 dis ...
ous
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
between the parietal and visceral pleurae. Contracting the
respiratory muscles The muscles of respiration are the muscles that contribute to inhalation and exhalation, by aiding in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during q ...
expands the chest 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 the alveoli and
bronchioli The bronchioles or bronchioli (pronounced ''bron-kee-oh-lee'') are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start o ...
, 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 t ...
surfaces and the hilar structures and extends
caudal Caudal may refer to: Anatomy * Caudal (anatomical term) (from Latin ''cauda''; tail), used to describe how close something is to the trailing end of an organism * Caudal artery, the portion of the dorsal aorta of a vertebrate that passes into the ...
ly 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 The root of the lung is a group of structures that emerge at the hilum of each lung, just above the middle of the mediastinal surface and behind the cardiac impression of the lung. It is nearer to the back (posterior border) than the front (an ...
. 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 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 ...
which is set apart from the
thoracic 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 ...
by the endothoracic fascia. The Parietal includes the inner surface of the
rib cage The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a se ...
and the upper surface of the diaphragm, 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 esophagu ...
, 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 The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a se ...
, 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 ch ...
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 an ...
s and intercostal muscles by the
endothoracic fascia The endothoracic fascia is the layer of loose connective tissue deep to the intercostal spaces and ribs, separating these structures from the underlying pleura. This fascial layer is the outermost membrane of the thoracic cavity. The endothora ...
. ** 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 The suprapleural membrane, eponymously known as Sibson's fascia, is a structure described in human anatomy. It is named for Francis Sibson. Anatomy It refers to a thickening of connective tissue that covers the apex of each human lung. It is an ...
. 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. Its junction with the costal pleura at the diaphragmatic margin is a sharp gutter known as the '' costodiaphragmatic recess'', 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 esophagu ...
, predominantly the
fibrous pericardium The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made of ...
,
thoracic aorta The descending thoracic aorta is a part of the aorta located in the thorax. It is a continuation of the aortic arch. It is located within the posterior mediastinal cavity, but frequently bulges into the left pleural cavity. The descending thoracic ...
,
superior vena cava The superior vena cava (SVC) is the superior of the two venae cavae, the great venous trunks that return deoxygenated blood from the systemic circulation to the right atrium of the heart. It is a large-diameter (24 mm) short length vein t ...
/
azygos vein The azygos vein is a vein running up the right side of the thoracic vertebral column draining itself towards the superior vena cava. It connects the systems of superior vena cava and inferior vena cava and can provide an alternative path for bloo ...
,
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 th ...
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 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, or o ...
( traumatic or
iatrogenic Iatrogenesis is the causation of a disease, a harmful complication, or other ill effect by any medical activity, including diagnosis, intervention, error, or negligence. "Iatrogenic", ''Merriam-Webster.com'', Merriam-Webster, Inc., accessed 27 ...
) 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 ar ...
process (e.g.
malignancy Malignancy () is the tendency of a medical condition to become progressively worse. Malignancy is most familiar as a characterization of cancer. A ''malignant'' tumor contrasts with a non-cancerous ''benign'' tumor in that a malignancy is not s ...
).


Neurovascular supply

As a
rule of thumb In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...
, the blood and nerve supply of a pleura comes from the structures under it. The visceral pleura is supplied by the
capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
that supply the lung surface (from both the
pulmonary circulation The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs. ...
and the bronchial vessels), and innervated by the nerve endings from the
pulmonary plexus The pulmonary plexus is an autonomic plexus formed from pulmonary branches of vagus nerve and the sympathetic trunk. The plexus is in continuity with the deep cardiac plexus. Structure It innervates the bronchial tree and the visceral pleura ...
. The parietal pleura is supplied by blood from the cavity wall under it, which can come from the
aorta The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes ...
( intercostal, superior phrenic and
inferior phrenic arteries The inferior phrenic arteries are two small vessels which supply the diaphragm. They present much variety in their origin. Structure Origin The inferior phrenic arteries usually arise between T12 and L2 vertebrae. They may arise separately ...
), 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 ...
, anterior intercostal and
musculophrenic The intercostal arteries are a group of arteries that supply the area between the ribs ("costae"), called the intercostal space. The highest intercostal artery (supreme intercostal artery or superior intercostal artery) is an artery in the huma ...
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 norma ...
. Similarly, its nerve supply is from its underlying structures — the costal pleura is innervated by the
intercostal nerve The intercostal nerves are part of the somatic nervous system, and arise from the anterior rami of the thoracic spinal nerves from T1 to T11. The intercostal nerves are distributed chiefly to the thoracic pleura and abdominal peritoneum, and diffe ...
s; 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 ...
in its central portion around the central tendon, and by the intercostal nerves in its periphery near the
costal margin The costal margin, also known as the costal arch, is the lower edge of the chest (thorax) formed by the bottom edge of the rib cage. Structure The costal margin is the medial margin formed by the cartilages of the seventh to tenth ribs. It attac ...
; the mediastinal pleura is innervated by branches of the phrenic nerve over the
fibrous pericardium The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made of ...
.


Development

The visceral and parietal pleurae, like all mesothelia, both derive from the
lateral plate mesoderm The lateral plate mesoderm is the mesoderm that is found at the periphery of the embryo. It is to the side of the paraxial mesoderm, and further to the axial mesoderm. The lateral plate mesoderm is separated from the paraxial mesoderm by a narrow ...
s. 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 sperm c ...
, each lateral mesoderm splits into two layers. The dorsal layer joins overlying
somite The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide in ...
s 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 the o ...
to form the
somatopleure In the anatomy of an embryo, the somatopleure is a structure created during embryogenesis when the lateral plate mesoderm splits into two layers. The outer (or somatic) layer becomes applied to the inner surface of the ectoderm, and with it (parti ...
; 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 gastru ...
to form the
splanchnopleure In the anatomy of an embryo, the splanchnopleuric mesenchyme is a structure created during embryogenesis when the lateral mesodermal germ layer splits into two layers. The inner (or splanchnic) layer adheres to the endoderm, and with it forms the ...
. 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, forms a pair of
intraembryonic coelom In the development of the human embryo the intraembryonic coelom (or somatic coelom) is a portion of the conceptus forming in the mesoderm during the third week of development. During the third week of development, the lateral plate mesoderm split ...
s anterolaterally around the
gut tube The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
during the fourth week, with the splanchnopleure on the inner cavity wall and the somatopleure on the outer cavity wall. 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 pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made o ...
. The
caudal Caudal may refer to: Anatomy * Caudal (anatomical term) (from Latin ''cauda''; tail), used to describe how close something is to the trailing end of an organism * Caudal artery, the portion of the dorsal aorta of a vertebrate that passes into the ...
portions of the coeloms fuse later below the
umbilical vein The umbilical vein is a vein present during fetal development that carries oxygenated blood from the placenta into the growing fetus. The umbilical vein provides convenient access to the central circulation of a neonate for restoration of blood vo ...
to become the larger
peritoneal cavity The peritoneal cavity is a potential space between the parietal peritoneum (the peritoneum that surrounds the abdominal wall) and visceral peritoneum (the peritoneum that surrounds the internal organs). The parietal and visceral peritonea are lay ...
, 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 The foregut is the anterior part of the alimentary canal, from the mouth to the duodenum at the entrance of the bile duct. Beyond the stomach, the foregut is attached to the abdominal walls by mesentery. The foregut arises from the endoderm, devel ...
called the ''pericardioperitoneal canal''. During the fifth week, the developing lung 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.


Function

As a
serous membrane The serous membrane (or serosa) is a smooth tissue membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous membrane ...
, the pleura secretes a
serous fluid In physiology, serous fluid or serosal fluid (originating from the Medieval Latin word ''serosus'', from Latin ''serum'') is any of various body fluids resembling serum, that are typically pale yellow or transparent and of a benign nature. The fl ...
(pleural fluid) that contains various lubricating
macromolecule A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. The ...
s 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 glycosaminoglyc ...
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 Fluid bearings are bearings in which the load is supported by a thin layer of rapidly moving pressurized liquid or gas between the bearing surfaces. Since there is no contact between the moving parts, there is no sliding friction, allowing fl ...
of the pleural fluid itself, reduces the
frictional coefficient Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
when the opposing pleural surfaces have to
slide Slide or Slides may refer to: Places * Slide, California, former name of Fortuna, California Arts, entertainment, and media Music Albums * ''Slide'' (Lisa Germano album), 1998 * ''Slide'' (George Clanton album), 2018 *''Slide'', by Patrick Glees ...
against each other during
ventilation Ventilation may refer to: * Ventilation (physiology), the movement of air between the environment and the lungs via inhalation and exhalation ** Mechanical ventilation, in medicine, using artificial methods to assist breathing *** Ventilator, a ma ...
, 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 ...
. 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 Oncotic pressure, or colloid osmotic-pressure, is a form of osmotic pressure induced by the proteins, notably albumin, in a blood vessel's plasma (blood/liquid) that causes a pull on fluid back into the capillary. Participating colloids displace ...
and the negative
fluid 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 and e ...
, also holds the two opposing pleurae in close sliding contact and keeps the
pleural space The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each lung. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication be ...
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 breathin ...
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 dis ...
. 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 and the
external intercostal muscle The external intercostal muscles, or external intercostals (Intercostales externi) are eleven in number on both sides. Structure The muscles extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they end ...
s (along with the
bucket A bucket is typically a watertight, vertical cylinder or truncated cone or square, with an open top and a flat bottom, attached to a semicircular carrying handle called the '' bail''. A bucket is usually an open-top container. In contrast, a ...
/
pump handle movement Pump-handle is a movement of the ribs that results in a change in the anteroposterior diameter of the thorax. Definition One of the most important functions of the ribs and diaphragm is the change in volume of the thorax that helps inspiration ...
s 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 ch ...
s and
sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sha ...
) 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). The def ...
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 and ...
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 dis ...
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 to th ...
pressure that causes an
airflow Airflow, or air flow, is the movement of air. The primary cause of airflow is the existence of air. Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric ...
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 Exhalation (or expiration) is the flow of the breath out of an organism. In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing. This happens due to elastic properties of the lungs ...
is however usually passive, caused by
elastic recoil Elastic recoil means the rebound of the lungs after having been stretched by inhalation,Sherwood, L. (2007). ''Human Physiology: From Cells to Systems'', 6th ed, Thomson Brooks/Cole, Belmont, CA. or rather, the ease with which the lung rebounds. Wi ...
of the
alveolar wall A pulmonary alveolus (plural: alveoli, from Latin ''alveolus'', "little cavity"), also known as an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where oxygen is exchanged for carbon dioxide. Al ...
s and relaxation of
respiratory muscles The muscles of respiration are the muscles that contribute to inhalation and exhalation, by aiding in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during q ...
. 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 Pleurisy, also known as pleuritis, is inflammation of the membranes that surround the lungs and line the chest cavity (pleurae). This can result in a sharp chest pain while breathing. Occasionally the pain may be a constant dull ache. Other sy ...
or
pleurisy Pleurisy, also known as pleuritis, is inflammation of the membranes that surround the lungs and line the chest cavity (pleurae). This can result in a sharp chest pain while breathing. Occasionally the pain may be a constant dull ache. Other sy ...
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 Chest pain is pain or discomfort in the chest, typically the front of the chest. It may be described as sharp, dull, pressure, heaviness or squeezing. Associated symptoms may include pain in the shoulder, arm, upper abdomen, or jaw, along with n ...
that is worse by breathing, known as ''pleuritic pain''. Pleural disease or
lymphatic Lymph (from Latin, , meaning "water") is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to ...
blockages can lead to a build-up of serous fluid within the pleural space, known as a
pleural effusion A pleural effusion is accumulation of excessive fluid in the pleural space, the potential space that surrounds each lung. Under normal conditions, pleural fluid is secreted by the parietal pleural capillaries at a rate of 0.6 millilitre per kilog ...
. 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 imme ...
), impairing ventilation and leading to type 2 respiratory failure. The condition can be treated by mechanically removing the fluid via
thoracocentesis Thoracentesis , also known as thoracocentesis (from Greek ''thōrax'' 'chest, thorax'—GEN ''thōrakos''—and ''kentēsis'' 'pricking, puncture'), pleural tap, needle thoracostomy, or needle decompression (often used term), is an invasive med ...
(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 substan ...
, or a thoracoscopic procedure. Infected pleural effusion can lead to
pleural empyema Pleural empyema is a collection of pus in the pleural cavity caused by microorganisms, usually bacteria. Often it happens in the context of a pneumonia, injury, or chest surgery. It is one of the various kinds of pleural effusion. There are three ...
, 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 be ...
and
fibrosis Fibrosis, also known as fibrotic scarring, is a pathological wound healing in which connective tissue replaces normal parenchymal tissue to the extent that it goes unchecked, leading to considerable tissue remodelling and the formation of perma ...
that require division and
decortication Decortication is a medical procedure involving the surgical removal of the surface layer, membrane, or fibrous cover of an organ. The procedure is usually performed when the lung is covered by a thick, inelastic pleural peel restricting lung exp ...
. For recurrent pleural effusions,
pleurodesis Pleurodesis is a medical procedure in which part of the pleural space is artificially obliterated. It involves the adhesion of the visceral and the costal pleura. The mediastinal pleura is spared. Uses Pleurodesis is performed to prevent recurr ...
can be performed to establish permanent obliteration of the pleural space.


See also

*
Pleural friction rub A pleural friction rub, or simply pleural rub, is an audible medical sign present in some patients with pleurisy and other conditions affecting the chest cavity. It is noted by listening to the internal sounds of the body, usually using a stethosco ...


References


Sources

*


External links

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