The pleurae (: pleura) are the two flattened closed sacs filled with
pleural fluid, each ensheathing each
lung
The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...
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
In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...
. 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 is called ''mediastinal'' (
fibrous pericardium,
oesophagus
The esophagus (American English), oesophagus (British English), or œsophagus ( archaic spelling) ( see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), colloquially known also as the food pipe, food tube, or gullet, ...
,
thoracic aorta and its main branches). The ''diaphragmatic'' portion covers the upper surface of the
diaphragm. The ''costal'' portion covers the inside of the
rib cage. Some authors also designate a cervical portion (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 to 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.
Anatomy
Each pleura comprises a superficial
serosa made of a simple
monolayer of flat (squamous) or cuboidal
mesothelial cells with
microvilli up to long. The mesothelium is without
basement membrane, and supported by a well-vascularized underlying
loose connective tissue containing two poorly defined layers of
elastin
Elastin is a protein encoded by the ''ELN'' gene in humans and several other animals. Elastin is a key component in the extracellular matrix of gnathostomes (jawed vertebrates). It is highly Elasticity (physics), elastic and present in connective ...
-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
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. (At the joints of long bones the bone's outer surface is lined with "articular cartila ...
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.
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 (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...
,
oncotic pressure, and the
fluid pressure
In physics, a fluid is a liquid, gas, or other material that may continuously move and deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot r ...
drop caused by the inward
elastic recoil of the
lung parenchyma
file:emphysema, bullous, subpleural and honeycomb fibrosis (4563270966).jpg, upright=1.6, Lung parenchyma showing damage due to large subpleural Focal lung pneumatosis, bullae.
Parenchyma () is the bulk of functional substance in an animal org ...
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 that acts as a functionally
vacuum
A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
ous
interface between the parietal and visceral pleurae. Contracting the
respiratory muscles 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, thus facilitating
inhalation.
Visceral pleura
The visceral pleura (from ) covers the
lung
The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...
surfaces and the
hilar structures and extends
caudally from the hilum as a
mesentery
In human anatomy, the mesentery is an Organ (anatomy), organ that attaches the intestines to the posterior abdominal wall, consisting of a double fold of the peritoneum. It helps (among other functions) in storing Adipose tissue, fat and allowi ...
-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 ) 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, as well as the side surfaces of the
mediastinum, 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
ribs/
cartilage
Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...
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 in many vertebrates that connects the head to the torso. It supports the weight of the head and protects the nerves that transmit sensory and motor information between the brain and the rest of the body. Addition ...
, 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. 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, predominantly the
fibrous pericardium, the
thoracic aorta, the first part of the
subclavian artery, the
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 vei ...
/
azygos vein, the
esophagus
The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...
and (very rarely) an enlarged
thymus
The thymus (: thymuses or thymi) is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. The thymus ...
. 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, but the left and right pleurae ''do not'' communicate unless there is a significant
injury
Injury is physiological damage to the living tissue of any organism, whether in humans, in other animals, or in plants.
Injuries can be caused in many ways, including mechanically with penetration by sharp objects such as teeth or with ...
(
traumatic or
iatrogenic) or
disease
A disease is a particular abnormal condition that adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...
process (e.g.
malignancy).
Neurovascular supply
As a
rule of thumb
In English language, 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 associat ...
, the
blood 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
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 lun ...
and the
bronchial vessels), and innervated by the
nerve endings from the
pulmonary plexus.
The parietal pleura is supplied by blood from the cavity wall under it, which can come from the
aorta
The aorta ( ; : aortas or aortae) is the main and largest artery in the human body, originating from the Ventricle (heart), left ventricle of the heart, branching upwards immediately after, and extending down to the abdomen, where it splits at ...
(
intercostal,
superior phrenic and
inferior phrenic arteries), the
internal thoracic arteries (
pericardiacophrenic,
anterior intercostal and
musculophrenic branches), or their
anastomoses. 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 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, 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 r ...
s. During the third week of
embryogenesis
An embryo ( ) is the initial stage of development for 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 ...
, 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 the o ...
to form the
somatopleure; and the ventral layer joins the underlying
endoderm 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, 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 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
The foregut in humans is the anterior part of the alimentary canal, from the distal esophagus to the first half of the duodenum, at the entrance of the bile duct. Beyond the stomach, the foregut is attached to the abdominal walls by mesentery. ...
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 pleura secretes a
serous fluid (pleural fluid) that contains various lubricating
macromolecule
A macromolecule is a "molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass." Polymers are physi ...
s such as
sialomucin,
hyaluronan
Hyaluronic acid (; abbreviated HA; conjugate acid, conjugate base hyaluronate), also called hyaluronan, is an anion#Anions and cations, anionic, Sulfation, nonsulfated glycosaminoglycan distributed widely throughout connective tissue, connective ...
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
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
*** Respirator, a ma ...
, thus help improving the
pulmonary compliance.
The
adhesive property of the pleural fluid to various cellular surfaces, coupled with its
oncotic pressure and the negative
fluid pressure
In physics, a fluid is a liquid, gas, or other material that may continuously move and deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot r ...
, 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
A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
. When
inhalation occurs, the contraction of the
diaphragm and the
external intercostal muscles (along with the
bucket/
pump handle movements of the
ribs and
sternum
The sternum (: sternums or sterna) 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 bl ...
) increases the
volume
Volume is a measure of regions in 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 and eve ...
within the pleural space. As long as the functional
vacuum
A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
remains intact, the lung will be drawn to expand along with the chest wall, relaying a negative
airway pressure that causes an
airflow
Airflow, or air flow, is the movement 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 air pressure is directly related to altitude, temperat ...
into the lung, resulting in
inhalation.
Exhalation is however usually passive, caused by
elastic recoil of the
alveolar wall
A pulmonary alveolus (; ), also called an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where pulmonary gas exchange takes place. Oxygen is Molecular diffusion#Biology, exchanged for carbon dio ...
s 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
Pleurisy, also known as pleuritis, is inflammation of the membranes that surround the lungs and line the chest cavity (Pulmonary pleurae, pleurae). This can result in a sharp chest pain while breathing. Occasionally the pain may be a constant d ...
is an
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
Lymph () 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 be recirculated. At the origi ...
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), 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, or a
thoracoscopic procedure. Infected pleural effusion can lead to
pleural empyema, which can create significant
adhesion 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
Respiratory system anatomy