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The pleural cavity, pleural space, or interpleural space is the potential space between the pleurae of the pleural sac that surrounds each
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 ...
. A small amount of serous pleural fluid is maintained in the pleural cavity to enable lubrication between the
membranes A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
, and also to create a pressure gradient. The serous membrane that covers the surface of the lung is the visceral pleura and is separated from the outer membrane the parietal pleura by just the film of pleural fluid in the pleural cavity. The visceral pleura follows the fissures of the lung and the root of the lung structures. The parietal pleura is attached to 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 ...
, 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 ...
, and to the inside of the
ribcage 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 sem ...
.


Structure

In
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
s, the left and right lungs are completely separated by the mediastinum, and there is no communication between their pleural cavities. Therefore, in cases of a unilateral pneumothorax, the contralateral lung will remain functioning normally unless there is a
tension pneumothorax A pneumothorax is an abnormal collection of air in the pleural space between the lung and the chest wall. Symptoms typically include sudden onset of sharp, one-sided chest pain and shortness of breath. In a minority of cases, a one-way valve i ...
, which may shift the mediastinum and the
trachea The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from t ...
, kink the great vessels and eventually collapse the contralateral cardiopulmonary circulation. The visceral pleura receives its blood supply from the
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 � ...
l
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: ...
of the underlying lung, which have input from both the pulmonary and the bronchial circulation. The parietal pleura receives its blood supply from whatever structures underlying it, which can be
branch A branch, sometimes called a ramus in botany, is a woody structural member connected to the central trunk of a tree (or sometimes a shrub). Large branches are known as boughs and small branches are known as twigs. The term '' twig'' usuall ...
ed 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 o ...
( intercostal, superior phrenic and inferior phrenic arteries), the internal thoracic (
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
anastomosis 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 ...
. The visceral pleurae are innervated by splanchnic nerves from the pulmonary plexus, which also innervates the lungs and bronchi. The parietal pleurae however, like their blood supplies, receive nerve supplies from different sources. The costal pleurae (including the portion that bulges above the thoracic inlet) and the periphery of the diaphragmatic pleurae are innervated by the
intercostal nerves 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 di ...
from the enclosing rib cage, which branches off from the T1-T12
thoracic spinal cord A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the ...
. The mediastinal pleurae and central portions of the diaphragmatic pleurae are 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. ...
s. which branches off the C3-C5 cervcial cord. Only the parietal pleurae contain somatosensory nerves and are capable of perceiving pain.


Development

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 the 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 ...
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 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 invertedly and apparently ''descends'' in front of the thorax, and is later encroached by the growing primordial heart as the pericardial cavity. 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 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 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 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. The tissue separating the newly formed pleural cavities from the pericardial cavity are known as the ''pericardiopleural membranes'', which later become the side walls of the fibrous pericardium. The transverse septum and the displaced somites fuse to form the ''pleuroperitoneal membranes'', which separates the pleural cavities from the peritoneal cavity and later becomes 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 ...
.


Function

The pleural cavity, with its associated pleurae, aids optimal functioning of the lungs during
breathing Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen. All aerobic creatures need oxygen for cellu ...
. The pleural cavity also contains pleural fluid, which acts as a
lubricant A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...
and allows the pleurae to slide effortlessly against each other during respiratory movements.
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 ...
of the pleural fluid also leads to close apposition of the lung surfaces with the chest wall. This relationship allows for greater inflation of 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 ...
during breathing. The pleural cavity transmits movements of the ribs muscles to the lungs, particularly during heavy breathing. During inhalation the external intercostals contract, as does 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 ...
. This causes the expansion of the chest wall, that increases the volume of the lungs. A negative pressure is thus created and inhalation occurs.


Pleural fluid

Pleural fluid is a serous fluid produced by the serous membrane covering normal pleurae. Most fluid is produced by the exudation in parietal circulation (
intercostal arteries 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 hu ...
) via
bulk flow Mass flow, also known as mass transfer and bulk flow, is the movement of fluids down a pressure or temperature gradient,Moyes & Schulte (2008). Principles of Animal Physiology. Pearson Benjamin Cummings. San Francisco, California particularly in ...
and reabsorbed by the
lymphatic system The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphatic or lymphoid ...
. Thus, pleural fluid is produced and reabsorbed continuously. The composition and volume is regulated by mesothelial cells in the pleura. In a normal 70 kg human, a few milliliters of pleural fluid is always present within the intrapleural space. Larger quantities of fluid can accumulate in the pleural space only when the rate of production exceeds the rate of reabsorption. Normally, the rate of reabsorption increases as a physiological response to accumulating fluid, with the reabsorption rate increasing up to 40 times the normal rate before significant amounts of fluid accumulate within the pleural space. Thus, a profound increase in the production of pleural fluid—or some blocking of the reabsorbing lymphatic system—is required for fluid to accumulate in the pleural space.


Pleural fluid circulation

The hydrostatic equilibrium model, viscous flow model and capillary equilibrium model are the three hypothesised models of circulation of pleural fluid. According to the viscous flow model, the intra pleural pressure gradient drives a downward viscous flow of pleural fluid along the flat surfaces of ribs.The capillary equilibrium model states that the high negative apical pleural pressure leads to a basal-to-apical gradient at the mediastinal pleural surface, leading to a fluid flow directed up towards the apex (helped by the beating heart and ventilation in lungs).Thus the recirculation of fluid occurs. Finally there's a traverse flow from margins to flat portion of ribs completes the fluid circulation. Absorption occurs into lymphatic vessels at the level of the diaphragmatic pleura.


Clinical significance


Pleural effusion

A pathologic collection of pleural fluid is called 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 k ...
. Mechanisms: # Lymphatic obstruction # Increased
capillary 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 bod ...
permeability # Decreased plasma colloid osmotic pressure # Increased capillary venous pressure # Increased negative intrapleural pressure Pleural effusions are classified as exudative (high protein) or transudative (low protein). Exudative pleural effusions are generally caused by infections such as pneumonia (parapneumonic pleural effusion), malignancy, granulomatous disease such as tuberculosis or coccidioidomycosis, collagen vascular diseases, and other inflammatory states. Transudative pleural effusions occur in congestive heart failure (CHF), cirrhosis or nephrotic syndrome. Localized pleural fluid effusion noted during pulmonary embolism ( PE) results probably from increased capillary permeability due to cytokine or inflammatory mediator release from the platelet-rich thrombi.


Pleural fluid analysis

When accumulation of pleural fluid is noted, cytopathologic evaluation of the fluid, as well as clinical microscopy, microbiology, chemical studies, tumor markers, pH determination and other more esoteric tests are required as
diagnostic Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine "cause and effect". In systems enginee ...
tools for determining the causes of this abnormal accumulation. Even the gross appearance, color, clarity and odor can be useful tools in diagnosis. The presence of heart failure, infection or malignancy within the pleural cavity are the most common causes that can be identified using this approach.


Gross appearance

* Clear straw-colored: If transudative, no further analysis needed. If exudative, additional studies needed to determine cause (cytology, culture, biopsy). * Cloudy, purulent, turbid: Infection, empyema, pancreatitis, malignancy. * Pink to red/bloody: Traumatic tap, malignancy, pulmonary infarction, intestinal infarction, pancreatitis, trauma. * Green-white, turbid: Rheumatoid arthritis with pleural effusion. * Green-brown: Biliary disease, bowel perforation with ascites. * Milky-white or yellow and bloody: Chylous effusion. * Milky or green, metallic sheen: Pseudochylous effusion. * Viscous (hemorrhagic or clear): Mesothelioma. * Anchovy-paste (or 'chocolate sauce'): Ruptured amoebic liver abscess.


Microscopic appearance

Microscopy may show resident cells (mesothelial cells, inflammatory cells) of either benign or malignant etiology. Evaluation by a cytopathologist is then performed and a morphologic diagnosis can be made. Neutrophils are numerous in pleural empyema. If lymphocytes predominate and mesothelial cells are rare, this is suggestive of tuberculosis. Mesothelial cells may also be decreased in cases of rheumatoid pleuritis or post-pleurodesis pleuritis. Eosinophils are often seen if a patient has recently undergone prior pleural fluid tap. Their significance is limited.De Mais, Daniel. ASCP Quick Compendium of Clinical Pathology, 2nd. Ed. ASCP Press, Chicago, 2009. If malignant cells are present, a pathologist may perform additional studies including
immunohistochemistry Immunohistochemistry (IHC) is the most common application of immunostaining. It involves the process of selectively identifying antigens (proteins) in cells of a tissue section by exploiting the principle of antibodies binding specifically to ant ...
to determine the etiology of the malignancy.


Chemical analysis

Chemistry studies may be performed including pH, pleural fluid:serum protein ratio, LDH ratio, specific gravity, cholesterol and bilirubin levels. These studies may help clarify the etiology of a pleural effusion (exudative vs transudative). Amylase may be elevated in pleural effusions related to gastric/esophageal perforations, pancreatitis or malignancy. Pleural effusions are classified as exudative (high protein) or transudative (low protein). In spite of all the diagnostic tests available today, many pleural effusions remain
idiopathic An idiopathic disease is any disease with an unknown cause or mechanism of apparent spontaneous origin. From Greek ἴδιος ''idios'' "one's own" and πάθος ''pathos'' "suffering", ''idiopathy'' means approximately "a disease of its own kin ...
in origin. If severe symptoms persist, more invasive techniques may be required. In spite of the lack of knowledge of the cause of the effusion, treatment may be required to relieve the most common symptom,
dyspnea Shortness of breath (SOB), also medically known as dyspnea (in AmE) or dyspnoea (in BrE), is an uncomfortable feeling of not being able to breathe well enough. The American Thoracic Society defines it as "a subjective experience of breathing di ...
, as this can be quite disabling.
Thoracoscopy Thoracoscopy is a medical procedure involving internal examination, biopsy and/or resection/drainage of disease or masses within the pleural cavity, usually with video assistance. Thoracoscopy may be performed either under general anaesthesia o ...
has become the mainstay of invasive procedures as closed pleural biopsy has fallen into disuse.


Disease

Diseases of the pleural cavity include: * Pneumothorax: a collection of air within the pleural cavity *
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 k ...
: a fluid accumulation within the pleural space. * Pleural tumors: abnormal growths on the pleurae.


See also

*
Coin test A coin test (or a bell metal resonance) is a medical diagnostic test used to test for a punctured lung. A punctured lung can cause air or fluid to leak into the pleural cavity, leading to, for example, pneumothorax or hydrothorax. In a coin test ...
, medical diagnostic test for a punctured lung


References


Sources

*


External links


Photo of dissection
at kenyon.edu {{Authority control Pleura