In human anatomy, the pleural cavity is the potential space between the two pleurae of
the lungs. A pleura is a serous membrane which folds back onto itself to form a two-layered
membrane structure. The thin space between the two pleural layers is known as the pleural
cavity and normally contains a small amount of pleural fluid. The outer pleura is attached
to the chest wall. The inner pleura covers the lungs and adjoining structures, via blood
vessels, bronchi and nerves. The parietal pleura is highly sensitive to
pain, while the visceral pleura is not, due to its lack of sensory innervation.
Structure In humans, there is no anatomical connection
between the left and right pleural cavities. Therefore, in cases of pneumothorax, the other
lung will still function normally unless there is a tension pneumothorax or simultaneous
bilateral pneumothorax, which may collapse the contralateral parenchyma, blood vessels
and bronchi. The visceral pleura receives its blood supply
from the bronchial circulation, which is the same as the lungs. The parietal pleura receives
its blood supply from the intercostal arteries, which is the same as the overlying body wall.
The costal and cervical portions and the periphery of the diaphragmatic portion of the parietal
pleura are innervated by the intercostal nerves. The mediastinal and central portions of the
diaphragmatic pleura are innervated by the phrenic nerves. The visceral pleura covering
the lung itself receives its innervation from the autonomic nervous system.
Development Initially the intraembryonic coelom is one
continuous space. During development this space partitions to form the pericardial,
pleural and peritoneal cavities. The diaphragm and the paired pleuropericardial membranes
separate the coelomic cavity into four parts. From the splanchnopleura develops the visceral
pleura and from the somatopleura develops the parietal pleura.
Function The pleural cavity, with its associated pleurae,
aids optimal functioning of the lungs during breathing. The pleural cavity also contains
pleural fluid, which allows the pleurae to slide effortlessly against each other during
ventilation. Surface tension 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
during breathing. The pleural cavity transmits movements of the ribs muscles to the lungs,
particularly during heavy breathing. During inspiration the External Intercostals contracts,
and so does the Diaphragm. This cause the expansion of the chest wall, that rise 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 parietal circulation via bulk flow and reabsorbed by the lymphatic
system. Thus, pleural fluid is produced and reabsorbed continuously. 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.
Clinical significance Pleural effusion
A pathologic collection of pleural fluid is called a pleural effusion. Mechanisms:
Lymphatic obstruction Increased capillary permeability
Decreased plasma colloid osmotic pressure Increased capillary venous pressure
Increased negative intrapleural pressure Pleural effusions are classified as exudative
or transudative. Exudative pleural effusions are generally caused by infections such as
pneumonia, malignancy, granulomatous disease such as tuberculosis or coccidioidomycosis,
collagen vascular diseases, and other inflammatory states. Transudative pleural effusions occur
in congestive heart failure, cirrhosis or nephrotic syndrome.
Localized pleural fluid effusion noted during pulmonary embolism results probably from increased
capillary permeability due to cytokine or inflammatory mediator release from the platelet-rich
thrombi. Causes of transudative pleural effusions
Congestive heart failure Hepatic cirrhosis
Hypoproteinemia Nephrotic syndrome
Pulmonary atelectasis Myxedema
Peritoneal dialysis Pulmonary embolism
Meig syndrome Obstructive uropathy
Causes of exudative pleural effusions Malignancy
Infection Trauma
Pulmonary infarction Pulmonary embolism
Autoimmune disorders Pancreatitis
Ruptured esophagus 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 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.
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: Mesothelioma.
Anchovy-paste: Ruptured amoebic liver abscess. Microscopic appearance
Microscopy may show resident 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 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.
If malignant cells are present, a pathologist may perform additional studies including immunohistochemistry
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. Amylase may be elevated in pleural effusions related to
gastric/esophageal perforations, pancreatitis or malignancy. Pleural effusions are classified
as exudative or transudative. In spite of all the diagnostic tests available
today, many pleural effusions remain idiopathic 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, as this
can be quite disabling. Thoracoscopy has become the mainstay of invasive procedures as closed
pleural biopsy has fallen into disuse. Diseases
Diseases of the pleural cavity include: Pneumothorax: a collection of air within the
pleural cavity Pleural effusion: a fluid accumulation within
the pleural space. Pleural tumors: abnormal growths on the pleurae.
Pleural plaques See also
Additional images
References
External links Photo of dissection at kenyon.edu