ANATOMY OF THE PLEURA The pleura develops in the embryo from the coelomic cavity (fetal body cavity). The coelomic cavity is where the vital organs (heart, bowels, and lungs) will develop. This cavity will then be divided into the peritoneal cavity and the pleural space by the septum transversum and the pleuroperitoneal membranes. The pleural space will later be separated into two distinct cavities by the pericardium. The lungs then develop from the premordial buds (central mass of me
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  ANATOMY OF THE PLEURA The pleura develops in the embryo from the coelomic cavity (fetal body cavity).The coelomic cavity is where the vital organs (heart, bowels, and lungs) will develop.This cavity will then be divided into the peritoneal cavity and the pleural space by theseptum transversum and the pleuroperitoneal membranes. The pleural space will later beseparated into two distinct cavities by the pericardium. The lungs then develop from the premordial buds (central mass of mesenchyme) and as they grow laterally, theyinvaginate each pleural space, thus taking its pleural covering. The pleura covers theentire thoracic cavity (parietal pleura) and the lungs (visceral pleura). The pleural layersare reflected back-to-back in the interlobar fissures, and at the pulmonary ligament whichextends from the hilum down to the diaphragm.The pleural space is a potential space that contains minute amounts of pleural fluidessential for lubricating expansion and movement of the lungs. The lung fills the pleuralspace during deep inspiration, while during expiration, the lung retracts and the costal parietal pleura and the diaphragmatic pleura may come in apposition and for the pleuralrecesses.The pleura is composed of five layers: () a single layer of mesothelial cells, (!) athin submesothelial connective tissue layer, ( ) a thin superficial elastic layer, (#) a looseconnective tissue layer, and ($) a deep fibroelastic layer. The surface of the mesothelialcells contains microvilli. These microvilli were thought to function in the absorption of fluid, but recently, it has been shown to enmesh glycoproteins to lubricate the gliding of  both pleural layers. The parietal pleura receives its blood supply from systemic capillaries. %mall branches of the intercostal arteries supply the costal pleura, whereas the mediastinal pleura is principally by the pericardiophrenic artery. The diaphragmatic pleura is supplied by the superior phrenic and musculophrenic arteries. The blood supply of the visceral pleura in humans and animals with thick pleura srcinates from the systemic circulationvia the bronchial arteries. &nvestigators have demonstrated that in sheep -an animal withthick pleura- the visceral pleura is supplied completely and exclusively by the bronchialartery. %ince humans have thick pleura, it is possible that the visceral pleura is supppliedsimilarly, although there is still controversy concerning this.  The venous drainage of the parietal pleura is through the intercostal veins(systemic veins) while the visceral pleura drains to pulmonary veins.The lymphatics of the parietal pleura drain the pleural fluid and any noxious particles that reach the pleura. The lymphatic system starts with small stomas thatcommunicate to lymphatic foci that will drain through lymphatic vessels to nodes alongthe internal thoracic artery and to the internal intercostal nodes (along the heads of theribs posteriorly). The parietal pleural lymphatics can remove about !' times the fluidformed under normal conditions (up to '.! m per kg per hour). The visceral pleuradrains through two systems: a) a superficial system that floats over the surface of the lungtowards the hilum, and b) a deep system that penetrates the lung parenchyma to reach thehilar nodes.%ensory nerves endings are present in the costal and diaphragmatic parietal pleura.The intercostal nerves supply the costal pleura and the peripheral part of thediaphragmatic pleura. hen either of these areas is stimulated, pain is referred to thead*acent chest wall. &n contrast, the central portion of the diaphragmatic pleura isinnervated by the phrenic nerve, and stimulation of this part of the pleura causes pain thatis referred to the ipsilateral shoulder. The visceral pleura contains no pain fibers  Physiology of The Pleural Space ) +leural pressure:The pleural pressure is subatmospheric due to the tendency of the lung to collapseversus the tendency of the chest wall to expand. This negative pleural pressure keeps theinflated. There exists a pressure gradient between the superior and the inferior portions of the pleura, with the superior portions being lowest (more negative). The magnitude of the pressure gradient appears to be approximately '.$ cm  !  per cm vertical distance.onsidering the si/e of the lung to be !$ cm in height in normal persons, the difference in pleural pressure between the apex and the base may be around ! cm  ! .!) +leural 0luid 0ormation:+leural fluid is normally formed from parietal pleural capillaries. The forcesgoverning this process are the pleural capillaries hydrostatic pressure and the oncotic   pressure of the pleural capillaries and the oncotic pressure of fluid in the pleura space.This can be summari/ed in the following schematic illustration. PARIETALP L E U R A LVISCERALPLEURAS P A C EPLEURA Hydrostatic pressure 30 - 5 +2435 296 029 2934 5 34   Oncotic pressure This illustration shows that the pleural fluid is normally formed from filtration fromthe parietal pleural capillaries with a gradient of 12 cm  ! . &t is noteworthy toremember that the fluid is also removed from the pleural space through the parietal pleural lymphatics through the lymphatic stomas. 3ormally, '.' m per 4g per hour isformed through pleural fluid lymphatics.!) ther rigins of +leural 0luid:a) &nterstitial origin: in conditions where the interstitial tissues of the lung areoverloaded with transudate (e.g., high pressure or high permeability pulmonary edema),fluid may escape in the pleural space. The fluid is then removed through parietal pleurallymphatics which have the ability to remove considerable amounts of fluid. b) +eritoneal cavity: ascetic fluid can pass to the pleural space through minuteopenings in the diaphragm. This can be seen in cases of hepatic hydrothorax, 5eig6ssyndrome, and peritoneal dialysis.c) Thoracic duct or blood vessel disruption: lymphatic vessel disruption (as in casesof trauma, or lymphoma) will lead to chylothorax. 7lood vessel disruption will result inhemothorax.  Pathogenesis of Pleural Flui For!ation The pathogenesis of pleural fluid accumulation can be summari/ed in the following table:   Pathogenesis of Pleural Effusions   A #ncrease pleural flui for!ation   $  #ncrease interstitial flui in the lung$  eft ventricular failure, parapneumonic effusion, pulmonary embolus, 89%, +ost lung transplantation.  #ncrease intra%ascular pressure in the pleura : 9ight or eft ventricular failure, %uperior vena caval syndrome (%;), and  pericardial effusion.  #ncrease pleural flui protein le%el : parapneumonic effusion  &ecrease pleural pressure 'e()%aco effusion *  ung atelectasis or increased elastic recoil of the lung.  #ncrease flui in peritoneal ca%ity$  8scites, peritoneal dialysis, and 5eig6s syndrome.  &isruption of the thoracic uct$ trauma, tumors (lymphoma) + &ecrease pleural flui rea,sorption   : ã O,struction of the ly!phatics raining the parietal pleura :   +leural malignancies ã Ele%ation of the syste!ic %ascular pressures$  %; syndrome or right ventricular failure.  -&isruption of the a.uaporins syste!$ a group of proteins that help transport water across the pleura ( still under investigation)
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