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How does the pleura work?
The pleura is composed of two layers of thin lining tissue. The layer covering the lung (visceral pleura) and the parietal pleura that covers the inner wall of the chest are lubricated by pleural fluid. Normally, there is about 10-20 ml of clear liquid that acts as a lubricant between these layers. The fluid is continually absorbed and replaced, mainly through the outer lining of the pleura. Pressure inside the pleura is negative (as in sucking) and becomes even more negative during inspiration (breathing in). The pressure becomes less negative during exhalation (breathing out). Therefore, the space between the two layers of pleura always has a negative pressure. The introduction of air (positive pressure) into the space (such as from a knife wound) will result in a collapse of the lung.
Fauci, Anthony S., et al. Harrison's Principles of Internal Medicine. 17th ed. United States: McGraw-Hill Professional, 2011.
3."Pleurisy and pneumothorax" by National Heart Lung and Blood Institute
4."2313 The Lung Pleurea" by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.. Licensed under CC BY 3.0 via Wikimedia Commons
7."Thoracentesis" by National Heart, Lung and Blood Institute - Thoracentesis. Licensed under Public Domain via Wikimedia Commons