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Respiratory Physiology - Notes .doc

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University of British Columbia
BIOL 153
Paul Hewitt

Notes Pressure Relationships in the Thoracic Cavity Respiratory pressures are always described in relation to atmospheric pressure the pressure exerted by the gases surrounding the body which is 760 mm Hg at sea levelIntrapulmonary pressure is the pressure within the alveoli of the lungs B it fluctuates during the process of breathing but always equalizes itself with the atmospheric pressureIntrapleural pressure is the pressure within the pleural cavity B it fluctuates with breathing but is always about 4 mm Hg less than intrapulmonary pressuretherefore it is always negative relative to intrapleural and atmospheric pressures How do the lungs remain inflatedopen2 forces act to pull the visceral pleura away from the parietal pleura hence collapsing the lungselasticity of the lungs gives them a natural tendency to recoil and assume their smallest possible sizefluid film on in the alveoli create surface tension which acts to draw the alveoli in to their smallest possible dimension this force is minimal due to surfactant which is discussed laterThese forces are opposed by the natural elasticity of the chest wall which tends to pull the thorax outward and enlarge the lungsa healthy person none of these forces overrides an otherthe pleura secrete pleural fluid into the pleural cavity creating a surface tension that secures the 2 pleural layers together and allow them to slide over one anotherseparating them requires great forceHow is the negative intrapleural pressure maintainedalmost as quickly as the pleura secrete it pleural fluid is pumped out of the pleural cavity into the lymphatic systemthis ensures that only a minimal amount of pleural fluid remains in the pleural cavityif the fluid is not adequately pumped out and excessive pleural fluid accumulates in the pleural cavity a positive pressure is created in the pleural cavity and the lungs will collapse atelectasis
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