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$la||$re|$ce|A QuizPlus DEMO File    ||$ca
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     This  DEMO  file  is  part of a  QuizPlus  teaching  disk  on 
Respiratory  Mechanics from the University of Toronto.   The  disk 
itself is available for $10 through the QuizShare  program.   This 
lesson  file  demonstrates how  complicated,  high-level  teaching 
material can be presented using QuizPlus.
      By  the  way,  you must buy (or  already  own)  QuizPlus  to 
purchase  the low-cost QuizShare disks.   For information  on  the 
purchase  of "Respiratory Mechanics" by Dr.  Duffin,  contact:
$re|$th||$ce|Mad Scientist Software.
$ca|$gr|$ce|2063 North 820 West
$ce|Pleasant Grove, UT 84062|
$ce|801-785-3028|$ca
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  T H E   M E C H A N I C S   O F   T H E   L U N G S

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          (c) Copyright 1987 by
          James Duffin Ph.D.
          Departments of Anaesthesia and Physiology
          University of Toronto

               This lesson examines  the mechanical properties  of  the 
          lungs, and answers the following questions:

          1. What is compliance?

          2. What factors determine compliance?

          3. What can go wrong?

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$centred|Lung Compliance

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          What  is  the  "springyness" property  of  the  lungs?  In  a 
          pressure-volume system it is called:
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$centred|Compliance = Change in Volume / Change in Pressure
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               Compliance describes how much the lungs change in volume 
          for an applied pressure,  i.e.  how easily the lungs "comply" 
          with the pressure's request for a change in volume.

               In  the case of the lungs the applied pressure  is  that 
          produced  in  the  intrapleural space by the  action  of  the 
          respiratory muscles in changing the size of the thorax.
          
                The  relation  between  lung  volume  and  intrapleural 
          pressure can be determined by subjecting an isolated lung  to 
          a series of pressures and measuring the resulting changes  in 
          lung volume.
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lungs
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$centre|Lung Volume vs Intrapleural Pressure

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               Two points should be noted about the pictured graph:
          
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          1.   The  graph is different depending on whether  volume  is 
          increasing or decreasing ( hysteresis ).  Normally, the upper 
          relation between -2 and -10 cm H2O is traversed,  with little 
          hysteresis,  but  in  disease states the  hysteresis  may  be 
          exaggerated.

          2.   The  lung  volume does not decrease completely  to  zero 
          when  intrapleural  pressure becomes zero  because  the small 
          airways collapse, trapping air within the lungs. While airway 
          closure  occurs  at very small volumes in  young  adults,  it 
          occurs  at  higher volumes with increasing age  and  in  some 
          disease states.  As a result, the minimum volume which may be 
          achieved  by  a maximum expiratory effort is limited  by  the 
          chest  wall  in the young but in the older person  by  airway 
          closure.                     
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$centre|Lung Compliance Determinants

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               Lung compliance is determined by two main properties  of 
          the lungs:
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          1.  The elasticity of the tissues due to an embedded  network 
              of elastin fibres.
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               These fibres are not so elastic themselves,  but because 
          they form a network, they act in an elastic manner, similarly 
          in  action  to a nylon stocking.  In age,  the  tone  of  the 
          elastin fibres declines (like old rubber bands), so the lungs 
          become more compliant;  whereas lung damage and the resulting 
          fibrosis makes the lung tissues stiffer and less compliant.
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          2.  The  surface tension of the fluid lining of  the  alveoli 
              tends to empty the lungs.
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               This  effect  is  more  difficult  to  understand,   and 
          involves an analysis of the forces of surface tension at  the 
          alveolar level.
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$centre|The Effect of Surface Tension
 
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               Consider the inflation of a balloon.  The larger it gets 
          the easier it is to inflate. The fluid lined alveoli are like 
          balloons  or  more acurately bubbles to which  LaPlace's  Law 
          applies:
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                                    P = 4T/r  
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          P is the intra-alveolar pressure,
          T is the surface tension for fluid lining the alveoli,
          r is the radius of the alveolus.
          
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               In a bubble the surface tension is constant so that  the 
          larger  the bubble the lower the pressure to inflate  it.  If 
          such  were the case in the lungs then small alveoli would  be 
          harder to inflate than large alveoli,  and the small  alveoli 
          would empty into the large alveoli.
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               This situation does not apply to the alveolus because of 
          the  presence  of  surfactant,  which  modifies  the  surface 
          tension so that it varies directly with alveolar radius.
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                          P = 4T/r     and      T = Kr     
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          K is a constant, so:
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                                     P = 4K

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               Therefore, alveolar inflation pressure does not increase 
          with increasing alveolar radius. In addition, surfactant also 
          reduces  the  surface tension of the watery film  lining  the 
          alveoli, making the lungs easier to inflate.
               Note however,  that the inflation pressure for the lungs 
          as a whole does increase with increasing lung volume, because 
          of the recruitment of alveoli, and other factors.
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               Without surfactant,  such as in the case of some newborn 
          infants,  the  lungs  do not  function  well,  producing  the 
          condition called respiratory distress syndrome.
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