Diffusion

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					             Diffusion

          John W. Kreit, M.D.
           Division of Pulmonary
        and Critical Care Medicine
University of Pittsburgh School of Medicine
Ventilation and Gas Exchange
               Diffusion

• The net movement of molecules from a
  region with a high partial gas pressure
  to a region with a lower partial pressure
• Diffusion is the process whereby O2 and
  CO2 are exchanged across the alveolar-
  capillary interface.
            Diffusion Path
• Oxygen diffuses
  through the alveolar
  epithelium, the
  capillary
  endothelium, the
  plasma, and the wall
  of the erythrocyte.
• Carbon dioxide
  diffuses in the
  opposite direction.
    Fick’s Law for Diffusion
             Vgas = A x D x (P1 – P2)
                           T
A = surface area of the alveolar-capillary interface
D = diffusion coefficient
   Directly proportional to gas solubility
   Inversely proportional to the square root of the gas molecular
   weight
T = Thickness of the alveolar-capillary barrier
P1 – P2 = Partial pressure gradient of the gas
   Fick’s Law for Diffusion
           Vgas = A x D x (P1 – P2)
                         T
A = approximately 100 M2
T = 0.2 - 0.5 µm
D  solubility/MW
   MW CO2 : MW O2 = 1.17:1
   Solubility CO2 : Solubility O2 = 24:1
P1O2 - P2O2 = 100 - 40 mmHg
P1CO2 - P2CO2 = 46 - 40 mmHg
          Diffusion of Oxygen
• Each erythrocyte
  spends ~ 0.75 second
  in the pulmonary
  capillaries.
• Equilibration between
  alveolar gas and
  capillary blood occurs
  within ~ 0.25 second.
   – Equilibration occurs even
     when transit time is
     reduced.
          Diffusion of Oxygen

• Any disorder that
  impairs diffusion will
  increase the time
  needed for equilibration
  to occur.
   – Alveolar - end-capillary
     PO2 gradient.
   – This gradient will
     increase during exercise.
  Diffusion of Carbon Dioxide
• Equilibrium between
  alveolar and capillary
  PCO2 also takes ~ 0.25
  second.
• Incomplete equilibration
  of PCO2 is not clinically
  relevant because of the
  small partial pressure
  gradient.
           Mechanisms of
         Arterial Hypoxemia
PAO2 = (PB – PH2O) x FIO2 – PACO2 / R

• Disorders causing a fall in PAO2 cause
  hypoxemia without an increase in PA-aO2.
  – Hypoventilation
     •  PACO2
  – Reduced barometric pressure (high altitude)
     •  PB
  – Reduced FIO2
           Mechanisms of
         Arterial Hypoxemia
Disorders that affect the airways,parenchyma,or
  blood vessels cause an increase in PA-aO2.
  – Ventilation-Perfusion inequality
     • Low V/Q units   PO2
  – Shunt
     • Mixed venous blood is added directly to the arterial
       circulation
  – Diffusion Impairment
     • Lack of equilibration between alveolar and end-capillary
       PO2
           Mechanisms of
        Arterial Hypercapnia
        PCO2 = K x VCO2 / VE - VD

• Decreased minute ventilation
• Inadequate increase in minute ventilation in
  response to:
  – Increased dead space ventilation
  – Increased CO2 production

				
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posted:10/7/2011
language:English
pages:14