# 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
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
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 CO2 production

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 views: 30 posted: 10/7/2011 language: English pages: 14