copyright © 2006 by elsevier, inc. determinants of diffusion rate of diffusion = (p 1 -p 2 ) * area...
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Copyright © 2006 by Elsevier, Inc.
Determinants of Diffusion
Rate of Diffusion = (P1-P2 ) * Area * Solubility
Distance * MW
• Pressure Gradient• Area• Distance• Solubility and MW are fixed
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Changes in Alveolar Gas Composition
Figure 39-3; Guyton & Hall
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Partial Pressure of Oxygen in Alveoli
Figure 39-4: Guyton & Hall
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Partial Pressure of CO2 in Alveoli
Figure 39-5; Guyton & Hall
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Alveolar and Blood Gases
PO2 = 100
PCO2 = 40
PCO2 = 40PCO2 = 45
PO2 = 40 PO2 = 100
PO2 = 159
PCO2 = 0PO2 = 149
PCO2 = 0
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Diffusion Capacity
• Oxygen• Diff capacity 21
ml/min/mmHg * gradient of 11 mmHg
• 230 ml/min diffusion of oxygen
0
20
40
60
80
100
120
0 0.25 0.5 0.75
Pulmonary Capillary Length %
Po 2
in b
lood
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Diffusion Capacity
• Carbon Dioxide– Diff capacity 400
ml/min/mm Hg * gradient < 1 mmHg
– 200 ml/min diffusion of carbon dioxide
40
41
42
43
44
45
46
0 0.2 0.4 0.6 0.8
Pulmonary Capillary Length %
blo
od P
co2
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Ventilation/Perfusion
• Relationship between adequate flow and
adequate ventilation
• Defined as V/Q
• V/Q = (4 L/min)/ (5 L/min) = 0.8
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Ventilation/Perfusion
pO2=?
pCO2=?
pO2=?
pCO2=?
pO2=?
pCO2=?
Shunt Normal Dead space
V/Q = 0 V/Q =
Figure 39-11; Guyton & Hall
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Regional Gas Exchange
Va/Q
Bottom lung Top lung
blood flow
ventilation
Blood flow and ventilation are not evenly matched
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Ventilation/Perfusion
• Physiologic shunt– Va/Q < normal
– low ventilation
• Physiologic dead space– Va/Q > normal
– wasted ventilation
• Abnormalities– Upper lung Va/Q ~ 2.5
– Lower lung Va/Q ~ .5