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Diffusion of Gases

Physiology and Pharmacology

QuestionAnswer
Ficks law Flux = diffusion constant x area x conc difference/thickness Henry's law states the conc = a x pressure Flux = d x a x area/thickness x pressure difference D = d x a x area/thickness Flux = D x pressure difference D is 250 ml/min/kPa for O2
Perfusion limitation Blood comes into equilibrium with alveolar gas Doubling diffusion capacity wont alter amount of gas exchanged Doubling blood flow will double amount of gas exchanges
Diffusion limitation Partial pressure gradient remains essentially constant along capillary Doubling blood flow wont change the amount of gas exchanged Doubling diffusion capacity will double exchange
What determines perfusion or diffusion limitation Diffusing capacity - larger = more likely to equilibrate Speed of chemical reaction with blood Solubility in blood B - larger D, larger B and less likely to get equilibrium as need to move more Blood flow - faster flow less time to equilibrate
Inert gases Do not react with blood Solubility in membrane is equal to solubility in blood Solubility does not affect speed of equilibration as soluble gases have same a/b ratio as non soluble gases Cross membrane very quickly but need a lot for equilibration
CO2 and O2 Solubility on blood is greater than in membrane as they react with blood They equilibrate slowly At rest they equilibrate over 1/3 of a capillary in 0.25 secs This is slowed in pulmonary fibrosis with thickened membrane
CO diffusing capacity Solubility in membrane greater than in blood Diffusion limited Can be used to measure diffusing capacity of lung as this affects rate of exchange Measure gas uptake, conc in alveoli and venous blood
Speed of chemical reaction Can only use b to include chemical reaction with blood is speed is fast 1/Dl = 1/Dm + 1/sigmaVc Sigma = diffusing capacity in blood Rate of reaction of O2 with free Hb is faster than RBCs, so limit is diffusion not reaction
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