dr_kh
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Out of the series of gas mixtures inhaled by a subject ,which will diffuse slowly from lungs to blood?
a) CO2 at PCO2 60mm Hg
b)CO at PCO of 0.5mm Hg
c) O2 at PO2 of 130mmHg
d)o2 at po2 150mmHg
Can someone please explain why the answer is not " C "
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| monam84
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what was the answer?
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| dr_kh
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Answer given in the book is "B"
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| new_n_lost
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dr_kh........can u plz tell me the source of this question. where is it from
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| monam84
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ok thats what i thought it was, but ive been wrong b4  so the way i reason it is the following: diffusion= (change in pressure) (surface area) /(thickness of membrane) so inthe question S.A. and thickness are the same as long as its the same person, so the only factor really affecting the rate of diffusion is the change in pressure.
choice A- change is pressure = 60 -47 (venous CO2 normally)=13mmHg
choice B- .5 - 0 (because the CO binds quickly to Hb so the conc is 0)=.5mmHg
choice c- 130 - 40 (venous 02)= 90 mmHg
choice d- 150-40= 110 mmHg
So essentially the one with the slowest rate of diffusion is the one with the lowest change in pressure i.e. choice B.
I hope this is rite.. thats kinda how i reasoned it.
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| dr_kh
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The source of question is Kaplans and I have a question
Will we not consider the solubility factor ? And u have explained it exactly as it is given in the book
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| monam84
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For step 1 you do not need to consider the molecular weight n stuff, they just want to know that you understand the basic principles. So just consider those 3 parameters when you look a diffusion (change in pressure, surface area, and thickness) hope that helps.
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| dr_kh
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Thanks a lot
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| tompat
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but the explanation is diff.
here we hav to think about the solubility of the gas. CO2 AND O2 are highly soluble gases in cell membranes and blood,so here the diffusion factor affecting exchange is negligible, thats why they make be called perfusion limited gases.
in case of CO , it is a diffusion limited gas meaning its exchange depends on its diffusion, solubility. it has high affinity for blood, hb so the perfusion factor is negligible,to make it easy less perfusion will still ensure that sufficient amount will reach systemic blood.
so CO is the only diffusion limited gas making the choic easier, diff pressures given here are good distractors
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