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Old 04-20-2012
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Lungs Perfusion/Diffusion Limited Confusion

I have read this short explanation in medEssentials in online Kaplan Qbank

NB: page 235 in 2nd Edition
page 249 in 3rd Edition.

-If low capillary velocity allows adequate time for diffusion, exchange is perfusion limited.

-If velocity is high, metabolic exchange may become diffusion limited.

what does that mean
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Old 04-20-2012
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Here is how I understand this ..
Perfusion limited.. means that the blood in the capillary is saturated with the gas before the end of the capillary.
Diffusion limited.. means that it will still be not saturated at the end of the capillary..

now if you increase the velocity in the capillary, even the perfusion limited gas will not be able to saturate the blood before the end of capillary, hence become diffusion limited.. this happens in cases of very heavy exercise.
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