564 FA 12 : DIFFUSION AND PERFUSION LIMITED

These videos are designed for medical students studying for the USMLE step 1. Feel free to comment and suggest what you would like to see in the future, and I will do my best to fulfill those requests.

[@nellyhoffman6194]

Basically when a gas gets in the alveoli, it goes across the alveolar membrane into the capillaries down it's concentration gradient. The key thing here is the gradient: if there is no gradient, it won't go across.
1. Perfusion limited- let's use N2O to illustrate. When blood comes by the alveoli, there is pretty much no N2O in it, so there is a huge gradient and N2O goes across into the capillary. However, after all that N2O rushes across, there is soon equilibration between the concentration of N2O in alveolar gas and in the capillaries, so there is no gradient. The only way more N2O can get across is if a fresh supply of blood with low N2O comes by and generates a gradient, hence the term "perfusion limited" (perfusion basically means blood coming in, so the gas getting across the membrane is limited by whether new blood comes in).
2. Diffusion limited is a little easier- it's simply limited by whether or not the gas can get across the alveolar membrane into the capillary (like if the membrane is fibrotic and stiff and it can't get across, it's diffusion limited). Classic example is CO. So, as I mentioned before, gradient is key so since the capillary blood doesn't have any CO in it and the alveoli have a lot of CO (presumably from smoke inhalation or whatever), CO goes down it's concentration gradient into the capillary. Now here is the kicker: once CO gets into the capillary, it binds tightly to hemoglobin, so the amount of free CO in the blood doesn't change much, hence no equilibration and you still have a large concentration gradient even after you've inhaled a ton of CO. So as long as the membrane allows diffusion, CO will continue to go into the blood, even when the patient has cherry red mucus membranes and is keeled over, the only way CO will stop going into the blood is if it can't diffuse across membrane anymore, hence "diffusion limited".
O2 is an interesting one- it can be either perfusion or diffusion limited. In a normal healthy lung, O2 is perfusion limited because you generally breathe in enough O2 for equilibration to occur. However, in a diseased lung like emphysema, these folks don't get enough O2 across to equilibrate the amount in the blood to the amount in the alveoli, so it's basically limited by the amount they are able to get across the damaged membranes, hence diffusion limited

[@euscieu4246]

Thanks! Great summary

[@nyile012]

THANK YOU!!!

[@argumentumadbaculum]

Wow thanks

[@femidaelettil1002]

Thanks a lot bro

[@brandonsaavedra8959]

a god amongst humans, thank you!!!!

[@remihamel4037]

Thank you. One note: at 8:00, when you're doing the example, I think that the drop in Pa02 is due to mixing with bronchiolar deoxygenated blood with the oxygenated blood from the pulmonary veins (I just had a UWORLD question that explained it this way).

[@memo20070]

Thanks from Egypt, you really helped me :))

[@Rafitch0]

Thank you! you make my life easier!

[@Elevatone]

At 1:34 Why are you saying CO binds to oxygen? don't you mean Hgb?

[@mizzk004]

thank you for explaining perfusion and diffusion limited. u simplified it so good.

[@rishsesh]

You make everything so easy and interesting thanks a lot :)

[@shanighias]

great video

[@LoveMedicine100]

My Pleasure ....so glad you liked it....

[@BigMikeWJ]

Thank you!!!

[@jeets2k2]

Thanx for ur videos

[@mahiroteles]

thanks a million

[@pixzers]

Good explanation

[@fatboy117]

thx i finally understand

[@Daniel-rk2qz]

So, can oxygen be BOTH diffusion and perfusion limited under pathological conditions? Or does do gases that are normally perfusion limited switch and become diffusion limited?

[@poonehfarhangi2589]

Daniel Rongo its both

[@obadatabbaa8136]

can i know what kind of application u r using for these notes

[@hereiswherethereisnt]

my confusion is in how CO is diffusion limited. i get that it wont be fully equilibrated by the end of the capillary so its diffusion limited but how is that so when it has such great affinity and provides maximal diffusion values? is it because we want to choose a diffusion limited gas as to not be capped by early equilibration to get the greatest DLco value possible when testing a patient? im confused

[@aurelcato7465]

+hereiswherethereisnt Hello, I believe it is the fact that CO is so highly bound to hemoglobinthat it does not reach equilibrium at the end of the pulmonary capillary. I could be wrong but equilibrium is when molecules moving between two compartments( the alveolus and pulmonary capillary) reach a balance of molecules moving back and forth between the compartments. CO just stays in the pulmonary capillary bound to hemoglobin mostly because of its high affinity. Hope this helps.

[@rishsesh]

Loads of luv from Pakistan :)

[@deeplyingplaymaker8804]

so whats flow limited? is it same as perfusion limited?

[@hereiswherethereisnt]

towards the end straight from uworld question 1522 if people need a reference

[@vasku5782]

I thought the explanation of the Uworld question at the end was very poor. I don't understand how you can determine diffusion vs. perfusion limited gas exchange without looking at both the PAO2 and PaO2 at the same time. I can you make the distinction by just looking at the tracheal and alveolar oxygen????

[@francismenendez5925]

I just want to add that CO2 it diffuses easily the membrane than O2 that is why reach faster than O2 the equilibrium
and the curve it is a more steepest than the O2 curve