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Old 05-04-2011
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Lungs Perfusion limited versus diffusion limited

Can any one explain to me what is this perfusion limited and diffusion limited in page no 507 in FA 2011??

.....pls help.......
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Old 05-04-2011
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I'll try to explain.
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.
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Old 05-04-2011
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Thanks a lot...now i understand it..thank u very much...
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Old 05-05-2011
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Question its always a confusion-correct me

The diffusion depending on equilibrium across the membrane.

If the concentration of the diffusing substance has achieved equilibrium across the membrane then there is no gradient in which case a gradient can only be established by an increase in blood flow ( PERFUSION LIMITED )which would increase further the concentration of the diffusing substance and establish a new gradient that will drive diffusion

On the other hand, if an equilibrium has not been achieved across the membrane then a gradient is present, which is maintaining the diffusion across the membrane and it does not require blood flow to maintain a gradient. Hence DIFFUSION LIMITED
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Quote:
Originally Posted by nevillenic View Post
The diffusion depending on equilibrium across the membrane.

If the concentration of the diffusing substance has achieved equilibrium across the membrane then there is no gradient in which case a gradient can only be established by an increase in blood flow ( PERFUSION LIMITED )which would increase further the concentration of the diffusing substance and establish a new gradient that will drive diffusion

On the other hand, if an equilibrium has not been achieved across the membrane then a gradient is present, which is maintaining the diffusion across the membrane and it does not require blood flow to maintain a gradient. Hence DIFFUSION LIMITED
Yes, that's correct
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Old 05-05-2011
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perfusion limited need a good blood supply tht creates a concentration difference across membrane n the gas crosses the membrane till equilibrium is reached
diffusion limited readily dissolve in blood cannot generate conc.difference so they depend on membrane property of diffusion only so wen membrane doesnt allow diffusion no gas movement across it
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Old 08-26-2011
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Question perfusion limited and diffusion limited

hi there, can anyone pls explain what is perfusion limited and what is diffusion limited, I read this in Kaplan physio, CVS chapter!
so in my understanding when there is equilibrium between vessel and interstitium, this is perfusion limitation and when there is no equilibrium, this is diffusion limited.
I don know how to apply this knowledge and what is its importance, although i read it many times, if anyone of u understood it.. please share!
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Old 08-26-2011
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Default what i understand

let me just say what i understand:

under normal circumstances the movement of oxygen in the lungs from the alveoli to the blood vessels is Perfusion limited, i.e under normal circumstances O2 is maximally extracted from the alveoli into the blood vessels, you want to increase this extraction, you can only do so by increasing the rate of Flow of blood through the vessels hence Perfusion limited. O2 dynamics in the lungs only becomes diffusion limited when there is a pathology eg the air-blood membrane is thickened so the extraction is now limited by diffusion
I don't really get the diffusion limited part but i will check it up and post something.

hope this has helped a bit
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Old 08-21-2012
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Quote:
Originally Posted by heights View Post
I'll try to explain.
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.


Thanks A lot.
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Old 01-27-2014
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kaplan says atelectasis is a perfusion limited situation, and i don`t really get why it`s not a diffusion problem. I thought any problem where lung can`t get air through will be diffusion problem and here we have collapsed lung-so no diffusion for sure.
is that `cause of collapsed lung we have constant conc. of gases in the lung and decreasing conc of gases in alveoli capillaries, so blood in alveoli will make a concentration difference so that`s why it`s a perfusion limited situation?
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Old 01-27-2014
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also is a piece of steak in trachea a diffusion imparment?
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Old 06-30-2014
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I have the same confusion with this topic.
I dont understand why atelectasis is a perfusion limited and not a diffusion limited...

please help!
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Old 07-01-2014
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Correct Answer DLCO

very hard concept to grasp. obscure.

I understand the following:

Perfusion limited: Oxygen carbon dioxide and nitrogen are perfusion limited. This term refers to capillaries because that is where perfusion takes place. So it means that these gases normally equalize on both the alveolar side and capillary side IN A LUNG based on diffusion potential, and that too, very,very quickly, hence they are equal on both sides. So to increase diffusion further, you need to increase their perfusion by increasing blood flow, which is essentially perfusion. So these gases are perfusion limited, meaning they are limited by blood flow, which would essentially increase their diffusion.

Diffusion Limited: Oxygen and Carbon Monoxide.These gases NEVER equalize between alveoli and capillary. Why is that? Well, simple. It is because of their inherent solubility (Pearl: Solubility of CO>CO2>O2).
Using this, take CO for example. It is SO SOLUBLE, it doesn't even have time to equalize between alveoli and capillary. It is immediately uploaded on the porphyrin and iron compound in Hb. Hence, it has a partial pressure of zero in arteries because it is quickly scooped up. Get it? It is limited by its own diffusion, which is in essence, its solubility…

Why is oxygen diffusion limited? Well because remember, the oxygen content is normally 20ml/100 ml blood. Out of this 19.7ml is seen in Hb and only 0.3 inn dissolved blood. Oxygen too is quickly taken up by Hb but not because of increased solubility, it is because eof increased affinity for the Hb. (Pearl: Hb Affinity CO>O2>CO2)
shoot i understand it better now. WOW! teaching is great!
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Last edited by 250orbust; 07-01-2014 at 03:35 PM.
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DrV0213,

i will give u an example so it helps u out.

PULMONARY EMBOLUS is seen, blocks the capillary in a lung, stopping perfusion. However, alveoli still have air but the air and the blood cannot go through gas exchange , which is their natural reason to even exist. This becomes DEAD SPACE. Hence pulmonary embolus is a perfusion limited disease because of blockage to perfusion by embolus. Now ATELECTASIS. In atelectatsis, there is blood flow but not ventilation (because alveoli are collapsed and shriveled). Using the same rationale as the above explanation, atelectasis shows decreased oxygen change, decreased oxygen diffusion, thickening of alveoli (due to fibrosis/scarring). This decreases the amount of nitrogen that can be exchanges via perfusion or ventilation.
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