Diffusion vs Perfusion

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So I am having trouble with this concept and I want to make sure I have it right. Perfusion has to do with the amount of blood and/or O2 reaching the alveoli, whereas diffusion is the ability of the gas exchange to occur at the alveoli such as the surface area available? It seems like they overlap a lot and I am not clear on how to differentiate the two. Thanks in advance for any help.

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I would look in YouTube for your question. I have found it to be very helpful.

Good Luck!

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Perfusion is BLOOD SUPPLIED.

Ventilation is the AIR SUPPLIED

DIFFUSION is the EXCHANGE.

So now let's apply this.

An example of a disorder causing PERFUSION issues ... A right to left shunt, this means that blood is going from the right ventricle to the left ventricle (stick with me), if blood is going "hey, I don't want to travel through the lungs", what going on? NO blood is reaching the lungs, so no CELLS will pick oxygen ... SUREE... you may be breathing in ALL the O2 you want, but hey ... it doesn't matter.

Ventilation ... think ... airway obstruction... ARDS is also a ventilatory disorder... and if I am not mistaken it is because the lungs become fibrotic ... the alveoli collapse and therefore ... even if blood is being supplied there ... the O2 can't be exchanged because the alveoli are collapsed (which is why we use PEEP- positive expiratory end pressure ... which a PUFF at the end of expiration)

Diffusion, once again is the exchange... I've never heard of diffusion being compared to perfusion..

Perfusion is simply BLOOD supplied.

Diffusion applied to pulmonology, I'm sure has to do with the high concentrations of oxygen on one side and high concentrations of CO2 on the other side ...

KayKay. I hope I somewhat helped. (=

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Perfusion is the exchange of gases at the tissue level. Diffusion is the exchange of gases between the capillaries and alveoli. Ventilation is the gas exchange in and out of the lungs. So hypoxia is a perfusion problem and hypoxemia is a diffusion problem. This is the simplistic explanation.

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Specializes in Adult Internal Medicine.
Perfusion is the exchange of gases at the tissue level. Diffusion is the exchange of gases between the capillaries and alveoli. Ventilation is the gas exchange in and out of the lungs. So hypoxia is a perfusion problem and hypoxemia is a diffusion problem. This is the simplistic explanation.

Not exactly.

Perfusion is the blood flow through the capillary bed (be it the lungs or the target tissues).

Ventilation is the filling of the alveoli with oxygen-rich air.

Diffusion is the passive movement across a gradient.

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Specializes in Adult Internal Medicine.
So I am having trouble with this concept and I want to make sure I have it right. Perfusion has to do with the amount of blood and/or O2 reaching the alveoli whereas diffusion is the ability of the gas exchange to occur at the alveoli such as the surface area available? It seems like they overlap a lot and I am not clear on how to differentiate the two. Thanks in advance for any help.[/quote']

There are three interrelated concepts: think of it in terms of a single alveolar sac and a single capillary. The alveolar sac needs to have air in it (ventilation) The capillary needs to have blood in it (perfusion). And there needs to be a membrane for a chemical process where oxygen moves in and co2 moves out (diffusion). All three of these concepts are required for effective respiration.

Simple:

If the alveolar sac has no air in it what kind of problem is it?

If the capillary has no blood in it what kind of problem is it?

Complex:

What might cause a diffusion problem?

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Not exactly.

Perfusion is the blood flow through the capillary bed (be it the lungs or the target tissues).

Ventilation is the filling of the alveoli with oxygen-rich air.

Diffusion is the passive movement across a gradient.

You and I are saying the same thing. Yes as the blood flows through the capillary it is exchanging gases be it the lungs or other tissues. The capillaries at the alveoli are diffusing gases to either be perfused or ventilated.

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Not exactly.

Perfusion is the blood flow through the capillary bed (be it the lungs or the target tissues).

Ventilation is the filling of the alveoli with oxygen-rich air.

Diffusion is the passive movement across a gradient.

I said perfusion was the supply... I think of perfusion as the blood flowing to the lungs or organ at hand .... Is my thought process correct? :p

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Thank you everyone for help clearing it up, I was confused by how my professor explained it.

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Perfusion is just blood movement and has nothing to do with diffusion, other than that diffusion occurs where there is blood (and in other places). If your lungs are full of water, for example, you can have terrific perfusion but you will not have any meaningful gas diffusion. If you have lousy blood oxygen because your lungs are really sick, you can have great perfusion to your tissues but you'll have lousy tissue oxygenation.

Gas (or any) diffusion depends on a gradient, which means that a substance moves through a semipermeable membrane from an area of higher concentration to an area of lower concentration. So gas exchange in the lung does move both CO2 (out, because it's higher in the blood than in the inhaled air in the alveoli) and O2 (in, because it's higher in the air than in the venous blood in the alveoli capillary bed) but NOT because there's "CO2 on one side and O2 on the other side." Those two are independent of each other, even though diffusion of both occurs at the same time.

Indeed, it is possible (and common) to have pulmonary conditions where blood oxygenation is poor but blood CO2 levels are normal.

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

Everyone has done a great job...this may help......http://wiki.ggc.edu/images/a/a5/Ventilation-Perfusion-Diffussion-and-More.pdf

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damrcngrl95 said:
You and I are saying the same thing. Yes as the blood flows through the capillary it is exchanging gases be it the lungs or other tissues. The capillaries at the alveoli are diffusing gases to either be perfused or ventilated.

No, you are not saying the same thing, because perfusion is categorically NOT the exchange of gases at the tissue level. Perfusion is nothing more, or less, than the presence of blood flow, no matter what's in it.

Diffusion makes the gas exchange occur, and perfusion makes that process possible by maintaining the gradient in which diffusion depends by constantly circulating more blood through all capillary beds. Oxygen goes into the blood and CO2 out of the blood in the alveoli, and oxygen goes out of the blood and CO2 into it at the tissue level).

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