Diffusion vs Perfusion

[BeautifulSky]

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.

[BeautifulSky]

Thank you again for all of the help. I was told for perfusion you had to have the right amount of blood and air that reaches the capillaries so I was confused on what exactly it was, but now I have read other sources and with your help I understand it much better.

[damrcngrl95]

Quote

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.

GrnTea that is such a great way to explain it. You are much better at explaining things than I ever could.

[damrcngrl95]

Quote

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).

I stand corrected on my explanation. I know what I was trying to say and what was said are two different things. I only added in the confusion. I do realize quiet clearly diffusion relies on a gradient where as perfusion in my mind has to do with the liquid moving from one place to the other. My understanding that when it comes to blood hydrostatic and osmotic pressures come into play. That's how I see perfusion but I could still be missing the mark.

[nurseprnRN, BSN, RN]

You're not so much missing the mark as overshooting it. ? Perfusion makes many things possible, but it's basically a mechanical (ok, you physicists, you're right, a fluid dynamic) event. No less, but no more, either.