Diffusion vs. Perfusion

[Scuba_Shane]

Hi there, I am reading an old article from 1989 "Physiological Basis of Decompression" (I know its old, but I like reading everything) and I came past a paragraph that struck me as odd. It states that gas in a bubble must diffuse back into tissue for it to be removed by the bloodstream. It goes on to state that perfusion controls the gas level in the tissue, while diffusion controls the rate at which gas leaves the bubble. I am just looking to confirm this, is this true?

 

[Dr Deco]

This is correct. I will write more tomorrow; right now I am watching the Lions game.:cool2:

 

[TSandM]

Perfusion is the blood supply to an area. Blood brings gases to the tissues from the lungs, and blood takes them away. If there is poor blood supply (poor perfusion) to an area, it will be difficult to remove inert gases from the tissues. On the other hand, good perfusion means tissue gas tensions will more closely approximate those in the lungs.

That has to do with gas dissolved in liquid, though (the liquid basically being plasma in the vascular system). Gas which is in bubbles is sequestered from removal by perfusion, unless the entire bubble can be swept away. Gas in bubbles must diffuse OUT of the bubbles to be eliminated (and that's true, whether the bubble is in the original tissue where it formed, or whether it has been swept elsewhere).

Therefore, I think it's pretty clear to see that free phase (dissolved) gas tensions are regulated by the level of perfusion, and gas in bubbles is diffusion-limited for elimination.

 

[Scuba_Shane]

Yeah I get all the basic physiology and physics of this. I am starting to think I misunderstood the article. I was referring to was bubbles formed within the plasma. I can understand that a bubble formed in tissue would only be able to perfuse back into tissue as fast as the gradient would allow it (i.e gas diffusing away from the bubble to create such a gradient). The hang up for me is bubbles in the pulmonary circulatory system. These bubbles will collapse in the capillaries of the lung as gradient should strongly slant towards inert gas moving across the membrane to the aveoli and out (during decompression that is). There should be no tissue transference here, just straight to the plasma and across the membrane. Is this article considering plasma to be tissue?

 

[Dr Deco]

Hello Scuba Shane:

Bubble Diffusion

Your question is not crystal clear to me. Any bubble in the capillaries of the lung system will shrink rapidly. The gas will be lost as a very tiny bubble transfers gas from the bubble to the plasma to the alveoli.

If the bubble is big enough to lodge in the lung capillary, the gas will diffuse from the bubble directly into the alveolus. Those are the only pathways.

Dr Deco :doctor:

 

[Scuba_Shane]

Thanks Dr. Deco, thats sort of what I started to realize. Thanks for helping me solve that brain block.