Help needed to understand gas switch and Fick's law

[rjack321]

Let's ignore everything else I said.

Why in this case the diffusion of helium from from the endolymph and perilymph compartments into the perfused vascular compartment exceeds:
1) The transfer of nitrogen in the opposite direction? because N2 is slow to enter these tissue and
2) The washout of helium from the vascular compartment by the blood flow away from the tissue? because helium is not "impeded" by their relative lack of vascularization and the arterial blood is helium free since you switched so it rushes out

inner ear tissues are highly vacsularized (there's a lot of blood in the ear/head in general). But the ear structures themselves are kind of chunky and not highly perfused. They are more like cartilage which means. N2 moves fairly slow in the ear. Helium moves fast everywhere.

 

[rjack321]

Yes, I should have written that in a different way.

ZHL16 does not have a compartment which models the inner ear tissue behavior.
Fick's law can probably describe the diffusion from tissue to tissue, but how complicated will a formula for the inner ear be?

ZHL16 does not model ANY individual cell, tissue or organ

 

[dorsal]

Who knew I would get to do vector calculus in scuba

the diffusion graduation shown in the full and simplified Fricks says the uptake of helium is faster than nitrogen. Not part of the op question but you really can’t use the simple form as there is a gas change as oxygen is metabolized and co2 released. But for this case the simple form works to show the general idea and avoid some partial differential equations.

So assuming the diver is ascending, switching to nitrogen would impart a new gas imbalance with the tissues adding nitrogen while removing helium until they are in saturation again. Each has its own rate.

 

[dmaziuk]

Let's ignore everything else I said.

Why in this case the diffusion of helium from from the endolymph and perilymph compartments into the perfused vascular compartment exceeds:
1) The transfer of nitrogen in the opposite direction? and
2) The washout of helium from the vascular compartment by the blood flow away from the tissue?

The model is not the real thing, it's a simplified view of the real thing that serves a purpose. The simplified view is each inert gas is separate; nitrogen goes in the direction of lower PPN2, helium goes in the direction of lower PPHe.

Not everyone's comfortable with abstract stuff, esp. when it relates to them personally getting bent. Hence all this "peri" and "endo" and lymph lingo. Ignore that, and you'll be fine.

 

[David Novo]

inner ear tissues are highly vacsularized (there's a lot of blood in the ear/head in general). But the ear structures themselves are kind of chunky and not highly perfused. They are more like cartilage which means. N2 moves fairly slow in the ear. Helium moves fast everywhere.

Why is nitrogen "slower" than helium?

Why does helium move faster from endolymph and perilymph compartments to the vascular compartment than from the latter to the blood if the concentration gradient seems to be larger in the second situation ("blood is helium free since you switched")?

 

[Angelo Farina]

Why is nitrogen "slower" than helium?

Why does helium move faster from endolymph and perilymph compartments to the vascular compartment than from the latter to the blood if the concentration gradient seems to be larger in the second situation ("blood is helium free since you switched")?

It all depends on the diffusivity D in the Fick's law. The diffusivity of Helium is much larger than the diffusivity of Nitrogen.
this is due to the fact that the Helium has a molar mass much smaller than Nitrogen. 4 kg/kmol instead of 32 kg/kmol.
But the diffusivity is a binary quantity, it depends both on the gas and on the medium through which the gas is diffusing, hence the same gas has different diffusivity in different tissues.
Finally, the blood is a liquid medium, which is flowing, this makes the mass transfer governed by diffusion to be overwhelmed by the convective mass transfer due to fluid motion.

 

[David Novo]

It all depends on the diffusivity D in the Fick's law. The diffusivity of Helium is much larger than the diffusivity of Nitrogen.
this is due to the fact that the Helium has a molar mass much smaller than Nitrogen. 4 kg/kmol instead of 32 kg/kmol.
But the diffusivity is a binary quantity, it depends both on the gas and on the medium through which the gas is diffusing, hence the same gas has different diffusivity in different tissues.

Understood. Thank you.

Finally, the blood is a liquid medium, which is flowing, this makes the mass transfer governed by diffusion to be overwhelmed by the convective mass transfer due to fluid motion.

Can you please explain in a simpler/more detailed way how this relate to question 2?

 

[Storker]

kg/kmol

While kg/kmol is an SI unit, g/mol is the normal unit used in chem. In chem.eng, kg/kmol is a better unit, though.

 

[rjack321]

Can you please explain in a simpler/more detailed way how this relate to question 2?

Every second, which far shorter than the shortest compartment half life in any model, there is fresh blood in the capillary next to the ear that in theory has no helium in it. Its arterial blood coming from the lungs so unless there a lung shunt with some venous crossover it has no helium because there's no helium in your inspired gas. That continuous supply of helium free blood maintains the highest possible gas gradient from the tissue to the blood. The rate of diffusion is always going to be at its highest in highly vascularized tissues. The degree of connectivity with the capillaries is controlling the offgassing more than the diffusion rate. The opposite would be poorly vascularized fat tissues or cartilage, also tissues like the cornea (although I don't think its really plausible to have corneal bubbles/DCS). In these kinds of tissues, diffusion rates are much more critical because the capillary bed is relatively sparse - so gases have further to travel before finding a blood vessel to carry them away.

 

[David Novo]

Every second, which far shorter than the shortest compartment half life in any model, there is fresh blood in the capillary next to the ear that in theory has no helium in it. Its arterial blood coming from the lungs so unless there a lung shunt with some venous crossover it has no helium because there's no helium in your inspired gas. That continuous supply of helium free blood maintains the highest possible gas gradient from the tissue to the blood. The rate of diffusion is always going to be at its highest in highly vascularized tissues. The degree of connectivity with the capillaries is controlling the offgassing more than the diffusion rate. The opposite would be poorly vascularized fat tissues or cartilage, also tissues like the cornea (although I don't think its really plausible to have corneal bubbles/DCS). In these kinds of tissues, diffusion rates are much more critical because the capillary bed is relatively sparse - so gases have further to travel before finding a blood vessel to carry them away.

I am not sure if we are talking about the same thing because this seems to justify the opposite of what is stated in the book.

Powell states that "the rapid diffusion of helium from the endolymph and perilymph compartments" (not perfused but connected to the vascular compartment) "into the perfused vascular compartment exceeds...the washout of helium from the vascular compartment by the blood flow away from the tissue."

My question is how is this possible if the "continuous supply of helium free blood maintains the highest possible gas gradient from the tissue to the blood."