Help needed to understand gas switch and Fick's law

[David Novo]

While continuing my reading of Deco for Divers, I came across another paragraph that puzzled me:

At a gas switch, the diver then changes from breathing a high helium mix to a high nitrogen mix. The blood quickly carries this nitrogen to the perfused vascular compartment of the inner ear. Here the rapid diffusion of helium from the endolymph and perilymph compartments into the perfused vascular compartment exceeds the transfer of nitrogen in the opposite direction and the washout of helium from the vascular compartment by the blood flow away from the tissue.

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, given that nitrogen is more soluble than helium (and according to Fick's law the rate of diffusion has a direct relationship with solubility)?
2) The washout of helium from the vascular compartment by the blood flow away from the tissue, given that the helium concentration gradient between the vascular compartment and blood is higher than between the former and the other compartments (and according to Fick's law the rate of diffusion has a direct relationship with the concentration gradient)?

Can anyone help me with this?

Thank you.

 

[rjack321]

Stop thinking in terms of solubility, it doesnt really fit how helium behaves in a fluid. (aka it doesnt really "dissolve" in anything, its more like a very fine oil-water emulsion throughout a tissue)

Both gases are just moving from their respective area/tissue of high concentration to areas of lower concentration consistent with Fick's. The rate they move is dependent on that concentration gradient. Blood "washes" away the helium because its moving and not static. So you have fresh low helium blood washing past the high concentration area.

 

[Angelo Farina]

I can just add that the solubility is not a factor in the Fick's law.
The law states that the mass flow per unit area [kg/(m2s)], Ji, of the i-th substance in a mixture is given by:

where "rho" is the density of the medium (kg/m3), Di is the binary diffusivity of substance i in the medium, measured in m2/s, and yi is the local value of the mass fraction of substance i.
The diffusivity Di has nothing to do with solubility S...

 

[David Novo]

Both gases are just moving from their respective area/tissue of high concentration to areas of lower concentration consistent with Fick's. The rate they move is dependent on that concentration gradient. Blood "washes" away the helium because its moving and not static. So you have fresh low helium blood washing past the high concentration area.

That was my understanding regarding point 2). As such my question remains.

I can just add that the solubility is not a factor in the Fick's law.
The law states that the mass flow per unit area [kg/(m2s)], Ji, of the i-th substance in a mixture is given by:

where "rho" is the density of the medium (kg/m3), Di is the binary diffusivity of substance i in the medium, measured in m2/s, and yi is the local value of the mass fraction of substance i.
The diffusivity Di has nothing to do with solubility S...

Thank you for the explanation, the book states otherwise, hence my statement.

 

[rjack321]

That was my understanding regarding point 2). As such my question remains.

Apart from the fact that decompression "compartments" do not directly associate with any actual biological cells, tissues, or organs...

The concentration gradient for helium is:
Tissue --->>> vascular compartment ---->>> blood

For N2 after the switch (not always but often):
Blood -->>> vascular --->> tissue

The vascular compartment is essentially the capillary wall for lack of a better analogy.

 

[David Novo]

Apart from the fact that decompression "compartments" do not directly associate with any actual biological cells, tissues, or organs...

The concentration gradient for helium is:
Tissue --->>> vascular compartment ---->>> blood

For N2 after the switch (not always but often):
Blood -->>> vascular --->> tissue

The vascular compartment is essentially the capillary wall for lack of a better analogy.

This was also my understanding but again I fail to see how this answers any of the two questions.

 

[Miyaru]

...
2) The washout of helium from the vascular compartment by the blood flow away from the tissue, given that the helium concentration gradient between the vascular compartment and blood is higher than between the former and the other compartments (and according to Fick's law the rate of diffusion has a direct relationship with the concentration gradient)?
...

I'm not sure if that law applies to the inner ear.

Perilymph, the fluid in the semicircular canals and the otolith organs, is not directly connected to the bloodstream. Gas diffusion into and out of the perilymph is through the inner-ear tissue. Helium off-gassing is not a problem anywhere in your body, except in this organ.
ZHL16 by itself does not compensate for this, neither does Fick's law for this specific biological tissue.

 

[Storker]

ZHL16 by itself does not compensate for this, neither does Fick's law for this specific biological tissue.

Huh? Fick's law doesn't compensate for anything. It just states that the rate of diffusion is proportional to the concentration gradient.

 

[David Novo]

I'm not sure if that law applies to the inner ear.

Perilymph, the fluid in the semicircular canals and the otolith organs, is not directly connected to the bloodstream. Gas diffusion into and out of the perilymph is through the inner-ear tissue. Helium off-gassing is not a problem anywhere in your body, except in this organ.
ZHL16 by itself does not compensate for this, neither does Fick's law for this specific biological tissue.

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?

 

[Miyaru]

Huh? Fick's law doesn't compensate for anything. It just states that the rate of diffusion is proportional to the concentration gradient.

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?