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"Thomas Kleemann (EED)" wrote:
> > > A previous posting (which I can't find now, but I don't think
> > >anyone argued against it) indicated that increased ppO2 will
> > >tend to block the Nitrogen molecules from coming out of the
> > >cells by osmosis.
> >
> > I will argue against it - that is not the way. There is no
> > osmosis by
> > gas. Gas can go freely on each side of the membrans of the cells.
> > So there is no osmosis. Osmosis occurs, when a part of the
> > fluid can=B4t pass the membran of the cell.
> >
> > > The only beneficial effect of the additional O2 that I can
> > >see is that (again from a previous Bill Wolk posting) "the
> > >additional osmotic pressure forces O2 into the surrounding
> > >tissue,
> >
> > That has nothing to do with osmosis but with diffusion.
> >
> > > Is that the only benefit of the higher ppO2, or is there
> > >something other way in which it aids off-gassing N2?
> >
> > Yes - I explained it up there.
> > The models of diffusion work with the DIFFERENCE between the
> > pressure of the gass inside the bubble and the pressure in the
> > blood. And this pressure is maximal, when the pressure in the
> > bubble is maximal and the pressure in the surrounding plasma is
> > nearly zero. And this nitro-zero-pressure can be made with any
> > nitrogen-free gass (heliox (see COMEX-deco-tables), oxygen etc.).
> >
> > HTH
> >
> > Bjoern
> >
>
> I think there are some terms stirred up here.
> AFAIR (but I'm not an expert)
> the process, when some stuff (soluted gas, salt or other)
> spreads in a _homogene_ medium by the gradient of concentration,
> is called diffusion.
>
Right here I think.
>
> If there is a membrane within the direction of the flow, jamming
> the process, then it is called osmosis.
>
> When a part of the fluid can't pass the membrane of the cell, then
> it is a _semipermeable_ osmosis.
>
> Am I right with that, or totally off track?
Almost. Osmosis is different from diffusion because it has a driving for=
ce,
osmotic pressure. Diffusion is driven solely by the tendency of all thin=
gs
to move towards a less organized state (lower their entropy), all other
things being equal (e.g. heat and stirring or currents would help diffusi=
on,
but without them...). Osmosis is driven by osmotic pressure. I don't
remember the exact mechanics of osmosis on a molecular level, but it
basically has to do with the "desire" (as if fluids and membranes had wil=
ls)
to achieve equal concentrations of dissolved substances on either side of=
a
membrane. Using the example of saline solutions, the problem comes when =
the
dissolved substance (e.g. salt) can't cross the membrane. The only way t=
o
equalize the concentrations on both sides of that membrane is by water
moving across the membrane in the direction of the higher salt
concentration. This movement is driven by osmotic pressure (I'm sure
someone on here must know all the math and mechanics). It will stop movi=
ng
once the solution on the other side of the membrane becomes diluted enoug=
h
that its concentration is the same as the one across the membrane.
You were on the right track, but I think that the fact that the solut=
e
can't pass through the membrane is a requirement for osmosis of any type,=
your first example would just be diffusion of the solute across a membran=
e,
driven by the difference in their concentrations (or partial pressures if=
we're talking gasses).
>
>
> OTOH, that was a good explanation of the 'ox window'.
> /Thomas.
>
I agree.
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