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Eddie, Rich, Bernie, and Hans,
Obviously O2 gets into a pressurized tissue somehow but I can not picture
it migrating passively by random walk diffusion against the onslaught of a
pressure gradient even though that gradient is surely quite small.
Before there can be diffusion between the tissue and the blood any
pressure gradients must be equalized. So, chances are that the tissue off-
gasses to the plasma as soon as it comes under it's influence and once this
pressure gradient is eliminated diffusion between the mediums proceeds.
Off-gassing is apparently the first event to occur as the blood approaches a
pressurized tissue - a situation that is not present (or not to any
appreciable extent) in normobaric conditions and which surely diminishes the
window of opportunity for diffusion. It will also instantly dilute the
plasma O2 content with inert gasses, lowering the PPO2 of the plasma as it
enters the capillary.
However, another factor that figures into this is that the release of O2
from Hb raises the total plasma gas tension. What does this do to the O2
window which depends upon a drop in blood gas tension through the capillaries
to create a vacancy into which inert gasses will supposedly "expand" (not
diffuse)?
The following pet hypothesis that came to me in a dream one day - a nightmare,
I'm sure! If there is anything to it it may play a part in all this as well.
The RBC's brush the walls of the capillaries as they pass through,
squeezing out the intervening plasma, shortening the diffusion distance
between their interior and the interstice and providing an alternate path to
the slow trip through the aqueous plasma to the tissue. If O2 is
dissociated from Hb quickly enough the restriction presented by various
factors of isolation within the bounds of the RBC may permit a sudden rise in
O2 pressure that causes the RBC to act as a small pressure vessel -
momentarily barometrically independent of the local ambient environment. O2
will be forced across the path of least resistance quickly, insuring a minimum
delivery of O2 in spite of tissue gas tensions.
In support of this crazy idea is the fact that most of the CO2 that enters
the plasma passes through the RBC's first where it is cayalysed by carbonic
anhydrase to form bicarbonate. If the RBC can manipulate the path of CO2
flow to insure this reaction within the RBC then it should be no problem to do
the same with the O2 it carries so that rather than being distributed
wholesale throughout the chaos of the plasma it is shot directly into the
interstitial fluid.
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Diffusion can not proceed against a pressure gradient (except perhaps in
the case of an extreme solubility disparity). If I take a container divided
by a permeable rigid membrane and inject 10 psi of 100% O2 into one side and,
at the same time, 20 psi of air into the other the air will expand into the
100% O2 side against a considerable O2 diffusion gradient in order to satisfy
the pressure gradient before any diffusion between the two sides occurs.
Diffusion is a forceless, directionless, passive phenomenon - a
redistribution of gasses between two adjacent environments or among molecules
of a single environment due to a random movement of molecules. There is no
driving force or energy prodding any body of molecules to move towards
distributive equilibrium down a concentration gradient. A "diffusion
gradient" is not a measure of any kind of force or pressure - only relative
concentration.
Nitrogen molecules do not "diffuse" into an area because there are fewer or
no nitrogen molecules there as if a vacuum or a force were sucking them in.
They do so because of the random migration of molecules (a mix or a single
species) in both directions across an interface. This bi-directional
exchange can not take place in the presence of a pressure gradient which
forces an expansion in one direction.
A pressure gradient, on the other hand, has the driving force of an entire
mass of molecules behind it and it will force the "expansion" of an "entire
mix" into and throughout a new environment with no regard for the distribution
of individual species within the mix - it indiscriminately drives a mix of
gasses to equalize an inequality in energy distribution.
Diffusion is species specific in that it can and does ultimately result in
a specific re-distribution of individual concentrations between communicating
environments depending on solubility. Though the concentrations of O2 and N2
in air are 20% and 80% the concentration in a lake is 36% and 64% due to the
differences in solubility. When a pressure gradient forces a mass of gas
into a liquid it simply pushes everything in that is available at the front
and diffusion will then proceed to re-distribute everything once the pressure
is equalized.
> Why do you want to know about oxygen delivery to tissue? <
Because I'm a diver, and because I am suspicious of many of the ideas floating
around the diving community about decompression, O2 treatment for the bends,
the O2 window, O2 narcosis, and other issues.
I'll check out the handbook next time I'm close to a big library.
Thanks,
Chuck Boone
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