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Terry,

Inert gas uptake and elimination is a function of the partial pressure
GRADIENTS between blood and tissue, and between blood and the
pulmonary alveoli.  Thus, inert gas uptake and elimination is
very much influenced by the gas mixture breathed.  Inert gas
uptake and elimination is concerned with the movement of 
DISSOLVED gas and the diffusion of this dissolved gas.  No matter
how large the difference between tissue and blood, or blood and lungs,
this gas will diffuse, not evolve into free gas phase.

However, free gas evolution (ie, bubble formation) is determined by
the difference between dissolved gas partial pressure and the ambient
(environmental) pressure.  When the ambient pressure exceeds the
dissolved inert gas pressure by a large enough margin, then
the dissolved gas can 'evolve" into free gas.

Thus, the ambient pressure and the the dissolved inert gas partial
pressure is the determinant of your "ceiling" (shallowest depth that
you can ascend to and not violate the decompression model).  However,
the gradient that exists between the inert gas concentration that you
are breathing and the dissolved inert gas partial pressures in your
tissues is what determines how long you will have to stay at each 
decom stop before it is safe to ascend to your next stop.

Hope this helps -

John Crea
Submariner Research, Ltd.
(johncrea@de*.co*) 

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