From:	Calvin Hsieh <u2172778@*******.ACSU.UNSW.EDU.AU>
Subject:	Re: Underwater breathing
Date:	Sun, 1 Dec 1996 13:18:43 +1100

SNIP!
>
> I'm not sure if this applies, as I've not been following the thread too
> closely, but I thought that a conventional "air" mix becomes quite lethal
> at very high pressure...something about the oxygen concentrations and
> pressure - net result is severe damage to the lungs. Thats why they
> routinely use a mix with helium for extreme depth excursions. (Please
> correct if I'm wrong, I'll try to remember where I read that..)
>
> ~Tim
>
Yeah. You see, the partial pressure for O2 has to remain at a high amount
in order for us to take O2 from the air and into our red blood cells in
haemoglobin. We use helium because it is unreactive and does not dissolve
nor change in volume too significantly compared to nitrogen (air is about
80% nitrogen). Hence, helium is a good "carrier" gas for O2 delivery.

I'm cutting this explanation short as I don't want to become to
medicalized here.

Shaman