It isn't totally clear from your post but I think you have it wrong. The
concentration of O2 is the same at all altitudes. At 18000' for instance
the air pressure is half, the concentration of O2 is the same as at sea
level so the total amount of O2 is half.

Mike
MU-2


"Mark Hansen" wrote in message
...
On 8/15/2005 09:54, wrote:

I am not a pilot, and as much as my email address sounds like I am a
doctor, I'm not...

That said, I thought someone here might be able to help me out - they
news reports talk about passing out within seconds at high altitudes.
I understand the air is thinner / less O2.

But if I go underwater, there's certainly no air there. And I can stay
down for a while, holding my breath. How is high altitudes different?
I would think you would rasp / fight to breath in but keep functioning
as your body uses up the 02 in the blood from your last breath? Not
like a sleeping gas in the movies where you just keel over as soon as
it hits you? THANKS!


There is a big difference between Air and Oxygen. With no Air,
you are unable to move your lungs (there is nothing to pull in).

With low oxygen, you can still move air, it just don't provide the
oxygen necessary to support your life.

This is called Hypoxia, and one of the side effects of this is
euphoria. What that means, is it causes you to feel like everything
is really great, when in fact, you're asphyxiating yourself.

Here is a link to an explanation of Hypoxia:

http://www.batnet.com/mfwright/hypoxia.html

This is the simple answer. Let me know if you would like more details.


--
Mark Hansen, PP-ASEL, Instrument Student
Sacramento, CA