My understanding is that the O2 in the cells and in the blood will actually
go back into the lungs at very low ambient pressures. It doesn't matter
whether you breath or hold your breath, the O2 will leave your body. It is
something like the bends where gases that will disolve in the body under
higher pressures will come out of solution at lower pressures.

Mike
MU-2


wrote in message
oups.com...
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!