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?

As others have posted in the other thread, it is all about pressures.
As you go up in altitude, pressures decrease, so gases come out of
solution (blood).
As you go deeper beneath the surface of the water, the pressure
increases, trapping gases in solution (blood).