There are other, important, limiting factors. The alveoli need to exchange
two gases--CO2 and O2 and pressure gradients are needed for this. The fly in
the ointment is that water vapor pressure in the alveoli remains near
constant at 47mm Hg (Torr). The CO2 comes from diffusion across the
capillary-alveolar barrier, from the blood, and therefore remains somewhat
high and at 30,000 ft is about 30 mm Hg. So the O2 must ovecome this
pressure and about 30 mm more to get into the blood effectively. So unless
the O2 is above about 107 mm Hg you don't get enough in your blood to do you
any good.

--
Hartley Falbaum,


"Brien K. Meehan" wrote in message
oups.com...
T o d d P a t t i s t wrote:
Wrong. the partial pressure has everything to do with it.

Wrong, the partial pressure has nothing to do with it.

The lungs (or more accurately the blood in the lungs) need
O2 pressure to pick up O2. That O2 pressure is referred to
as the partial pressure of O2. You can be at altitude on
100% O2 and have the same partial pressure as at sea level
on partial O2

The lungs (not the blood) need pressure to allow gas exchange
(respiration) in the alveoli. The pressure (or lack thereof)
determines the effectiveness of respiration. Reduced effectiveness can
be compensated for with supplemental oxygen.

Sounds like you don't understand the process.

I think we're talking about different parts of the process. It sounds
like you don't know what I mean by respiration.

You may be referring to the breathing reflex
that requires sufficient CO2 to trigger breathing ...

I did not refer to that.

The original question was why you lose consciousness faster
at altitude than if you hold your breath at sea level.

The original question is why you lose consciousness at altitude with
supplemental oxygen when the partial pressure of oxygen appears to be
the same as at sea level. It had nothing to do with holding one's
breath.

True - because the PP drops too low for consciousness or
life. It's all about PP of O2.

True because the lungs become unable to respirate effectively at
reduced atmospheric pressure, which leads to hypoxia.