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> 100% oxygen will delay the point at which the dropping
> pressure results in hypoxia, but eventually you WILL
> reach an altitude where the ambient pressure no longer
> forces a gas exchange in the lungs. Not even 100% oxygen
> will help at that point.
This is true, but the physics behind it has not been
explained properly. The oxygen exchange between the
blood and the oxygen in the lungs does not require
an overpressure as previously stated. Instead, the
problem is getting oxygen into the lungs at high
altitude.
Note that we all have about the same body temperature,
98.6 F or 36 C. At that temperature, the vapor pressure
of water is 47 Torr (47 mmHg = 1.85 inHg).
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/watvap.html#c1
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/watvap.html#c2
At what altitude is the outside
air pressure 47 Torr?
At that altitude, expanding your lungs will cause
water from your lungs to evaporate and completely fill
the lung volume. No air enters the trachea as a result
of lung expansion. That happens at an altitude of 18500 m
or 60,000 feet. At that altitude is is not possible to
get *any* oxygen into the lungs by exercising the
diaphragm -- while the body is still at 37C, which
won't be long.
At 35,000', the outside air pressure is sufficient to
push pure oxygen into the lung spaces -- just barely.
Pressure breathing helps keep some oxygen in the
mixture as well as all that water vapor. People can
tolerate an overpressure of only about 0.5 psi (1 inHg), so a pressure mask can only help a little bit.
At 25,000', breathing 100% O2 at ambient pressure is
sufficient for healthy adults to maintain blood saturation
in the 90+% range. Smokers may be completely
hypoxic.
BTW, my vocal cords quit working at about 25,000'.
There isn't enough air density for me to drive the
vibrations that make voice sounds. At 25-30K, I can
croak a bit, but cannot make myself understood over
a radio. I didn't notice this in the Miramar pressure
chamber, but found out later over the Sierras in a
sailplane.
-bob
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