"Morgans" wrote in message
...

"Bill Denton" wrote

I believe airline cabin pressure is somewhat below 10,000 ft...

It can be set at anything the pilots want.

Not true, it is limited by the amount of differential pressure the plane can
take.

It is normal to operate at a cabin pressure of 8,000 ft and a pressure
differential across the aircraft skin is normally designed not to exceed 8-9
psi.

As far as partial pressures are concerned, its the partial pressure in the
lungs that matters.

At sea level the partial pressure of oxygen in the atmosphere is 160 mm Hg
yet in the lungs it is only 103 mm Hg. At 10,000 ft that drops to 55 mm Hg
which is considered the minimum for normal operations. Above cabin
altitudes of 10,000 ft oxygen needs to be added to the pilots air supply.
The oxygen which is added should be enough to maintain a pressure of oxygen
in the lungs (alveolar partial pressure) of 103 mm Hg.

At lower levels less oxygen will need to be added but as altitude increases
more will need to be added. A stage will be reached when the 100% oxygen is
required to maintain the 103 mm Hg. This is reached at about 33,700 ft.

After this whilst breathing 100% oxygen one can continue to operate
normally with an alveolar partial pressure of 55 mm Hg (equivalent to
breathing air at 10,000 ft). This is reached at 40,000 ft. From here
oxygen needs to be supplied under pressure.

Summary

Upto 10,000 ft air only
10,000 - 33,700 ft Oxygen/air mix
33,700 - 40,000 ft 100% oxygen
40,000 + 100% Oxygen under pressure