Oxygen partial pressures and lifde support.

[Genghis the Engineer]

Can somebody bail me out here, I'm tying myself in knots.

(1) Air is 21% Oxygen

(2) Based upon ISA, the partial pressure of Oxygen at 10,000ft would be about 0.687668 x 0.21 = 0.145.

(3) So, 100% Oxygen at this partial pressure should give an equivalent level of human body performance – in this case at about 45,000ft from my standard atmosphere tables.

(4) However, I know that 25,000ft is generally quoted as the limit for breathing 100% unpressurised oxygen.


Why the difference? Is it just down to a reduced efficiency of the lungs at low pressures, or am I missing something?

G

[SRB]

Genhis, when it comes to physiological gas pressures I was trained in mmHg so you'll have to do the conversions. (It's a cut 'n' paste job from an explanation I prepared for non-techy people so forgive any noddy terms - I'm not trying to talk down to anyone, just save time).

 Sea level pressure is about 760 mmHg.
 As oxygen is only about one fifth of the constituents of air, the pressure of oxygen at sea level is about one fifth of the total pressure, ie about 150 mmHg.
 This is called the “partial pressure of oxygen” (because it’s only a “part” of the total pressure).
 The missing 10mmHg is exerted by the other trace gases in air, like argon (1%)


The bullets above describe the pressure in the outside atmosphere, but it is the pressure that’s inside our lungs that determines how much oxygen gets pushed into our bloodstream.

We are about 75% water, therefore the air in our lungs is always fully saturated with water vapour. This means the air in our lungs is different to the air outside because it contains a much higher proportion of water vapour. This water vapour exerts a partial pressure too, and this competes with the other gases in our lungs. The partial pressure exerted by water vapour in our lungs is 47 mmHg. It is always 47 mmHg, at any altitude, whatever you are breathing in, because it is always fully saturated with water vapour.

The partial pressure of oxygen in our lungs at sea level is 103 mmHg. (150 mmHg from the atmosphere, but take away the constant 47 mmHg from the water vapour and this leaves 103 mmHg). So in our lungs:

 Sea Level partial pressure of oxygen is about 103 mmHg
 At 10,000 ft this pressure drops to 55 mmHg, but this is enough for normal fit people to get by on.
 Above 10,000 ft the oxygen concentration breathed in has to be increased to maintain the oxygen partial pressure at 103 mmHg, ie more oxygen is added to the air mix in the mask.
 At 33,700 ft breathing 100% oxygen still provides a partial pressure of 103 mmHg. (Just like being at sea level as far as our bodies are concerned).
 Between 33,700 ft and 40,000 ft the partial pressure of oxygen in your lungs decreases to 55 mmHg. (So, although you are now breathing 100% oxygen through a mask, the pressure this oxygen exerts in your lungs is only 55 mmHg). A normal, fit person is still ok, as he is at the equivalent altitude of about 10,000 ft (but people with heart or lungs problems may start to feel the strain. And many do).
 Above 40,000 ft even the 100% oxygen in your mask cannot provide enough pressure to push the molecules into the blood stream. You need positive pressure added to your 100% oxygen to force it across the lung wall.

We are ok up to 10,000 ft because haemoglobin has cleverly adapted its behaviour with respect to the absorption and release of oxygen. It is still able to gobble up oxygen from the lungs and almost fully saturate the blood even at the lower partial pressures experienced at 10,000 ft. This allows humans to live at these altitudes. Above 10,000 ft though, the haemoglobin struggles to absorb sufficient oxygen. Humans living above these altitudes (Peru etc) have other adaptations, but note there are almost no humans who live above about 12,000 ft.

Cabin altitudes are kept below 10,000 ft to add a bit of safety factor – hence 8,000 is the maximum allowed.

The limit of breathing 100% oxygen is normally quoted as about 39 - 40,000 ft, not 25k. The 25k limit may be referring to the upper limit for unpressurised cabins because of the risk of decompression illness (the bends) above these altitudes. (This limit has subsequently been reduced down to about 18,000 ft. You'd be unlucky to get the bends down there but Health & Safety rules all these days).

You weren't missing anything. It's a tricky subject to get your head around and many struggle with it at first. Read the Helios thread again and you'll realise that there is a lot of ignorance on the subject of hypoxia, sadly by people who should know better.

I think I'm done. I hope this makes some sense.

[Genghis the Engineer]

The 25k limit may be referring to the upper limit for unpressurised cabins because of the risk of decompression illness (the bends) above these altitudes.

That is almost certainly the missing bit of information; I really don't recall that ever being mentioned at various aeromed courses I've done - thank you very much indeed.

G