Re: Mt.Wilson

On Wed, 14 Dec 2005 10:11:36 -0800 (PST), Brian Tung wrote:

> Mike Simmons wrote:
>> A lot less oxygen. The decrease in atmospheric pressure is approximately
>> exponential so the higher you go the more of a difference each incremental
>> increase makes. Think of going from sea level to 4000 feet. Most people
>> won't notice any change. 4000 to 8000 is noticeable but not huge. 8000 to
>> 12,000 matters a lot and acclimatization really matters (which certainly
>> isn't true for going from sea level to 4000 feet).
>
> Not quite. I mean, I think your subjective characterization feels
> right, but the physics isn't. The atmospheric pressure does scale
> roughly exponentially, but it's a negative exponential, with a scale
> height of about 8 km. In other words, atmospheric pressure (in atm) is
> approximately exp(-h/8 km).

Hi Brian,

Other than not saying that it's negative -- which I think is obvious in the
context -- what isn't right about what I said regarding the physics? (BTW,
scale height for Earth is about 7.4km.)

> I think the reason we don't notice a whole lot from 0 feet to 4,000 feet
> unless we exert ourselves is that we normally have a surplus of oxygen
> in the air we breathe; otherwise, we wouldn't be *able* to exert any
> effort. At some point in increasing altitude, though, the surplus runs
> out and we feel the effects, something that NBA players who visit the
> Denver Nuggets can attest to.

Rather than physics, you're referring to physiology and what you say isn't
right. We don't have a "surplus of oxygen" to use. There is a physiologic
equivalent to what you say, though, which is the sigmoid shape of the
oxygen disocciation curve -- (the relation between partial pressure of
oxygen [pO2] in inspired air and the percentage of hemoglobin that is
saturated with oxygen (i.e., oxyhemoglobin) [sO2]. The curve is at a
plateau for normal people at sea level and for some drop in pO2 but once
you reach the steep part of the curve a small change in pO2 makes a big
difference in sO2 (this point is called desaturation). It's the amount of
oxyhemoglobin in the arterial blood that matters, and it's not a linear
relation (as, e.g., for dissolution) because of the affinity of hemoglobin
for molecular oxygen.

But this isn't that big a factor in normal people because respiratory drive
isn't affected much by a decrease in blood oxygen anyway. This is contrary
to common belief and doesn't seem to make sense at first so it's normal to
assume hypoxia drives respiration. But the partial pressure of dissolved
carbon dioxide (CO2) in the arterial blood is the primary factor that
drives respiration. This makes sense since as you exercise you produce
more CO2 but because of the above sigmoid nature of the oxygen disocciation
curve your available oxygen in arterial blood doesn't change. If you were
to breath air with a little CO2 in it you'd find yourself breathing very
hard to try to reduce your arterial pCO2 even though your pO2 was normal.
Reduce the O2, though, and there's a far smaller drive to breath harder,
which is eventually overcome by the decrease in CO2 that works the
opposite.

When you go to altitude the amount of oxygen in the blood drops once the
partial pressure of oxygen in the air drops enough to get to the steep part
of the oxygen disocciation curve (which gives an effect like what you said
above) but the carbon dioxide doesn't change. Thus mountaineers have to
learn to breath harder even at rest because the oxygen drive is so weak.
Thus you end up with too little CO2 in the blood which itself can be a
problem but it's better than being woozy all the time. You can't overdo it
even to try and get more O2 because the CO2 level will overcome it (and
there are strong effects of too little CO2 that are detrimental anyway,
like constricting arterial blood flow to the brain). Purse-lipped
breathing -- puckering your lips on exhalation to maintain a higher
pressure in the mouth and thus in the lungs -- is one way that lung
patients and mountaineers can increase oxygen in the blood. It makes it
look to the lungs like you're at lower altitude.

If you're still unconvinced of the difference in the hypoxic and
hypercapnic drives then let me know and I'll arrange for you to breath a
mixture of 21% O2 with 5% CO2. That's convince you *real* fast. ;-)

Your assumption that we have a surplus of oxygen that allows us to exercise
is not right. You're ignoring the physiologic effect of exercise on
breathing and blood flow. The oxygen available to the blood is thus
increased many-fold. The pressure is almost the same (the pressure in the
alveoli drops as you use more oxygen) but blood flow is greatly increased,
which brings blood in contact with the alveoli faster so it can pick up
oxygen quicker and thus carry more to the muscles. Try keeping your
breathing the same while increasing your exercise and you'll see it doesn't
work. Actually, what you'll notice there is the hypercapnic drive -- the
increase in CO2 that causes you to breath harder -- not a decrease in
oxygen.

> It doesn't take much to acclimate if you're just sitting around. Cabins
> on airplanes are routinely pressurized only to 0.74 atm, the equivalent
> of an altitude of 8,000 feet.

That's not acclimatization. That's just not using much oxygen or producing
more than normal CO2. There is a long list of physiologic response to
chronic exposure to altitude that take place, chief among them (in this
discussion) a shift of the oxygen disocciation curve so that a lower pO2 in
the lungs (i.e., in the atmospher) produces a higher sO2 in blood than it
would at sea level, as though the hemoglobin's affinity to oxygen
increases. There also changes in response to the decreased CO2 in the
blood, which is a problem because it raises the blood's pH (acidity), which
is kept in very strict balance on a short-term basis by adjusting breathing
to increase or decrease blood CO2 (the kidneys do this by adjusting the
metabolic rather than respiratory pH). If you fly to 8000 feet and you're
comfortable sitting around you'll feel differently if you start to exercise
heavily. After acclimatizing for a few days you can not only rest
comfortably but exercise as well.

>> This is what makes it so amazing that people can go to great heights. The
>> higher you go the greater the effect of going even higher. How some
>> climbers can do what they do is beyond me.
>
> Again assuming a scale height of 8 km, the atmospheric pressure at the
> top of Mt Everest is only about 0.33 atm!

The pCO2 of arterial blood in Everest summiters not using supplemental
oxygen has been measured at 7 mmHg. Normal is 40 mmHg. A change of 2 mmHg
is enough to make you start breathing harder. A change of 5 mmHg will
cause you to pant. They're breathing so hard to get the oxygen they need
-- partly due to training (you learn to breath more when you feel woozy)
and partly because the hypoxic drive definitely has an effect at that kind
of unreal altitude -- that they've put themselves in a state that would be
fatal to the average person. You know how you get dizzy when you
hyperventilate at rest? That's due to a drop in CO2 in the blood of a few
mmHg, which causes constriction of the peripheral arteries including the
carotid to the brain. I can't figure out how they stay concsious, let
alone alive. I'm not sure if that's been explained yet. My theory is that
they aren't human -- nothing else fits the observed data.

Mike Simmons
.

Relevant Pages

  • Blood Pressure Treatment
    ... Medscape review supports effectiveness and safety of new non-drug blood ... trials evaluating the efficacy of device-guided breathing. ... authors conclude that RESPeRATE, an FDA-cleared over-the-counter ... significantly lowered blood pressure with no adverse side ...
    (sci.med.cardiology)
  • Re: Mt.Wilson
    ... I have to say that I must have been breathing too low a percentage ... ambient pressure declines, which of course decreases the partial pressure ... > I personally experinenced recognizable degradation on 100% ... have at sea level. ...
    (sci.astro.amateur)
  • Re: Chris: The Actual Data about the Relationship of Tight Control & Retinopathy
    ... that controlling blood sugar and blood pressure, over time, is the only ... aqueous humour to soak up ... a reversal and avoid risky eye surgery. ...
    (alt.support.diabetes)
  • [breathing pressure limits]: forming composit space station skin in situ
    ... breathing pressure for work, not "emergency ... practice breathing thin air. ... the displacement effect within the lungs of the ... you are confusing the "emergency" situation ...
    (sci.space.tech)
  • Re: I knew I needed a layoff but...
    ... someone holds their breath during the exertion phase, ... Somethingsomething building up intrathoracic pressure which impedes ... the blood flow to and from the brain and might cause you to black out. ... Actually I was running a gauntlet of wheelchair-bound ninja panhandlers. ...
    (misc.fitness.weights)