From: jenkins@fritz (Steve Jenkins)
Subject: Re: Man in space...
Organization: Jet Propulsion Laboratory (NASA)
Distribution: sci
Date: Wed, 11 Nov 1992 17:24:10 GMT

In article <BxHL1F.Dvs.1@cs.cmu.edu> roberts@cmr.ncsl.nist.gov (John Roberts) writes:
>The world record for voluntarily remaining underwater (and conscious) is
>something like 13 minutes. The fellow hyperventilated pure (1 Atm) oxygen
>(don't try this at home!) ahead of time. Does this fit in with your model?
>(I realize his lungs weren't exposed to vacuum, but he didn't have any way
>of getting more oxygen except perhaps very slow diffusion from the water.)

First of all, it's not *my* model, it's standard first-year med school
physiology.  So none of this should be controversial.  :-)  I'm
assuming you haven't yet seen my reply to Henry, so I'll try not to
repeat too much.  It's well known that hyperventilation can extend the
limit of voluntary breathholding.  It's also well known that this is
extremely dangerous.  Last night I was checking up on myself by
reading some reference material (I was right :->) and found a couple
of good paragraphs about this.  I'll type them in tonight.

Now, hyperventilation doesn't add any appreciable amount to the stores
of oxygen outside the lung, but it does reduce the CO2 stimulus to
breathe.  Holding a big breath of O2, however, provides at
least 5 liters of O2.  Normal resting O2 consumption is about 250
ml/min, so 5 l gives you plenty of O2 to hold your breath for 13
minutes.

The point of all this is that breath-holding is not a good model for
depressurization.  When you hold your breath, you have a few minutes's
worth of O2 in the lungs so you can maintain arterial oxygen partial
pressure.  In zero ambient pressure, your arterial PO2 will be zero,
and it's lights out.

>What's the mechanism by which the hemoglobin gives up its oxygen where
>it's needed? And is there a 100% oxygen turnover on each pass of the
>blood through the lungs?

Oxygen moves from the lungs into arterial blood purely by
diffusion--it moves from a region of higher to lower PO2.  The magic
of hemoglobin is that it binds O2 such that the PO2 on the blood side
rises more slowly than the O2 content.  (If O2 were merely dissolved in
blood, the relationship would be linear.)  The consequence is that
more O2 can be loaded into the blood before partial pressure
equilibrium is reached.  (In normal humans equilibrium is always
reached before a red blood cell leaves the lung.)  At the tissues, the
reverse process occurs.  The tissue PO2 is lower than the blood, 
so O2 diffuses into the tissue.  This time, hemoglobin holds the PO2
"artificially" high so that more O2 unloads before equilibrium.

There are also secondary effects, in which the low arterial PCO2
assists O2 loading into blood, and the higher tissue PCO2 assists
unloading O2 into tissue.  O2 and CO2 have this mutual relationship;
one is called the Bohr effect, the other is the Haldane effect.
pH also shifts the dissociation curve.

The PO2 of mixed venous blood is about 40 mmHg, so there is not
complete unloading in the tissues.  However, as O2 delivery to tissue
is by diffusion, the low partial pressure prevents effective uptake.
This is especially problematic in the brain, whose O2 demands are very
high.

>People seem to do "reasonably" well breathing air at around half normal
>pressure, which would imply an oxygen partial pressure of 80mm Hg or not
>much over. What's the lowest pressure of pure oxygen that people can
>manage on? And what is the air pressure on the top of Mt. Everest? (It
>has been climbed (strenuous physical activity) without supplementary
>oxygen. It's very dangerous, but it has been done.)

I don't believe the limits are well understood.  *Slow* adaptation to
low PO2 allows certain compensatory mechanisms (in particular renal
adjustment of bicarbonate and left shifts in the hemoglobin
dissociation curve).  If you were to helicopter directly from sea
level to Everest, you'd be unconscious long before the summit.  On the
other hand, it's clearly possible to ascend it one step at a time
without supplementary oxygen.  There is evidence to indicate that such
feats engender brain damage.  A standing joke among climbers is that
you'd have to be brain-damaged to climb Everest anyway, so what the
hell!

>There's an article in a recent Scientific American on altitude sickness -
>I haven't had time to more than scan it. Apparently it's not really healthful
>to breathe air at any pressure noticeably less than sea level pressure, and
>some people are much more susceptible than others to altitude-related
>health problems.

We're getting a little beyond my knowledge here.  Altitude sickness is
a complex of phenomena, at least some of which is related to low PO2,
not merely low ambient pressure.  It's certainly true that there is
wide individual variation in response to altitude, and no good
predictors of response that I'm aware of.

-- 
Steve Jenkins      			jenkins@devvax.jpl.nasa.gov
Caltech/Jet Propulsion Laboratory	(818) 306-6438
Newsgroups: sci.space