From: jenkins@fritz (Steve Jenkins)
Subject: Re: Man in space ... 
Organization: Jet Propulsion Laboratory (NASA)
Date: Tue, 10 Nov 1992 17:40:20 GMT

In article <BxGxF6.50n@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
>In article <1992Nov9.182037.19085@jpl-devvax.jpl.nasa.gov> jenkins@fritz (Steve Jenkins) writes:
>>... At normal arterial O2 partial pressure (about 100 mmHg),
>>the blood is almost completely saturated with oxygen.  You can raise
>>the partial pressure by hyperventilating, but not the oxygen content.
>
>How do the breath-holding effects of hyperventilation work, then?
>Flushing CO2 out to suppress the desire to breathe, as opposed to
>providing more internal oxygen to eliminate the need to breathe?

Precisely.  The neural ventilatory drive generated by the carotid
bodies (located in the carotid artieries conveniently placed near the
brain) is proportional, at a given PO2, to the PCO2 of the arterial
blood.  The slope of the line rises as the PO2 decreases.
Hyperventilation raises the PO2 somewhat, perhaps from 100 mmHg to 140
mmHg or so, but can lower the PCO2 from its normal 40 mmHg down to near
zero, thereby nearly abolishing the carotid ventilatory drive.  This
is unwise.  Hyperventilation also causes alkalosis, as the pH of
arterial blood is determined in the short term by the PCO2.  That's
what causes the dizzyness if you overdo it.

>Certainly the desire to breathe can be suppressed for surprisingly
>long periods by hyperventilating first, even if you're holding
>breath *out* to minimize oxygen in the lungs.

Normal breath-holding causes a potent respiratory stimulus, as the
simultaneous increase in PCO2 and decrease in PO2 potentiate each
other.  This condition is called asphyxia.  Hyperventilation delays the
onset.  In addition, the functional residual capacity (normal exhale
position lung volume) of a normal adult is about 3 liters, so even
"breath out" you've got a considerable supply of O2 on board.

As an aside, I always find the images of (U.S.) football players
breathing oxygen on the sidelines amusing.  Sprinting and other forms
of anaerobic exercise cause the release of lactic acid into the blood.
The buffering of lactic acid by bicarbonate causes a dramatic rise in
the PCO2, which causes the carotid bodies to scream out "breathe, you
fool!"  Pure oxygen does nothing to solve the problem (which can by now
only be solved by ventilation), but instead shuts off the neural
signals.  It's like turning off the fire alarm because it keeps you
awake.  There are not many athletes who can out-exercise the lung's
ability to oxygenate.  Most of us can (easily) exceed the
cardiovascular system's ability to deliver O2 to muscle tissue and the
lung's ability to get rid of carbon dioxide.  Sometimes just reaching
for the TV remote control is enough. :-)

>>The best you could do in anticipation is to prepare to hold your
>>breath (carefully!) so as to maintain some O2 pressure in the lungs...
>
>Probably a bad idea, since it doesn't take very much internal overpressure
>to cause serious lung damage.

Only recommended as a gedankenexperiment. :-)  The point is that,
without some O2 in the lung, you'll be out cold in 30 seconds or so,
hyperventilation or no.

A good reference on all this is John B. West's little monograph
"Respiratory Physiology--the essentials".  I think it's up to two
volumes now.  Highly recommended.

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