[Beowulf] HVAC and room cooling...
Gerry Creager (N5JXS)
gerry.creager at tamu.edu
Sat Jan 31 17:30:47 EST 2004
At the risk of turning this technical, the real problem is usually the
partial pressure of available oxygen. Generally, if the PaO2 (arterial
blood partial pressure) drops below about 80 mmHg, one begins to suffer
the effects of hypoxia. Most folks (University administrators seem to
be immune to this) start dying at or below a PaO2 of 40 mmHg.
In general, if one considers a standard pressure to be 760 mmHg at sea
level, and noting that, in fact the normal concentration of oxygen
available ambiently is roughly 20.95%, one sees the ambient PO2 at sea
level is approximately 159 mmHg. Upon inspiration, one adds significant
water vapor to the inhaled air (for reference, the medical and aviation
world tend to assume the partial pressure of water in ambient conditions
is about 4 mmHg), usually, about 40 mmHg at saturation... which is how
the inspired air is supposed to be when it hits the lungs.
So, taking that off-the-top from the partial pressure of oxygen in
inspired air we have 159 - 40= 119 mmHg. Now, there's a gradient that
helps control the transfer of oxygen across the alveolar-capillary
barrier, where gas exchange occurs. Further, there's some transfer at
the same interface of carbon dioxide leaving the blood cells and plasma.
This combines to a net pressure gradient (I'm working from memory
here, and I've not had to give this lecture for several years, so the
exact numbers could be off) of 20-25 mmHg. The net result is simple:
The PaO2 (arterial O2 concentration) is about 95 mmHg for normal
Nitrogen in an enclosed space tends to act simply as an oxygen
displacer. Thus, you go from roughly 78% N2 in ambient air to however
much you ramp it up, and the O2 concentration decreases from a nominal
21% (rounding error, you understand) to whatever's left. If you note,
while there's trace gases in ambient air, we can generally ignore them
and worry about N2 and O2. YMMV for increased concentrations of CO2 and
That's the end of gas exchange physiology I. There will be a short quiz
Monday. We'll continue with the next module. I encourage everyone to
have read the Pulmonary Medicine chapters in Harrison's for the next
Pfenniger Daniel wrote:
> Per Lindstrom wrote:
>> Liquid Nitrogen is deadly dangerous as it will shut off the breathing
>> function of the human body when the Oxygen content falls below 19.5 %
>> and thereby causing suffocation.
> Yeah sure, this remind me the joke about dihydrogen oxyd (H2O) deadly
> dangerous when inhaled in too large quantities.
> The quoted 19.5% are probably referring to the human body (blood?)
> concentration, not the inhaled air. Otherwise most of the people in
> pubs or small rooms would rapidly die as the normal O2 concentration
> in air is 20%.
> Note that in the responded message John was confusing N2 and NO2.
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Gerry Creager -- gerry.creager at tamu.edu
Network Engineering -- AATLT, Texas A&M University
Office: 979.458.4020 FAX: 979.847.8578
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