[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.
>     Dan
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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
Cell: 979.229.5301    Pager: 979.228.0173

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