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Re: Partial pressures



Responders on this thread have been correct when they have pointed out that
an increase in overall pressure does not change the percentage of oxygen in
the air. If the overall air pressure doubles, the partial pressure of
oxygen doubles, but so does the partial pressure of nitrogen. So doubling
the overall air pressure does indeed double the partial pressure of oxygen
in that air, but it does not change the percentage of oxygen in that air.

However, this may does address the question that Herb Gottlieb asked when he
said, "Please tell me what the writer is trying to say?"

The writer has made some errors... here is what he should have been saying.

The binding of oxygen to hemoglobin in the lungs, and the release of carbon
dioxide in the lungs, is dependent upon the partial pressures of oxygen and
carbon dioxide in the lungs. It is not dependent on the relative
percentages of oxygen, nitrogen, carbon dioxide. This is also true for the
amount of gas (such as nitrogen) that might be "dissolved" in the blood
(where "dissolved" is not the same as being "bound" to a carrier such as
hemoglobin).

Let's repeat this. The interaction between oxygen and blood is determined
by the partial pressure of oxygen. It is not determined by the percentage
of oxygen.

Therefore, when a diver (or astronaut, or whatever) is subjected to a
breathing atmosphere different than one atmosphere, it is not the
percentages of oxygen, nitrogen and CO2 we have to worry about; rather, we
have to worry about is the partial pressure(s). If the overall pressure
doubles, the partial pressure of oxygen will double. That could be bad. To
fix this we can remove oxygen from the breathing air. In fact, at two
atmospheres we can cut the percentage of oxygen in half (down to about 10%)
in order to maintain the partial pressure near the desired 0.2 atm.

I would reword the quotation Herb gave as follows. In view of not having
colored type, strikeout, etc., I will use the following convention: I will
place removed text and substituted text in parentheses in the format
(removed / substituted).

"Ordinary air is about 21 percent oxygen and 78 percent nitrogen. (Under
water, each / Each) gas exerts its own pressure, independently of the other.
( / At one atmosphere this means the partial pressure of oxygen is 0.21
atmospheres.) That means the compressed air that a diver breathes at 10
meters ( / (two atmospheres total pressure)) actually has (42 percent of the
total pressure due to oxygen alone / 0.42 atmospheres partial pressure of
oxygen if it were compressed from normal air). That's approaching the
danger level. Much higher than (42 percent / 0.42 atmospheres) partial
pressure of oxygen, and the diver may go into convulsions. Prolonged
exposure to high (concentrations / partial pressure) of oxygen (are / is)
dangerous. The solution is to use less oxygen in the mixture of compressed
gases that divers breathe. Around 300 meters, the gas mixture may contain
as little as 2 percent oxygen ( / in order to keep the partial pressure of
oxygen near 0.2 atmospheres)."

In case that's too hard to read, here's the same paragraph with just the
corrections:

"Ordinary air is about 21 percent oxygen and 78 percent nitrogen. Each gas
exerts its own pressure, independently of the other. At one atmosphere this
means the partial pressure of oxygen is 0.21 atmospheres. That means the
compressed air that a diver breathes at 10 meters (two atmospheres total
pressure) actually has 0.42 atmospheres partial pressure of oxygen if it
were compressed from normal air. That's approaching the danger level. Much
higher than 0.42 atmospheres partial pressure of oxygen, and the diver may
go into convulsions. Prolonged exposure to high partial pressure of oxygen
is dangerous. The solution is to use less oxygen in the mixture of
compressed gases that divers breathe. Around 300 meters, the gas mixture
may contain as little as 2 percent oxygen in order to keep the partial
pressure of oxygen near 0.2 atmospheres."

Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817