Oxygen therapy [was Migraine question]

Richard Norman rsnorman at mediaone.net
Sat Feb 17 11:22:17 EST 2001


"Andrew T. Austin" <slightlynervous at NOSPAMbtinternet.com> wrote in message
news:96k87r$ru24 at eccws12.dearborn.ford.com...
>
> > Certainly hyperventilating lowers blood pCO2.  But considering air
> > has such a low pCO2 already, breathing pure O2 shouldn't really
> > make a difference.  Or is it that breathing anything "different"
> > produces hyperventilation?
>
> Good point.  It is not so much the atmospheric partial pressure of CO2,
but
> the rate that the bodily CO2 is expired.  My understanding is that in the
> Hb, raising the partial pressure of O2 will increase the Hb exchange of
CO2
> for O2.
>

I have always thought (and taught) that Hb-CO2 exchange is a result
of Hb-O2 binding and unbinding, not of pO2.  That is, according
to the Haldane shift, Hb-O2 binding causes a change in the pKa of Hb
which causes it to release H+ which helps convert HCO3- to CO2.  At
the same time, there is a change in the ability of Hb to carry Co2 in
the carbamino form.

In any event, the increased CO2 loading with increased O2 is related
to O2 binding, not to the raw pO2.  Since arterial blood is normally
almost already saturated, increasing the pO2 level will not significantly
increase Hb-O2 binding and therefore will not significantly increase
CO2 unloading.

Increasing pO2 will definitely increase dissolved O2.  But blood saturated
with O2 at 37C and 760 mm Hg pO2 holds only 2.4 ml O2/100 ml.  This
is much larger than the 0.3 ml O2/100 ml at 100 mm Hg pO2, but still
only 10% of the amount carried by hemoglobin.









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