RE: Lung ventilation rates

[Nathan Myhrvold <nathanm@MICROSOFT.com>]

 From:  John Ruben[SMTP:rubenj@BCC.ORST.EDU]

>Re the notion that high Mesozoic atmospheric oxygen concentration would
>alter mammal/reptile lung ventilation rates.  This is broadly incorrect: 
>  ALL tetrapod lung ventilation rates are governed primarily by lung
>CARBON DIOXIDE pressures, not by oxygen pressures. Regardless of lung
>oxygen pressure, at any given metabolic rate significant reduction of lung
>ventilation rate would result in unacceptably high lung carbon dioxide
>levels.  The result would be chronic systemic hypercapnia and acidosis -
>physiologically unacceptable at any level of oxygen availability. 
>
>It is also worth noting that, given the respiratory physiology of all 
>extant tetrapods, upward alteration of Mesozoic atmospheric 
>concentration to 30% oxygen would have had little significant effect on 
>oxygen consumption levels in Mesozoic tetrapods, either ecto- OR 
>endothermic.  Tetrapod blood hemoglobin affinity for oxygen is such that 
>blood oxygen loading is virtually 100% at oxygen pressures well below 
>those of current levels of atmosperic oxygen. Thus, even doubling of 
>atmospheric oxygen would only increase oxygen available to tissues by 
>a small fraction.  

This is interesting, however it does beg the question of why 100% oxygen
is used for humans in medical situations (and in some veternary cases) .
 
I have naively assumed that this is done because the (approximately)
five fold increase in oxygen partial pressure allows better oxygenation
in individuals with impared ventilation.  

Granted, pure oxygen is a LONG way from having a 32% enhancement.
Still, I am curious about why folks would bother with pure oxygen if in
fact the blood loading is already nearly maxed out, and carbon dioxide
exhalation is the gaiting factor.   

Nathan