Lung ventilation rates

[John Ruben <rubenj@ava.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.  

Consequently, alteration of Mesozoic oxygen availability would have had 
little effect on either lung ventilation rates OR oxygen consumption rates.

Significantly, a variety of other physiological data suggest that current
tetrapod blood oxygen affinity/pH and carbon dioxide regimes are extremely
ancient, dating back probably to the Paleozoic Era. -John Ruben