My experience with scaling of physiological parameters is the same as Paul's. I
find that quantities such as cardiac output, ventilation rate, gastrointestinal
absorption rates, etc. scale as A*body weight^0.7 with little difference in the
value of A across species. For ventilation rate as an example, A is 20.2 in the
mouse and 20.0 in the rat; for cardiac output, I find A is 15.3 in the mouse
and 14.7 in the rat. Tissue weights as % of body weight and capillary volume as
% of tissue weight and blood flow rates as % of cardiac output appear to be
quite similar across species. However, enzyme tissue capacities can be hugely
different even among closely related species, resulting in highly idiosyncratic
metabolic profiles.
Paul's posting does bring up an interesting point that I've never been sure of.
Some laboratory animals have been killed by decapitation and then (presumably)
exhaustively exsanguinated by gravity. Blood volumes so determined are 5-7% of
body weight. When you subtract the tissue capillary volumes (determined by
measuring residual blood volume in tissues by radioactive tracers) from this
number, you find that there is only about 40% of the total measured blood
volume left to put in the major arteries and veins. Is this reasonable? Some
people have treated measured blood volume as arterial plus venous only and
capillary blood as being in addition to the measured amount, but the blood
comprising 9-11% of body weight seems too high to me. The skeleton of rats is
less than 6% of body weight.
--
Michael C. Kohn
Laboratory of Computational Biology and Risk Analysis
National Institute of Environmental Health Sciences
P.O. Box 12233, Mail Drop A3-06
Research Triangle Park, NC 27709-2233
Telephone:
919-541-4929 (voice)
919-541-1479 (fax)
e-mail:
kohn at valiant.niehs.nih.gov
WWW home page:
http://valiant.niehs.nih.gov
