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Re: Breathing



>Date: Sat, 17 Apr 1999 15:42:17 -0400
From: "Patrick Norton" <ptnorton@email.msn.com>
To: "Dinolist(message)" <dinosaur@usc.edu>
Subject: Re: Breathing
Message-ID: <004301be890a$676e1b20$5c5efbd0@oemcomputer>


I'll open myself up to correction again and ask why an (arguably) close to
optimal gas exchange sytem (gills), which evolved in a fluid medium that was
approximately 50% oxygen (H2O), needed to be abandoned for a *less*
efficient exchange system (the lung) in a medium (the atmosphere) that
contains *less* than half (21% ?) the oxygen concentration? Terrestoriality
apparently had a huge cost, energetically speaking.  Perhaps endothermy was
unavoidable, and perhaps the continued taxonomic dominance among vertebrates
of the archosaurian lineage that includes birds can in part be ascribed in
some degree to the fact that they are the only lineage to *reinvent* an
approximation of the counter-current (birds, I am told, have only a
cross-current) respiratory system.<

I think, (as Dinogeorge pointed out), that water holds a much less
concentration of dissolved O2. That`s probably why most aquatic forms
started out ,and remain being, "cold-blooded" (warm-bloodedness requiring
much oxy consumption). To have gills in aterrestrial enviorn, they would
have to be "bathed" in water, which might not be impossible, but the size of
the "tank" would be too small to store enough o2 for the gills to extract .
You can put only so many tropical fish
in a small tank, without adding an aerator.
The hypothetical "tank" within the organism would ( by definition of being
"within"), have to be way too small to hold enough oxygen.