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This isn't strictly a dinosaur question, but it is prompted by discussions
on this list about avian v other types of respiratory systems.

As I understand it, the avian respiratory system is arguably more efficient
than the mammalian system in part due to the unidirectional flow of gas
through the avian lung, even though bidirectional flow still occurs in the
trachea.  Unidirectional flow makes sense to me as a more efficient gas
exchange process, but wouldn't this suggest that the more primitive gill,
which ventilated the organism using an entirely one way flow of oxygen
bearing fluid over a vascularized surface (no bi-directional flow
whatsoever), was even more efficient? Has anyone investigated the gas
exchange efficiency of gills v avian or mammalian lungs?  If it turns out
that the more primitive unidirectional system is more efficient, it raises
the question as to why terrestrial vertebrates abandoned this system in
favor of the "in and out" breathing of birds, mammals and reptiles (after
all, we didn't [thank goodness] abandon the unidirectional flow of the
digestive process).  Perhaps the density and oxygen concentrations in the
differing fluids has something to do with it.