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In a message dated 7/26/98 3:08:07 AM Eastern Daylight Time,
jjackson@interalpha.co.uk writes:

<< The avian one-way system doesn't quite include the trachea >>

My understanding of the avian respiratory system leads me to believe that it
does, to a large degree, eliminate the problem of dead air space posed under
other respiratory schemes by an elongated trachea.  By a long shot, I'm not
the best person to make this case, but here are my thoughts:

Unlike mammals and non-dinosaurian reptiles that have a bellows-like
ventilation system, it takes *two* inhalation/exhalation cycles for air to
move in and out of a bird.  In a bird lung, new air comes in one end of the
lung and the used air goes out the other end.  One set of air sacs store air
before it's ready to enter the lungs, and another set of air sacs store air
until it's ready to be breathed out.  The sequence of bird breathing, as I
understand it, is as follows:

Inhalation #1: Air inhaled into posterior air sacs;
Exhalation #1: Spent air is expelled via the mouth from anterior sacs and
previously inhaled air moves from posterior sacs into the lungs;
Inhalation #2: New air is inhaled into posterior sacs and previously inhaled
air moves out of lungs to anterior sacs; 
Exhalation #2: Inhaled air #1 is expelled from anterior sacs via the mouth.

This means that, at any one time, air flow is essentially unidirectional in
the trachea; thereby eliminating (or, at least greatly reducing) the dead air

As I said before, maybe sauropods had this type of lung and maybe they didn't.
I have no idea.  But it certainly seems obvious that the avian lung evolved
either within the Saurischia or perhaps even prior to the divergence of the
Saurischia from the Ornithischia.  Personally, I think an avian-like lung was
an evolutionary innovation of the basal dinosauria.  Sereno and Novas have
certainly shown that Herrarosauridae and Eoraptor--perhaps two of the earliest
known dinosaurs--had already evolved pluerocoels (or at least hollowing) in
the long bones and the centra of the vertebrae.