Re: breathing

["Patrick Norton" <ptnorton@email.msn.com>]

>Why does air exit back through the trachea instead of a unidirectional tube
evolutionarily created someplace else on the body?<

That's the nub of my question.   Except that I might phrase it as why such a
unidirectional tube wasn't 'preserved' among terrestrial vertebrates rather
than why it wasn't 'created' within that group, since such a unidirectional
counter-current gas exchange system was the primitive condition in that
group.