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Re: breathing



>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.