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Re: Avian respiration
Thanks Judy, your ref confirms what I know about avian air sacs not
being involved in gas exchange. Air passes from the trachea to the
mesobronchus and then to the parabronchi which serves as the
equivalent of alveoli in mammals. Unlike mammals where flow is
bidirectional thruout the respiratory system, birds flow air
unidirectionally primarily between the dorsibronchi and ventrobronchi
areas of the parabronchi. They can bypass the parabronchi passing air
directly from the trachea to the air sacs and either cool or warm
themselves dependent on ambient temperature without causing
hypocarbia. The rib cage is relatively quite and air flow is
accomplished by compression of the air sacs by a rocking motion of
the sternum against the vertebral column and some rib movement.
Reptiles create negative pressures to ventilate by expanding their
thoraic cage primarily by rib movement(generalized statement).
Mammals do it with a diaphragm and somewhat expanded thoracic cage.
Turtles are unique and crocs have a cross between reptiles and
mammals with a "pseudodiaphragm" and rib movement.
Your thoughts were interesting. However, I'm still confused on how
avian respiration evolved anatomically. Reptilian and avian and
mammalian respiratory systems are significantly different although
they do their jobs well.
If you look at the kidney, you can see a relatively nice progression
in complexity and function from reptile to bird to mammal.