Re: Avian respiration

["Michael" <tons@ccs.netside.com>]

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.  

Michael Teuton