The Amazing Efficiency Of The Avian Respiratory System

I've been entranced by birds for my entire life. I've read tons about different aspects of bird life but never really understood the avian respiratory system until this morning when we covered it in our overview lecture of the respiratory system.

We watched a beautiful animation of bird breathing in class that I just loved but unfortunately it is password protected and so I cannot link to it:( I'll describe what it showed below, though.

In the avian respiratory system, air is moving through the lung (the arches in the middle of the diagram) in the same direction during both inspiration and expiration (airflow is unidirectional)! This means that birds have air with a high partial pressure of oxygen available to their lungs all the time. That's why birds can fly at altitudes that people cannot survive at, even at rest-- because they have a constant supply of fresh air to their lungs, unlike people whose lungs are filled with waste gases during expiration.

How exactly does this happen? On inspiration some of the inspired air moves into the caudal thoracic and abdominal air sacs while an equal portion flows into and then across the lung into the cranial air sacs. On expiration, the fresh air in the caudal thoracic and abdominal air sacs is expelled and flows into and across the lung while the used air in the intraclavicular and cranial thoracic air sacs exits the respiratory system via the trachea. (For images and description by the same professor who did the animation-- Dr. Ludders,one of the anesthesiologists at Cornell-- see chapter 8 of the 12th edition of _Duke's Physiology Of Domestic Animals_).