"Birds have one of the most complex respiratory systems of all animal groups. Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation."
By utilizing a unidirectional flow of air, avian lungs are able to extract a greater concentration of oxygen from inhaled air. Birds are thus equipped to fly at altitudes at which mammals would succumb to hypoxia. This also allows them to sustain a higher metabolic rate than an equivalent weight mammal.
For those who wish to know more about it, please check here.
Here are references to pterosaur breathing:
"In this report we present various lines of skeletal evidence that indicate that pterosaurs had a highly effective flow-through respiratory system, capable of sustaining powered flight, predating the appearance of an analogous breathing system in birds by approximately seventy million years."
"A 2009 study showed that pterosaurs had a lung-air sac system and a precisely controlled skeletal breathing pump, which supports a flow-through pulmonary ventilation model in pterosaurs, analogous to that of birds. The presence of a subcutaneous air sac system in at least some pterodactyloids would have further reduced the density of the living animal."In current evolution theory thinking, birds evolved from dinosaurs. That means that according to the current "dino to bird" evolution thinking, birds and pterosaurs both evolved this unique and complex breathing system completely INDEPENDENTLY.
In evolution theory, this idea is called "convergent evolution". The similar characteristics are called "analogous".
In the position I am presenting in this blog, we do not need to rely on a (miraculous) convergent evolution. Modern birds developed from pterosaurs so the development was not independent.
Postcranial skeletal pneumaticity and air-sacs in the earliest pterosaurs
Richard J. Butler1,*,
Paul M. Barrett1 and
David J. Gower2
Pterosaurs breathed in bird-like fashion and had inflatable air sacs in their wings
Posted by Darren Naish on February 18, 2009