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Bird and dinosaur respiration

I've read Guy Leahy's article in Greg Paul's book which discusses, among other things, dinosaur lungs.

He describes bird respiration as being mostly one-directional, with some of the air flowing into the air sacs upon inhalation. Then, upon exhalation, some of the air sacs compress air through the parabronchi into the bronchus. The lungs are stiff because they don't have to expand and contract.

I understand this only up to a point.

How is this mostly one-directional? Doesn't the air utlimately get exhaled out through the bird's mouth? Or is it coming out some other way?

Why do they breath this way? Is it just a way to keep the air sacs hollow for weight-saving purposes? What's the benefit of stiff lungs? If this way of breathing is primarily a flight-based feature, why did dinosaurs apparently breath this way long before they were flying?

Larry Dunn
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