Air Sacs
As we all know that the birds' lungs obtain air during the respiratory process (during both the inhalation as well as exhalation process).The air sacs do all the "pumping" and the lungs simply absorb oxygen.
Since 1870 onwards scientists have generally accepted that the post-cranial skeletons of many dinosaurs contained many air-filled cavities especially in the vertebrae. Pneumatization of the skull is found in both synapsids and archosaursu, but postcranial pneumatization is found only in birds, non-avian saurischian dinosaurs, and pterosaurs.
For a long time these cavities were regarded simply as weight-saving devices, but Bakker proposed that they contained air sacs like those that make birds' respiratory systems the most efficient of all animals.
John Ruben et al. (1997, 1999, 2003, 2004) disputed this and suggested that dinosaurs had a "tidal" respiratory system (in and out) powered by a crocodile-like hepatic piston mechanism - muscles attached mainly to the pubis pull the liver backwards, which makes the lungs expand to inhale; when these muscles relax, the lungs return to their previous size and shape, and the animal exhales. They also presented this as a reason for doubting that birds descended from dinosaurs.
Researchers have presented evidence and arguments for air sacs in sauropods, "prosauropods", coelurosaurs, ceratosaurs, and the theropods Aerosteon and Coelophysis.
Since 1870 onwards scientists have generally accepted that the post-cranial skeletons of many dinosaurs contained many air-filled cavities especially in the vertebrae. Pneumatization of the skull is found in both synapsids and archosaursu, but postcranial pneumatization is found only in birds, non-avian saurischian dinosaurs, and pterosaurs.
For a long time these cavities were regarded simply as weight-saving devices, but Bakker proposed that they contained air sacs like those that make birds' respiratory systems the most efficient of all animals.
John Ruben et al. (1997, 1999, 2003, 2004) disputed this and suggested that dinosaurs had a "tidal" respiratory system (in and out) powered by a crocodile-like hepatic piston mechanism - muscles attached mainly to the pubis pull the liver backwards, which makes the lungs expand to inhale; when these muscles relax, the lungs return to their previous size and shape, and the animal exhales. They also presented this as a reason for doubting that birds descended from dinosaurs.
Researchers have presented evidence and arguments for air sacs in sauropods, "prosauropods", coelurosaurs, ceratosaurs, and the theropods Aerosteon and Coelophysis.
Three explanations have been suggested for the development of air sacs in dinosaurs:
• Increase in respiratory capacity.
• Improving balance and maneuverability by lowering the center of gravity and reducing rotational inertia.
• As a cooling mechanism. It seems that air sacs and feathers evolved at about the same time in coelurosaurs. If feathers retained heat, their owners would have required a means of dissipating excess heat. This idea is plausible but needs further empirical support.
• Increase in respiratory capacity.
• Improving balance and maneuverability by lowering the center of gravity and reducing rotational inertia.
• As a cooling mechanism. It seems that air sacs and feathers evolved at about the same time in coelurosaurs. If feathers retained heat, their owners would have required a means of dissipating excess heat. This idea is plausible but needs further empirical support.
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