Australia: The Land Where Time Began

A biography of the Australian continent 

Dinosaur Biology - Respiration and Circulation

Turtles, lizards and snakes have 3-chambered hearts that cannot pump blood at high pressure and large internally simple lungs that are dead-end structures operated  by the action of the ribs and in which the gas exchange is inefficient. The incipiently 4-chambered hearts of crocodilians are still only low pressure pumps. Crocodilian lungs are dead-end though they may have airflow that is unidirectional being ventilated by a sophisticated mechanism. The liver of crocodilians fills the full height and width of the rib cage. The lungs are expanded by muscles attached to the pelvis pulling on the liver which results in expansion of the lungs. The rib cage has an unusually smooth ceiling to allow the liver to move back and forth. Immediately in front of the pelvis the lumbar region has no ribs and to a lesser extent, at least in advanced crocodilians, in the pelvis a mobile pubis enhances the action of the attached muscles.

The lungs of mammals and birds have 4-chambered hearts that are double pumps capable of pumping large volumes of blood at high pressure. The lungs of mammals are fairly large dead-end structures in which the surface area for gas exchange is greatly enlarged by their intricate internal structure. The vertical muscular diaphragm and rib action combine to operates the lungs. The well-developed rib-free lumbar region is preceded by a border to the rib cage that plunges steeply on which the vertical diaphragm is attached.

According to the author1 it is widely agreed that dinosaurs had hearts that were fully 4-chambered and were capable of pumping at high capacity and high pressure. It is believed their respiratory complexes were probably much more diverse.

As the ornithischians left no living descendants it has been difficult to reconstruct the respiratory system of this group of dinosaurs. Gregory1 suggests the problem is made worse by the ornithischians having rib cages that differ from those of all living tetrapods as well as varying among different groups of ornithischians. It has not been possible to determine the complexity of ornithischian lungs. Gregory1 suggests all that can be said about ornithischian lungs is that they should have been internally complex if they had high aerobic capacity. He also suggests that they probably had high volume dead-end lungs, though they could have been at least partly unidirectional, as there are no known ornithischian bones that are pneumatic, and their ribs were not highly mobile. In the ankylosaurs the ribs were actually fused to the vertebrae. In the ceratopsids the ribs were packed together tightly and attached to the pelvis which prevented them from moving. Gregory1 suggests that the abdominal muscles in most ornithischians, that were anchored on the ventral pelvis, may have been used to push the viscera forward in exhalation, the lungs expanding when these muscles relaxed.

There was a different arrangement in one group of ornithischians. A large rib-free lumbar region is present in ornithopods in which the rib cage plunges steeply immediately ahead. He suggests that it is possible that the ornithopods may have had a diaphragm that may have been muscular as in mammals as the lumbar region is very similar to the arrangement in mammals.

As birds are living members of the saurischian group the restoring of the respiratory system of  saurischians, especially the theropods, is much easier than with ornithischians. The respiratory system of birds is the most efficient and complex of all vertebrates. The chest ribs encasing the lungs of birds are fairly short because the lungs are small but the gas exchange surface area of the lungs is very large because of their internally intricate system. The rather stiff lungs are set deeply into the ceiling of the rib cage that is strongly corrugated. Rather than being dead-end structures the lungs are connected to large complex of air sacs that are more flexible than the lungs and the volume of the air sacs greatly exceeds that of the lungs, some of these air sacs invading pneumatic and other bones, though the sides of the trunk are where the largest air sacs are located. In some birds, especially flightless birds, these trunk air sacs are limited to the chest, though they extend back to the pelvis in most birds. The air sacs in the chest and abdomen are partly operated by the ribs, and the belly ribs have a tendency to be very long in birds that have abdominal air sacs that are well developed. As the ribs attach to the vertebrae of the trunk region by hinge articulations that are well developed all the ribs are very mobile. The ribs swing outwards as they swing backwards because of the orientation of the hinging, thereby inflating the air sacs within the rib cage, and when the ribs swing forwards and inwards they deflate. Ossified uncinate processes forming a series along the sides of the rib cage enhance the movement of the ribs in most birds. The muscles operating the ribs attached to each uncinate process use it as a lever. The big sternal plate also helps ventilate the air sacs in most birds. Ossified sternal ribs attach the ribs to the sternum allowing the plate act as a bellows on the ventral air sacs. The sternum is of less importance in the ventilation system on birds with short sternums, such as the flightless ratites and active juveniles.

In the respiratory system of birds most of the inhaled air moves directly to the air sacs without passing through the gas exchange portion of the lungs, and from the air sacs it is injected through the lungs in one direction as it is exhaled. The stale air that remains in dead-end lungs at the end of each exhalation is eliminated in the respiratory system of birds because the airflow is unidirectional allowing the airflow and blood flow to work in opposite, countercurrent, directions to maximise gas exchange, making the system very efficient. This system is so efficient it allows some birds to sustain cruising flight at altitudes greater than that of Mt. Everest.

According to the author1 evidence of air sacs have not been found in the earliest theropods or prosauropods. Though their lungs are believed to have been of the dead-end types little is known of their respiration. He also says some of the vertebrae of the earliest avepod theropods are pneumatic, indicating the presence of at least some air sacs. There is also some indication of lung ventilation being present as the hinge jointing of the ribs had increased further indicating that they were probably involved in inflating and deflating air sacs. The hinge jointing of the ribs increased further as theropod evolution progressed, as well as the invasion of the vertebrae eventually reaching the hips. Another change that occurred was the shortening of the chest ribs because, as the author1 suggests, the size of the lungs was decreasing and their stiffness was increasing as the air sacs took over more of their role in ventilation. He suggests that the air sac complex was probably approaching the condition seen in birds, and the airflow in the lungs should have been largely unidirectional. The sternum was small, though the gastralia may have been used in ventilating the ventral, belly air sacs. He also suggests an alternative condition in which the air sacs were limited to the rib cage, a condition found in some flightless birds. Theropods lacked the extra long belly ribs present in birds with big abdominal air sacs.

The sternum of many avepectoran theropods was as large as it is in ratites and juvenile birds and was attached by ossified sternal ribs to the ribs, so the sternal plate was combining with the gastralia to inflate and deflate the air sacs. There are also uncinate processes that are often present, which the author1 suggests indicates that the bellows-like action of the rib cage had improved. He suggests that the respiratory complex was probably as well developed as it is in some modern birds.

He suggests that there are still a few researchers who do not believe that birds are dinosaurs, denying that theropods breathed as birds do. It has been proposed by some that the respiratory system of dinosaurs was of the liver pump type as found in crocodilians. He suggests that as well as not being closely related to crocodilians theropods lacked the specialisations that allow the liver-pump system to function, such as a rib cage ceiling that was smooth, a lumbar region, or a pubis that was mobile. Some of the adaptations of theropods for the avian air sac system, such as the rib cage ceiling that was corrugated that resulted from the hinged rib articulation, the belly ribs that were elongated, both would have prevented the mobile liver pump system from operating. Advocates of the liver pump system have pointed to the alleged presence of a deep liver within the skeletons of some small theropods. The author1 suggests that the presence of these large livers is questionable, suggesting that there is a tendency among predators to have big livers, as occur in some birds. He also suggests that the presence of the liver pump system for lung ventilation can be ruled out.

According to the author1 there is strong evidence that the sauropods evolved the air-sac system independently, their vertebrae usually being highly pneumatic. All their ribs were hinge jointed, including the belly ribs, a situation different from the expected condition in which the ribs would be solidly anchored to support the belly. It has been agreed by most that the vertebrae that were air-sac filled and the mobile belly ribs indicate that an air-sac driven respiratory system was present in sauropods that was complex, probably involving airflow that was unidirectional. The air sacs should have been limited to the rib cage as there was no gastralia in sauropods. The fact that most sauropods needed to breathe through very long tracheas means there would have been a large dead air space that needed to be cleared with each breath.

Unlike mammals that have red blood cells that lack a nucleus, allowing the carrying of more oxygen, the red blood cells of birds, reptiles and crocodilians all have a nucleus, so it is assumed that dinosaurs would also have had a nucleus in the red blood cells.

Sources & Further reading

  1. Paul, Gregory S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press.




Author: M. H. Monroe
Last updated 24/01/2012 



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