Air sacs and pneumatized bones

Birds, like reptiles, have septate lungs that are small and somewhat rigid. Thus bird lungs do not greatly expand and contract as ours do on each breath. But the rib cage is very much involved in respiration, and especially those ribs closest to the pelvic region are very mobile in their connection to the bottom of the sternum, and this mobility is quite important in allowing respiration. But these are not the biggest differences. Very much unlike extant reptiles and mammals, these lungs have appendages added to them known as air sacs, and the resultant system of respiration is highly efficient. When a bird inspires air, it goes first into the series of air sacs. It then passes into the lung tissue proper, but in so doing the air passes but one way over the lung, since it is not coming down a trachea but from the attached air sacs. Exhaled air then passes out of the lungs. The one way flow of air across the lung membranes allows a counter current system to be set up - the air passes in one direction, and blood in the blood vessels within the lungs passes in the opposite direction. This countercurrent exchange allows for more efficient oxygen extraction and carbon dioxide venting than are possible in dead-end lungs.

In Figure 2.3 the various air sacs are shown with their communication to the lungs. It is clear that the volume of air sacs far exceeds the volume of the lungs themselves. The air sacs are not involved in removing oxygen; they are an adaptation that allows the countercurrent system to work.

There is no question that the greater efficiency of this system compared to all other lungs in vertebrates is related to the two-cycle, counter current system produced by the air-sac-lung anatomy in birds as evidenced by the well known, higher tolerance to hypoxia of birds compared to mammals and reptiles: birds cannot only exist but can undertake the costly exercise of flying to at least 33,000 feet (bar headed goose, observed above the Himalaya Mountains by high flying airplanes), where there are oxygen levels that are fatal to mammals. It has been estimated that a bird is 33% more efficient in extracting oxygen from air than a mammal, at sea level. But at higher altitude this differential increases: a bird at 5000 feet in altitude may be 200% more efficient at extracting oxygen than a mammal. This gives the birds a huge advantage over mammals at living at altitude. And if such a system were present deep in the past, when oxygen even at sea level was lower than we find today at 5000 feet, surely such a

Air Sacs The Bird

Fig. 2.3 The air sac system in the lungs of birds (above) and respiratory cycles (below). The cranial, cervical, and vertebral air sac positions are based on new data, while the caudal air sacs are partially encased in bones of the bird, thus creating "pneumatized" bones. On the first inspiration, air is taken into the air sacs. It then moves to the lungs on the second inspiration.

Fig. 2.3 The air sac system in the lungs of birds (above) and respiratory cycles (below). The cranial, cervical, and vertebral air sac positions are based on new data, while the caudal air sacs are partially encased in bones of the bird, thus creating "pneumatized" bones. On the first inspiration, air is taken into the air sacs. It then moves to the lungs on the second inspiration.

design would have been advantageous - perhaps enormously so - to the group that had it in competing or preying on groups that did not.

But when did this system first appear, and in how many groups? We know that birds evolved from small bipedal dinosaurs that were of the same lineage as the earliest dinosaurs - a group called Saurischians (Chapter 1). The first bird skeletons come from the Jurassic. But the air sacs attached to bird lungs are soft tissue, and would fossilize only under the most unusual circumstances of preservation. Thus we do not have direct evidence for when the air sac system came about. But we do have indirect evidence, enough to have stimulated the "air-sac in dinosaurs group" to posit that all saurischian dinosaurs had the same air sac system as do modern birds. The evidence for an air sac system in ancient organisms can be deduced entirely from well-preserved skeletons. The evidence is the presence of:

1 pneumatic bones, especially in the vertebral column;

2 shortening of the trunk of the body;

3 shortening of the first dorsal ribs;

4 elongation and increased mobility of posterior ribs - this mobility is enabled by the presence of ribs with double heads at their ends;

5 uncinate processes on several of the ribs (these are small, hook-shaped bones attached to the trunk ribs);

6 a hinge joint making up the attachment of the ribs with the sternum.

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Responses

  • darryl
    Do reptiles have air sacs in bones?
    2 years ago
  • christian
    What bones are pneumarised in bird?
    1 year ago
  • yolanda smallburrow
    Do non flying birds still have air sacs?
    1 year ago
  • MITCHELL
    Which bones interact with air sacs in birds?
    11 months ago

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