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AW: Bird and dinosaur respiration

Hi Larry,

I'll take a crack at this one:

>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
>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?

I think it is referred to a "uni-directional" because the air passes through
the trachea, to the posterior air sacs, lungs, the anterior air sacs, and
exits via the trachea. This contrasts mammalian respiration, whereby air
rushes into the lungs and then "turns around" and to leave the lungs again.
So, the air birds breath enters and exits via the trachea, but only has one
pass through the lungs.

>Why do they breath this way?  Is it just a way to keep the air sacs hollow
>for weight-saving purposes?

One explanation is that this system allows the animal to have a nearly
constant flow of oxygen overs its lungs. This apparently gives them an
efficient system to support high metabolic demands. It also allows them to
take in much more oxygen per breath than mammals, which is probably why they
can fly at high altitudes.

There's two processes happening during each inhalation and exhalation.
Here's the cycle:

1.Inhalation: Fresh air passes from the trachea to the posterior air sac,
but stops short of going into the lungs. Meanwhile, another "packet" of air
(used) is passing from the lungs to the anterior air sac.

2. Exhalation: The fresh air sitting in the posterior air sac passes into
the lungs and stops. Meanwhile, the old air in the anterior air sac passes
into the trachea and leaves out the mouth.

>What's the benefit of stiff lungs?

Contraction of the dome-shaped diaphragm and of the external intercostal
muscles cause the rib cage in mammals to expand and the elastic lungs with
them. This creates a pressure difference between the outside environment and
the lungs causing air to rush in. During exhalation, internal intercostal
muscles pull the ribs back and the diaphragm relaxes to original dome shape.
This causes another pressure difference pushing the air out. With birds,
compression and expansion of the rib cages act on the air sacs and not the
lungs, so the lungs don't need to elastic, apparently. Don't know if there's
a benefit to being stiff. That's all I remember on the subject.

>If this way of breathing is primarily a flight-based feature, why did
>apparently breath this way long before they were flying?

As to the flight assumption: Bats, with their typical mammalian lungs, are
fairly good fliers, sometimes able to migrate long distances. Therefore, to
answer your question precisely: I dunno.

I'm sure someone will want to correct this, but hope it helps. You'll find a
thorough, probably more accurate, explanation in "Vertebrates" by K.
Kardong.  I'm fairly certain I learned it from there, although I don't have
my copy handy at the moment.

Laura L.B. Schulz
Munich, Germany