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Re: Bird and dinosaur respiration



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

Good explanation (AFAIK)!

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

This doesn't save weight... it just reduces density.

> >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.

There is a benefit: Lungs that constantly change shape must be robust enough
not to collapse during this, therefore the walls of the alveoli must be
thicker than a certain limit. Diffusion is easier through thin walls. If the
alveoli were smaller than a certain size, they would stick together during
exhalation, so their size -- means, the size of the lung's surface -- is
limited. The parabronchi of birds hardly move and can therefore be much
thinner than mammalian alveoli, means more surface for gas exchange.

> >If this way of breathing is primarily a flight-based feature, why did
> dinosaurs
> >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.

It is, of course, not a flight-related feature (primarily -- the demands of
flight have surely enhanced it). IMHO it is related to the demands of
endothermy: Amniotes started with fairly simple septate lungs like seen in
many recent "reptiles", mammals/cynodonts/therapsids/whatever improved this
by evolving the mammalian lung, and (apparently) archosauromorphs evolved
the avian lung & air sacs. Coincidentally, the latter is much more efficient
than all other types.
Crocs secondarily lost pneumatization, it seems, and evolved the famous
hepatic piston.