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Re: Bird and dinosaur respiration



At 20:52 21-06-2001 +0200, you wrote:
>> >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.
>

That's well said.

But where to put in _Scipionyx_ with its diaphragm?

Fred Ruhe