GT> I do not see such a problem -- the only difference between bird and
GT> other vertebrate lungs is primarily improved capacity, something which
GT> has no problems with intermediary steps.
Hmm, actually, bird lungs are fundamentally different from the lungs of
other vertebrates. Mammal lungs, for example, form a bronchial tree with
the air tubes splitting into finer and finer sub-divisions until they end in
the alveolus. Each alveolus is like a tiny balloon and is the site of gas
exchange. The key point here is that air flow into the alveolus is two-way:
inspired air inflates the alveolus when the volume of the chest cavity is
increased by the muscles of breathing and the alveolus deflates upon
expiration. Inspired air and expired air travel the same path, up and down
the same tubes in a tidal fashion (indeed, the volume of air moved in and out
of the lung in one inspiratory/expiratory cycle is called the tidal volume).
Also the lung is very compliant, such that the negative pressure induced
in the chest cavity upon contraction of the external intercostals and the
diaphragm can couple the muscle contraction to lung expansion and, thus,
inspiration of fresh air.
Bird lungs are quite different from the ventilatory system described above.
Firstly, bird lungs are non-compliant; they don't change volume upon changing
pressure in the chest cavity. Instead, the air sacs connected to the lungs
change volume and hold the inspired and expired air. Secondly, bird lungs
don't have tiny balloon like alveoli with two-way air flow; instead they
have air capillaries with one-way air flow. This one-way air flow is a
more efficient method of gas exchange.
However, what really sets the system apart is that the blood capillaries
that run along side of the air capillaries to exchange gases between blood
and air, run counter-current to the air capillaries. (i.e. if the air flow
in the air capillary is running right to left, the flow of blood in the
associated blood capillary is running left to right.
This one-way flow counter-current exchange mechanism is _vastly_ more
efficient than the two-way tidal flow of other vertebrates.
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