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Re: Breathing



At 01:49 PM 4/10/99 -0400, Patrick Norton wrote:
>As I understand it, the avian respiratory system is arguably more efficient
>than the mammalian system in part due to the unidirectional flow of gas
>through the avian lung, even though bidirectional flow still occurs in the
>trachea.  Unidirectional flow makes sense to me as a more efficient gas
>exchange process,

Strictly speaking, what makes it more efficient is the counter-current
exchange system.  That is, it is the fact that the air and blood flow
opposite directions that improves the efficiency.

> but wouldn't this suggest that the more primitive gill,
>which ventilated the organism using an entirely one way flow of oxygen
>bearing fluid over a vascularized surface (no bi-directional flow
>whatsoever), was even more efficient?

In a sense, yes, but it is unable to extract oxygen from *air*.  A gill,
for some reason, can only exchange gases effectively with water.  This
makes it unusable for land animals.  [Note, most gills are counter-current
systems].

> Has anyone investigated the gas
>exchange efficiency of gills v avian or mammalian lungs?  If it turns out
>that the more primitive unidirectional system is more efficient, it raises
>the question as to why terrestrial vertebrates abandoned this system in
>favor of the "in and out" breathing of birds, mammals and reptiles (after
>all, we didn't [thank goodness] abandon the unidirectional flow of the
>digestive process).

The difference here is in prior anatomy.  The digestive system was a tube
with two valves at opposite ends already in the ancestral vertebrate.  The
lungs of early fish were blind sacks from the start, thus requiring two-way
flow at least at the entry/exit point.

>  Perhaps the density and oxygen concentrations in the
>differing fluids has something to do with it.

A good deal to do with it, I expect.

--------------
May the peace of God be with you.         sarima@ix.netcom.com