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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
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
> 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
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. firstname.lastname@example.org
- From: "Patrick Norton" <email@example.com>