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respiratory questions

I opined:

>>IMO the best terrestrial gas exchange system is the much-maligned
>>tracheal system of insects...
>>It is also highly efficient, because air is
>> much less viscous than blood.

John Jackson wondered:
>Makes one wonder why birds etc never quite got that far.  Probably
>blood carries more oxygen than air, and acts as a better internal 
>cooling system.

Per volume, oxygenated blood has about the same amount of O2 as fresh
air.  Cooling is a good point, although many vertebrates, including
birds, need countercurrent exchangers to reduce internal heat

I think vertebrates are stuck with an inefficient respiratory system
just as insects are stuck with inefficient eyes.  Having more
numerous, smaller airways would increase the resistance to the flow
of air, and wouldn't do any good unless the tubes had very thin
walls.  I can think of one disadvantage of the tracheal system: with
air flowing through every tissue rather than confined to a few
organs, there would be more opportunities for inhaled pathogens.

Matt wrote:

> We just started respiratory in my biology class and we got a sheet
> from my 
> teacher label "Evolution of Respiratory Systems." It just starts
> with 
> sponges, I think, and talks about their lack of a system...

Sponges are _all_ respiratory system!  :)

> I was wondering if
> someone 
> could give a brief description of how lungs accually developed that
> I could 
> bring in for my class, or, if I understand the string right,
> there's not a 
> certain way that we're sure it happened, so maybe something about
> the 
> theories.

For a start, this is what I know.  I expect someone else will be able
to add to it.  It's pretty certain that our lungs originated from
fish guts (ugh).

Most bony fish and tetrapods have one or two gas-filled pouches which
develop from the embryonic digestive system.  In many cases they
remain contiguous with it.  It's agreed that these structures are
homologous with each other and evolved from the digestive system.

Tetrapods develop two separate lungs, used mainly for breathing. 
Lungfish have one or two lungs, used for breathing and probably
buoyancy.  Teleosts (i.e. most fish) usually have a single swim
bladder, used for buoyancy.  Many teleosts have lost the connection
between the swim bladder and gut (gas is added and removed by a
gland), some have lost the bladder altogether and others use it for
breathing as well as flotation.  

I don't know about the condition in non-teleost actinopterygians. 
The extant coelacanth has no lung(s)/swim bladder(s), but there is a
big lump of fat where it ought to be.  This suggests it used to have

The paragraphs above don't shed much light on whether the original
state was one organ or a pair, nor whether its original function was
respiration or flotation.  I've read that two sacs is the primitive
condition, but I can't remember any evidence for this.

Many freshwater fish, especially in the tropics, swallow air because
there isn't enough disolved oxygen in the water.  Lungfish take the
air into their lungs, of course, and some teleosts are able to use
their swim bladders.  Others have evolved new vascularised pouches
from the digestive system.  These respiratory organs have appeared
many times, so it's perfectly feasible that lungs evolved this way.

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