Re: human lungs: second best

r norman wrote:

> On Sat, 24 Sep 2005 14:33:12 GMT, John Harshman
> <jharshman.diespamdie@xxxxxxxxxxx> wrote:
>
>
>>If we're supposed to be the pinnacle of creation, the reason for it all,
>>how come we were designed with substandard parts? Birds have lungs that
>>are in all respects superior to ours. Why don't we have bird lungs?
>>
>>Birds have one-way airflow through their lungs. This has two advantages
>>in extracting oxygen from the air:
>>
>>1. No dead space. Your lungs can't completely collapse (well, if they
>>do, you're in trouble) and thus some old air is always left in the lung
>>to mix with air from each new breath; in humans this is about 30%. In
>>birds, every breath completely replaced the air in the lungs.
>>
>>2. Countercurrent exchange. Your lungs can only extract about half of
>>the oxygen in the air (which is already depleted by #1), because oxygen
>>can only diffuse into blood until air and blood have an equal oxygen
>>concentration. Countercurrent exchange means that blood and air flow in
>>opposite directions, so the most oxygenated blood is in contact with the
>>most oxygenated air, and vice versa. This means that oxygen can always
>>diffuse into blood with a lower oxygen concentration, and birds are able
>>to extract nearly all the oxygen from the air.
>>
>>Bird lungs are superior in all respects to your lungs. What happened? I
>>thought a creator was able to re-use parts as he wished. Surely his
>>favorite species would get nothing but the best parts. Why are we stuck
>>with inferior parts?
>>
>>I can think of two theories to explain this:
>>
>>1. We aren't the purpose of creation; birds are.
>>
>>2. This creation theory is just wrong. Bird evolution came up with a
>>solution that no other lineage has hit upon, because the series of steps
>>leading to one-way airflow is unlikely and contingent.
>
>
> Aha - comparative physiology!
>
> The intent is very fine, but this may not be the best example of human
> imperfection.

Do you have a better one? It needs to be not just a human imprefection,
but something present in some other species that could replace what we
have. I suppose a better color vision system would be good, as would the
ability to synthesize vitamin C. What else you got?

> True, bird respiratory systems are more highly
> specialized in structure and in function than mammalian or any other.
> However, human (mammalian) systems are in no way "inferior" in
> virtually all reasonable habitats.
>
> First, birds do have a "flow through" parabronchial system rather than
> a "tidal" alveolar system for the actual gas exchange surfaces.
> However, birds do have substantial "dead space" in their trachea which
> do, indeed, act in tidal fashion. All long-necked birds have a long
> trachea, hence a large dead space. The trumpeter swan in particular
> has "excess" trachea curled around in its thorax.

However, we have deadspace in trachea and lungs both.

> Second, birds do not have a true counter-current exchange system.
> Fish do in their gills. But birds have a "cross-current" exchange
> that is not as effective. Blood leaving the inhalant end of the
> parabronchii leaves at a very high pO2 while blood leaving the
> exhalant end have a lower pO2. The resulting blood leaving the avian
> lung is a mixture.

Still, this takes advantage of partial pressure differentials to extract
more oxygen from the air, which is the point.

> Third, the "inefficient" tidal mammalian system is essential for pH
> regulation. True, the air in the alveoli has a much lower oxygen
> level than outside air, but that is associated with the fact that is
> also has a much higher CO2 level than outside air. This, in
> combination with the bicarbonate pH buffer system in the blood, is
> essential in regulating the acid-base balance of the blood.
> Hyperventilation does virtually nothing to increase the oxygen content
> of arterial blood, normally 95% or more saturated to start with. But
> it does substantially lower the CO2 content, increasing the pH and
> causing dizziness and all sorts of bad symptoms. The fastest way of
> controlling our blood pH level over the short run is to adjust our
> breathing. In fact, our breathing is regulated primarily by the CO2
> level (and pH) of the blood, not the oxygen level. The kidneys play a
> secondary backup role, although are essential for long-term
> regulation. I haven't seen mention of avian pH regulation in my
> comparative physiology references but I assume that it uses
> alternatives to bicarbonate.

Why? I'm not clear on why birds can't use the same system we do. They
tend to take fewer breaths per unit time than mammals of the same size.

> Fourth, as already mentioned, our arterial blood is virtually
> saturated with oxygen at all normal levels of activity and all
> reasonable habitats. Our atmosphere is so rich in oxygen that our
> "inferior" respiratory systems are quite adequate for our needs. True,
> at rest we only extract a small fraction of the oxygen from the air we
> breathe -- exhaled air has almost as much oxygen as inhaled. But what
> we do extract is more than adequate for our needs. It is only at
> extremely high altitude (or with serious lung disease) that an
> alternative would prove better.

We could however get by with smaller lungs if we had a more efficient
system, even counting our air sacs. And are you saying that god doesn't
want us climbing Everest?

> Birds are very nice animals, often pretty to look at and to listen to
> (excepting starlings and Canada geese!) But they not the "pinnacle of
> creation" either.

Sez you. Ask a parrot.

> References: (probably newer editions of these exist, but these are
> what I have available)
> P.C. Withers
> Comparative Animal Physiology
> Saunders - Harcourt Brace Jovanovich, 1992
>
> K. Schnidt-Nielsen
> Animal Physiology: Adaptation and Environment, 4th Ed.
> Cambridge Univ. Press, 1990
>
>
>
>
>
>

.

Relevant Pages

  • Re: human lungs: second best
    ... Why don't we have bird lungs? ... >Birds have one-way airflow through their lungs. ... you're in trouble) and thus some old air is always left in the lung ... >can only diffuse into blood until air and blood have an equal oxygen ...
    (talk.origins)
  • Re: human lungs: second best
    ... Why don't we have bird lungs? ... >>Birds have one-way airflow through their lungs. ... you're in trouble) and thus some old air is always left in the lung ... >>can only diffuse into blood until air and blood have an equal oxygen ...
    (talk.origins)
  • Re: human lungs: second best
    ... Why don't we have bird lungs? ... > Birds have one-way airflow through their lungs. ... you're in trouble) and thus some old air is always left in the lung ... > can only diffuse into blood until air and blood have an equal oxygen ...
    (talk.origins)
  • Re: human lungs: second best
    ... Why don't we have bird lungs? ... >> Birds have one-way airflow through their lungs. ... you're in trouble) and thus some old air is always left in the lung ... >> can only diffuse into blood until air and blood have an equal oxygen ...
    (talk.origins)
  • Re: human lungs: second best
    ... Why don't we have bird lungs? ... > Birds have one-way airflow through their lungs. ... you're in trouble) and thus some old air is always left in the lung ... > can only diffuse into blood until air and blood have an equal oxygen ...
    (talk.origins)
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