Re: RT water and heat retention in theory and life

[Martin Baeker <martin.baeker@tu-bs.de>]

> There was discussion about respiratory turbinates some years ago, but
> it apparently missed (AFAIK) some very important points.

Interesting ideas, but I sense some errors in them:

> For water vapour to condense, its pressure should be higher that
> *saturated vapour pressure* at given point of temperation.
> Depencity is close to being exponential. So for water retention
> an organism needs to have RTs to be significantly cooler than air
> (does it?) Ergo, at ambient low temperatures (like -5 deg C)
> there is no way to stop water loss, as organism can't let its RTs
> to cool for so low temperatures. It is well known that many mammals
> do quite fine at such temperatures.

I think you miss the point here: RTs serve to cool down the
air (by using some counter-current mechanism as in a heat exchanger)
and thus allow for heat and moisture retention. Imagine air coming up
from the lungs at 100% humidity and T=36ÂC. If the air gets cooled
down by the RT's to 20ÂC before exhaling, water will condense inside
the nose - I don't have exact formula at hand, but there is a graphic on
the German wikipedia site that lets me estimate that about 50% of the
water in the air might get out this way. So the temperature difference
that is of importance is that between tip of nose and lung.


> Well compare two almost identical animals, one endothermic and other
> one ectothermic at same temperature, about 36 deg C. Ectotherms
> typically have metabolic speed of about 1/4 of endotherm of same size
> (provided that ectotherm is as warm as endotherm).

Isn't the number more like 1/10? At least, 1/10 is the number in the
scaling relation given by Schmid-Nielsen.

> Well... someone linked that RTs help heat retention and through that
> promote endothermy.
> I did a lot of physical activity (like running distances like 5--40 km)
> at temperatures as low as -25 deg C and as high as +30 deg C.
> At temperatures at -25 deg C, human can overheat if wearing
> heavy clothes. THIS MEANS THAT BODY GAINS MORE HEAT THROUGH METABOLISM
> THAN IT LOSES THROUGH RESPIRATION. (UNLESS YOU'RE AMPHIBIAN AND BREATHE
> THROUGH YOUR SKIN).

I don't think this follows, cause animals don't jog all the time -
cannot afford the energy. So the fact that you get hot when you are
jogging is irrelevant.

> Most mammals have sweat glands.
> If respiratory heat loss is not overhelming at
> T diffs (lung-ambient) about 50 deg C, why should it
> be for reptile for which T diff is hardly over 3?

Respiratory heat loss is overwhelming in this case - that's why people
in winter tend to muffle their mouths with a shawl (when not jogging,
that is). In addition, the counter-current mechanism also serves to
heat up inbreathed air and the RTs are needed for that as well (and
that's why your mom told you always to breathe through your nose when
outside in winter back when you were a kid).

> Mammals are plainly not designed to save water. Rather reverse.
> And it is very likely that early dinosaurs have been quite adapted to
> dry climates (Pangea).
> Also, when endotherm spends water to cool itself, it is not significant if
> water is lost though respiration or other sweating.

That's plain wrong. In sweating, you get cooled by the latent heat of
the phase transition between liquid and gaseous states, and that's
quite a lot. You won't get this if you loose water just by exhaling.

> I haven't heard warning from doctors about possible dehydratation in
> case of using medicines that make nasal passages to extend.

No, but if you ever had a cold you might have realised that it tends
to dry the body (unscientific personal observation, but I tend to
drink a lot when having a cold and am forced to breathe through my
mouth only, and the same is true for other people I know).

Cheers,

Martin.

                   Priv.-Doz. Dr. Martin BÃker
                   Institut fÃr Werkstoffe
                   Technische UniversitÃt Braunschweig
                   Langer Kamp 8
                   38106 Braunschweig
                   Germany
                   Tel.: 00-49-531-391-3073                      
                   Fax   00-49-531-391-3058
                   e-mail <martin.baeker@tu-bs.de>