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Re: Ruben was right
> For some period of time our generic animal produces enough CO2 to make its
> pressure in
> its lungs 4.04 kPa and is now ready to exhale it.
> Remember that glucose brutto-formula is C6H12O6.
> Recall ideal gas equation. So if animal somehow produced
> CO2 in amount to make 4.04 kPa pressure in a given volume (lungs), it
> has to produce H2O in amount that would make 4.04 kPa pressure
> in same volume.
I don't understand this argument (note that I don't claim it is wrong,
I just don't understand it): The metabolising happens inside the
body. How much of its end-products end up in the exhaled air depends
on how permeable the lung tissue is to the substances, doesn't it?
Imagine the blood-air-barrier were completely impenetrable to water,
then there would be no water exhaled. Imagine it were completely
impenetrable to CO2, there would be no CO2 exhaled. So is it clear
that partial pressures must be the same?
BTW, these are questions that could be easily answered by
experiment. Here's an abstract I found with google:
M Berger, JS Hart - Journal of Comparative Physiology A: Sensory, Neural,
In hovering hummingbirds (Amazilia fimbriata fluviatilis, mean weight
5.7 g) oxygen consumption, CO2 production, breathing frequency,
respiratory water loss and wing frequency were measured at various
environmental temperatures from 0 to 35 Â C.
The oxygen consumption above 20 Â C reached 4.1 ml/min = 43 ml/gÂhr,
and was 14 times the calculated basal rate. Oxygen consumption
increased about 6% for a 10 Â C fall in environmental temperature
(Fig. 3). During flight the thermoregulatory heat production at low
temperatures was largely substituted by the heat that is produced by
contraction of the wing muscles.
The respiratory frequency was estimated to be 280/min, the tidal
volume 0.63 ml (BTS), the ventilation 0.18 1/min (BTS) and the oxygen
utilization as 2.2%. The respiratory heat loss at temperatures of 20
Â C and below was less than 20% of the heat production, while at 35 ÂC
a maximum loss of 40% was reached (Fig. 6). In dry air at 0-20Â C the
water loss measured 2.9 to 4.5% of body weight per hour while at 35 Â
C the loss was 11%. At 0 Â C the respiratory water loss and metabolic
water production were equal, but at all other temperatures the loss
exceeded production (at 35 Â C the loss exceeded production by 350%).
The final paragraph shows it all, I think: If ambient temperature is
increased, water loss by breathing (admittedly, heavy breathing) is
larger than metabolic water production. A water loss of 11% body
weight/hour is quite a lot, I think (although they surely don't hover
for an hour). According to your theory, this should not be possible,
Another paper is
Greogry K. Snyder
Respiratory metabolism and evaporative water loss in a small tropical
lizard Oxygen consumption (VO2 ) and evaporative water loss were
determined in the 0.45-g mesophilic lizard,Sphaerodactylus macrolepis,
between 21 and 33ÂC. varied directly with temperature at low and high
temperatures but was temperature independent between 24 and 27ÂC. At
30ÂC, VO2 was 0.25 ml O2 gâ1 hâ1, within 10% of the value predicted from
the weight-specific relation of oxygen consumption in lizards. Total
evaporative water loss varied directly with temperature in a manner
similar to oxygen consumption. At 30ÂC, total evaporative water loss
was 5.4 mg H2O gâ1 hâ1. Nonrespiratory evaporative water loss was 5.1
mg H2O gâ1 hâ1 and accounted for 94% of total evaporative water
loss. Breathing rate increased from 23.7 to 38.5 cycles minâ1 between
21 and 27ÂC, but remained essentially constant between 27 and
33ÂC. Respiratory efficiency and evaporative water loss in this
mesophilic lizard are compared with similar values in a xerophilic
one,Xantusia vigilis, previously reported.
Oxygen consumption is 0.25mlO2/gh. At normal pressure, one mol takes a
volume of 22.4l, so this corresponds to 0.25E-3/22.4=1.1e-5 mol, at
16g/mol, that's 0.18mg.
Whereas water loss is 5.4mg which is 30 times larger (the difference
gets bigger if you convert to mols). So it seems not to be true that
for each molecule of O2 consumed (and thus for each molecule of CO2
exhaled) there is exactly one molecule of O2 exhaled as well.
Or did I miss something (again)?
Priv.-Doz. Dr. Martin BÃker
Institut fÃr Werkstoffe
Technische UniversitÃt Braunschweig
Langer Kamp 8