sdw at lig.net
Thu Mar 4 11:14:11 PST 2010
Dave Long wrote:
>> Though those adaptations make humans and our immediate ancestors
>> better runners, it is our ability to run in the heat that Lieberman
>> said may have made the real difference in our ability to procure game.
>> Humans, he said, have several adaptations that help us dump the
>> enormous amounts of heat generated by running. These adaptations
>> include our hairlessness, our ability to sweat, and the fact that we
>> breathe through our mouths when we run, which not only allows us to
>> take bigger breaths, but also helps dump heat.
> Men perspire, but horses sweat.
> Rather than lose their hair, equids adapted a salivary protein to
> produce latherin, which is currently believed to act as a wetting
> agent, promoting evaporative cooling even through fur...
> (horses can move 1'800L/min of air when working at 6,5kW. What's the
> equivalent for human heat exchange?)
We only weakly use our lungs for heat exchange, but for comparison, an
estimate of the absolute maximum air exchange would be:
6L  of lung capacity max
2 full breaths per second max (empirical testing)
720L / minute - This is highly inflated, probably 1.5-3x, because you
can't fully exhaust air in your lungs during a breath.
Weight of a healthy athletic man: 150-200lbs
Weight of a typical horse: 1354lbs , or 7x the weight for 2x the air
movement. Possibly shows a worse ratio.
For humans, there is evaporation, radiation, and convection which are
detailed well here: 
The max effects in high heat are for sweating / lung evaporation at -95
W/m^2 at 39C. Total dissipation max seems to be at 16C: -99+-12, plus
-40 of storage for -111W/m^2 long term and -154W/m^2 for short term
periods (until the body overheats).
 doesn't have running in their metabolic table. It would range up to
and past their highest sport: tennis at 8.7 "met". A met == 58.2
W/m^2. A human male has a surface area of about 1.8m^2 .
So a human can produce at least 9*58.2*1.8 = 942.84 ~= 1KW of heat and
can bleed it off at 0.10 KW or handle 0.15 KW for a short time.
Most numbers published are for average people, not trained athletic
ones. Few doctors care about those, since they don't give as much
More information about the FoRK