See hed. I’m thinking about the opening and closing of pores; the “pumping” – cellular release, or however the body does it–of sweat; the actions of sweat glands: all are at some level mechanical actions and must create heat.
Or, say, muscle cells, when stretched out in use.
Note that I’m also working with basic non-knowledge of, as for both of those questions (pores and muscles moving) as mechanical heat, as I’ve posed the question, compared with non-knowledge of chemically generated heat–if that’s a stupid way to say it, tell me–exothermic reactions in general in our biochemistry.
Which made me realize I have no idea to begin with why our bodies (or any other warm-blooded beast) are warm when we are alive. My high school biology and the words “Krebs cycle” are now coming back to snicker at me as I type.
I’m not sure I fully understand the question here, but everything that goes on within a body is indeed the result of chemical reactions within cells, which consume energy and release heat.
Hmm… Thanks, Xema and Ranger Jeff. It’s funny how some time I think “Yeah, this is a perfect GQ question which will garner wide interest, excite helpful and super knowledgable readers and crack scientists, and will be recognized as not a truly idiotic question but showing a spark of creativity in its initial formulation and the fact that I’m honestly trying to flesh out my knowledge by putting some more examples of where I’m coming from, which I’ll put in self-deprecating style to show my good intentions.”
And some times nice longish threads ensue. And sometimes not.
It’s sort of like, if we were short on electricity, and so we decide to start the turbines in the Hoover Dam (shut up, people who like realistic scenarios!) by opening gates to allow water to flow through the, uh, turbine channel things. And then worrying about how much electricity is being used up by the motors that are opening those gates. It’s not wrong, just largely irrelevant.
Something like the Krebs cycle involves the catabolism of nutrients, which means there are lots of individual chemical reactions going as the big molecule get broken down into small molecules. Every step along the way releases some waste heat because, as Chronos points out, everything always produces waste heat. (Second Law of Thermodynamics and all that.)
But sweat is a little different. There’s no chemical transformation of one compound to another.
Sweat is pretty much just your body moving fluid from the bloodstream through the skin to the outside of your body. It’s done through a “sieve” so that big things like red blood cells stay put in your body, but it’s not done with any great deal of osmosis to try to change the concentration of dissolved chemicals like salt. Osmosis is a pretty expensive process, energetically speaking, while letting water leak out through a sieve is pretty much free.
I think that explains why losing valuable salt through sweat is an acceptable loss compared to the energy cost of trying to sweat only pure water. Plus, you’re sweating out some of the blood solutes that might otherwise wind up in urine, so you’re saving a little of both the water and the energy used by the kidneys.
So there is very little energy involved in sweating, but not quite zero.
Thinking this over, i believe there is more to it than sweating causes water to evaporate and thereby cools the body. Here are my questions :
What is the thermal conductivity of skin ? Evaporation is a surface phenomenon; for the body to realize the cooling benefits, somehow there has to be heat transfer from the surface through the skin through the cells and to the blood vessel. Having designed heat exchangers, this does not pass the smell test - or maybe I am missing something.
Why have we not evolved for not sweating through the pores that are not exposed to the air - like the armpits ? Since sweating at these places does not result in appreciable evaporation ?
Fair observation Chronos - and it gets more interesting. Instinctively speaking, I would say that the boundary layer in the armpits (lack of moving air) would make evaporation very slow. But hang on - it gets more interesting.
[QUOTE=Wikipedia]
In hoofed animals and marsupials, apocrine glands act as the main thermoregulator, secreting watery sweat. For most mammals, however, apocrine sweat glands secrete an oily (and eventually smelly) compound that acts as a pheromone, territorial marker, and warning signal.
[/QUOTE]
So the sweat from the armpits is not intended for ooling us down - or maybe I am reading this wrong.
Anyways - I think the OP is right - ignorance is not well fought off on the cooling mechanism of the human body. It is evaporation, I agree - but there is more to it.
When you get hot, you also get flushed. Flushing is the observable evidence that blood vessels are delivering more blood to the surface of the skin. The purpose of this is so that when all that sweat evaporates, the blood is right there any read to carry away the cooled blood, and to deliver more hot blood. In fact, many people flush even when physical activity is merely anticipated - when nervous, embarrassed, aroused, etc. - because it’s part of the body’s general fight-or-flight mechanism.
The opposite happens in cold weather. You look pale or blue when cold because there is less blood near the surface. This slows down the exchange of heat, helping you to stay warm.
Now, sweating near the armpits likely has more to do with sexual display than cooling; for example, the odor is a sign of sexual maturity and the hair helps to disperse the odor. But still, there is plenty of air-skin contact there for someone who is actively moving and not just sitting around. Cooling is still part of the function.
If the ambient temperature is above 98.6F, then evaporative cooling is the only conceivable mechanism available. You may get some evaporative cooling due to humidification of dry air entering your lungs, but this will be minor compared to evaporative cooling due to sweat (unless you’re in a position to go swimming in a nearby body of water).
Your body certainly has a mechanism for moving heat to the skin where sweating can cool it (i.e. blood flow) - but if it’s hot out, and you’re not sweating, your body can’t actually get rid of that heat.
Convective cooling can only possibly work at temps below 98.6. And you won’t get much convective cooling until ambient temps are well below that, and/or the wind picks up.
You’re right, body core -> skin isn’t a very good heat exchanger. What you’re missing here is it’s an open cycle system. You’re taking the coolant (saltwater in your blood) and just dumping it. This would be like cooling a car engine by spraying radiator fluid onto your hood and letting it evaporate. (letting it evaporate causes the water molecules with the most thermal energy to preferentially leave)
One positive note is that water is a combustion byproduct of human metabolism. Unfortunately, open cycle cooling is so inefficient that humans need to consume huge* quantities of water daily just to stay alive.
*huge relative to our power output. As engines, we don’t even develop 1 horsepower.
Evaporative cooling will work at all temperatures as long as the air around you is not saturated with water (i.e. 100% relative humidity). I agree that evaporative cooling will cool the skin but how is that cooling translated to the cooling of internal organs - especially blood.
The skin can only cool the part that reaches the skin by evaporative cooling - it will have to rely or conduction (or other mechanisms) for the cooling of the blood in the vessels and internal organs. The question is - is this mechanism conduction or is there something else ?
Thanks Habeed. The example you give will not cool the car engine ! the temperature of the coolant in the engine will not change by pumping some of the coolant to the hood !! The temperature of the hood will drop - and then you’ll need a conduction path from the hood back to the engine.
Dumping hot water from the bloodstream to the surface of the skin will not cool the bloodstream by itself.
Bolding mine - so you are suggesting that conduction is at play here ? That heat is also transferred from the blood vessels through the skin cells to the surface by conduction ? In that case I’d like to see how much heat can be moved through the skin per unit area when say the temperature difference between skin and blood is say 4F ?
Conduction will only be necessary for the very short distance between the sweat-cooled surface of the skin and the capillaries just below the surface. The blood in those capillaries gets cooled and then travels deeper to larger veins. Along the way there will be conduction to nearby arteries and tissues. With blood flowing just about everywhere in the body, conduction distances should never need to be very far at all.
You can do the math, but you’ll need to nail down more parameters. You could assume that blood which visits the skin is cooled by 4 degrees before being pumped back into the body’s interior. Further assume that the specific heat of blood is comparable to that of water. After that, you’ll just need to know what the mass flow rate is for blood undergoing this temperature change. Then you’ll know what the total cooling rate is.
An alternative approach is to look at how much mechanical power a human can generate, assume a certain degree of inefficiency, and calculate the total waste heat. A healthy adult can sustain 150 watts of mechanical power for about an hour. If we assume the human body is 25% efficient at producing mechanical work, then there must be 450 watts of waste heat - and the only place for it to depart the body is at the skin. With 1.75 square meters of skin, that works out to 250 watts per square meter of heat dissipation.