Does the body *generate* heat when you sweat?, and body heat questions in general

[Habeed]

@am77494

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.

Actually, it will if you then reabsorb the coolant. Sweat glands must be bidirectional in order for this to work. The coolant, missing the component that evaporated, is now cooler.

But that’s not how it works. Hmm. Apparently the skin itself is the heat exchanger. That would explain it fully. The blood carries the coolant to blood vessels near the surface of the skin, and apparently those vessels are so close to the surface that they can conduct heat to the water layer (for sweating) or to the air layer (if you are not sweating). That’s how it works.

[am77494]

@Machine_Elf

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.

**Machine Elf **- thank you. Please consider ignorance fought. I ran the numbers and here’s what I got :

Q/A = K.dT/s (Q/A = 250 W/m2, dt = 2.2 C {4F}, s = 2.5mm)

References : For the average adult human, the skin has a surface area of between 1.5-2.0 square metres (16.1-21.5 sq ft.), most of it between 2–3 mm (0.10 inch) thick.

That gives K = 250x2.5x10-3/2.2 = 0.28 W/mK, which is consistent with the human skin conductivity measurements published on Page 3.

I understand this better now and also understand why the blood vessels are closer to the skin where it sweats.

[Smeghead]

Why put so much effort into solving a “mystery” that has been fully worked out for decades, if not centuries?

[am77494]

@Habeed

Actually, it will if you then reabsorb the coolant. Sweat glands must be bidirectional in order for this to work. The coolant, missing the component that evaporated, is now cooler.

But that’s not how it works. Hmm. Apparently the skin itself is the heat exchanger. That would explain it fully. The blood carries the coolant to blood vessels near the surface of the skin, and apparently those vessels are so close to the surface that they can conduct heat to the water layer (for sweating) or to the air layer (if you are not sweating). That’s how it works.

Thanks Habeed - I really understand it now and think that I understood it partially all along my life. Evaporation has a role here but so does thermal conduction. Its very important to have those blood vessels closer to the skin

[dracoi]

I think you’re overestimating the thickness of the skin. Capillaries are literally just a few cells below the surface in some places. Here’s a diagram. If you have designed heat exchangers yourself, then you can’t help but look at that diagram and see “Heat Exchanger!” written all over it. The outermost layer, the epidermis, can be as little as 0.05 mm thick.

So when those capillaries are engorged with blood, there’s an enormous heat exchanger operating a tiny distance away from the air. When the capillaries contact and prevent blood moving into the exchange system, you make it so that both the dermis and the epidermis are providing insulation, and the dermis is a much thicker layer.

But maybe an even better way to appreciate the heat exchange potential is to compare a person to a dog. Dogs get evaporative cooling from their tongue. That’s it. So we’ve got hundreds of times more heat-exchange surface for cooling.

[Machine_Elf]

@am77494

I understand this better now and also understand why the blood vessels are closer to the skin where it sweats.

Even if sweat isn’t involved, skin is where heat gets rejected to the environment; without sweat, you’re left with ordinary convection, and then you keep cool by packing a lot of blood vessels near the skin, and having a large area of skin. Classic example? the gigantic ears of an elephant. They don’t sweat, so these are its liquid-to-air heat exchangers. An elephant’s overall size gives it a low surface-area-to-volume ratio; it would have a difficult time keeping cool without it’s ears. It’s got a fair bit of surface area elsehwere on its body, but that other skin is all pretty tough/thick and not packed with blood vessels to anywhere near the degree that the ears are; those babies are made for rejecting heat. Hot out? Flap your ears, get the air moving a bit; this is the equivalent of your car turning on the electric fan in front of its radiator. No sweating required.

@Smeghead

Why put so much effort into solving a “mystery” that has been fully worked out for decades, if not centuries?

Isn’t that why this forum exists? For explaining things to people who don’t yet understand them?

[Machine_Elf]

@dracoi

I think you’re overestimating the thickness of the skin. Capillaries are literally just a few cells below the surface in some places.

Even the shallowest of paper cuts will serve to demonstrate this.

[barbitu8]

@am77494

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.

Apocrine sweat gland - Wikipedia

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.

We have two types of sweat glands: apocrine and eccrine. The apocrine glands produce the musk odor, a pheromone. However, the eccrines do not and they are the ones producing the perspiration providing a cooling effect: Eccrine Sweat Gland - Anatomy Pictures and Information