# How does sweat work?

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1. Alright, so I know that sweat works by using thermal energy from the body in order to evaporate, thus cooling down the body. But what I don't understand is how it does that. The way I understand it is I think about it as particle diffusion. There's a high concentration of heat in the body and a lower concentration on the surface of the body. The heat inside of the body then succumbs to a high-to-low concentration gradient and gives off heat to the water on the surface - the two balance out. This water then uses the heat to evaporate. But if the surface of the body is hotter than the inside of the body, the high-to-low concentration gradient would work in the opposite direction, thus actually heating up the internal temperature of the body. This is where my confusion begins. If the water, i.e. sweat, comes from inside of the body, how on earth can it be cooler than the core temperature of the body in order to enable the "heat diffusion" to happen in such way that it cools the body down as opposed to heating it up? I'm obviously missing something or the fundamental idea that I'm basing this on is wrong.
I made that post for Reddit a while ago but I couldn't find an answer as no one replied... Can someone help me, please? I am genuinely confused as to how this works and where I'm going wrong.
2. (Original post by Nichita)
I made that post for Reddit a while ago but I couldn't find an answer as no one replied... Can someone help me, please? I am genuinely confused as to how this works and where I'm going wrong.
You seem to have this idea that the water is somehow colder? Of course it isn't - its body temperature*.

Heat is removed because the sweat evaporates. This directly absorbs energy (just like melting, evaporation is an endothermic process), but the main reason it cools is because it only happens to the most energetic particles of water present on your skin. The less energetic i.e. colder particles are left behind. Look up the Boltzman distribution.

This is of course only one of many ways mammals adapt to stay cool.

*and with the circulation of blood, that temperature is pretty even, never really varying by more than a degree or so from outside to inside or wherever.
3. Let me try to explain my perspective better. The water (sweat) is in the body before it's needed. When it's too warm outside e.g. 30C, the water makes its way out of the body onto the skin. However, as the water came out of the body, its temperature can't be much lower than the body's, except for the fact that it's 30C outside as opposed to 37C inside. Now, because of this difference, I can see how the water on the surface on the skin can cool down using the environment and then use heat from the body to heat back up again and then evaporate, cooling down the body. But what if the temperature outside was hotter, like 40C? Now the water on the skin surface would be hotter than the water inside the body. What stops the water inside the body using the temperature of the water outside to evaporate as opposed to the water outside using the temperature of the water inside to evaporate?

*once again, I know I am going wrong somewhere, I just need help figuring out where

*2 upon re-reading your reply over and over again, I understood that sweat particles on my skin of ~37C would use water to evaporate from particles of temperature significantly greater than that, from behind the skin. Say 40C? Am I just now coming to the realisation that different particles in the body have different temperatures?
4. Okay here goes, water needs an air current to evaporate into, in this case air flowing over the surface of the skin. Water can't just evaporate from any place in your body. As high energy molecules evaporate, the average energy of the sweat remaining on your skin decreases. This directly reduces your peripheral temperature, which reduces the temperature of venous blood which flows back into your body and reduces your core body temperature. Let me know if this made sense
5. (Original post by Nichita)
Let me try to explain my perspective better. The water (sweat) is in the body before it's needed. When it's too warm outside e.g. 30C, the water makes its way out of the body onto the skin. However, as the water came out of the body, its temperature can't be much lower than the body's, except for the fact that it's 30C outside as opposed to 37C inside. Now, because of this difference, I can see how the water on the surface on the skin can cool down using the environment and then use heat from the body to heat back up again and then evaporate, cooling down the body.
No. This is not how it works. This process will happen whether or not there is water on the skin.

*2 upon re-reading your reply over and over again, I understood that sweat particles on my skin of ~37C would use water to evaporate from particles of temperature significantly greater than that, from behind the skin. Say 40C? Am I just now coming to the realisation that different particles in the body have different temperatures?
Yes, this is how it works. Like I said, Boltzman distribution.

So most of the particles on the skin are around 36C. Some are around 20C, some are 40C, some few are even 70C. The process of evaporation gets rid of the 70C and 40C ones and leaves behind the 36C and 20C ones. Which of course means the average temperture of the water is now much cooler and its ready to absorb some body heat.

This allows your body to lose heat even at 40C. Though at those kind of temperatures your behavioural adaptations are going to be more important in your survival.
6. (Original post by nexttime)
No. This is not how it works. This process will happen whether or not there is water on the skin.

Yes, this is how it works. Like I said, Boltzman distribution.

So most of the particles on the skin are around 36C. Some are around 20C, some are 40C, some few are even 70C. The process of evaporation gets rid of the 70C and 40C ones and leaves behind the 36C and 20C ones. Which of course means the average temperture of the water is now much cooler and its ready to absorb some body heat.

This allows your body to lose heat even at 40C. Though at those kind of temperatures your behavioural adaptations are going to be more important in your survival.
I understand what I came here to understand, but you said something quite fascinating to me in your second to last paragraph. Does this mean that the core body temperature only increases because of residual thermal energy from those extremely high energy particles?

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