eli5 Why does our body sweat out salt when we need it?


Since sodium is so important to keep electrolytes up in our body why does our body sweat it out instead of just simply water?

In: Biology

I think we’re thinking a little circularly here! We only “need” extra salts *because* we are sweating them out. We are told to keep our electrolytes up because we sweat.

The reason why we sweat is to cool down the body. Since it takes energy to turn water into a vapor, biology has evolved to use “evaporative cooling” as a way to stop our bodies from overheating. When we are hot, the heat from our bodies can be put into the water in our sweat, which will cause it to evaporate and leave our bodies.

Have you ever heard the idea that salt draws water, e.g. in cooking? We’re using the same principle to get water out of our bodies, so it can wick away the heat from our bodies. Sweat contains salts because it is the best way to get the water out of our bodies. This is due to a property called “osmotic pressure”, which basically says water moves into solutions that contain more solutes, like salt or sugar.

We sweat out a saltwater solution because it is similar to the concentration of it in your blood plasma (and therefore cells).

You do not *need* sodium transit in order to have water transit (although it can be an energy efficient way to make that happen so the body often does it that way).

However, arguably the more important reason you sweat out some sodium is to maintain sodium concentration *in* your body. A high body sodium concentration is dangerous, especially for your brain. Say, for example, your body’s sodium to water is 20 parts sodium and 80 parts water, ie 25% sodium concentration. If you sweat out pure water on a hot day, we’ll say now you have 20 parts sodium and 60 parts water, or 33% sodium. If instead you sweat out some sodium with that and end up with 17 parts sodium and 60 parts water, your sodium concentration is now 28% sodium, closer to the original 25%.

A lot of other things come into play here so an explanation this simplified has some holes, but the bottom line is you sweat out some sodium to maintain the natural tonicity in your body.

**Fun fact**: our sweat is actually *less* salty than the fluid in our cells and blood. Our bodies do already reabsorb some of the electrolytes from our sweat so as not lose too much willy-nilly.

Typically fluid moves around to different compartments in our bodies by following salt (via electrolyte gradients made by salt pumps). The fluid follows the salt out of our sweat glands cells to the ducts, then our bodies re-absorb some of the salt back into the cells, leaving less-salty (hypotonic) fluid behind to evaporate and cool us down.

**Summary**: our sweat fluid follows salt out of our cells and into our sweat gland ducts; our cells then reabsorb some of the salt but not quite all of it.

* [https://en.wikipedia.org/wiki/Perspiration](https://en.wikipedia.org/wiki/Perspiration)
* [https://www.researchgate.net/figure/Mechanism-underlying-elevated-sodium-and-chloride-levels-in-the-sweat-of-CF-patients_fig3_263808741](https://www.researchgate.net/figure/Mechanism-underlying-elevated-sodium-and-chloride-levels-in-the-sweat-of-CF-patients_fig3_263808741)
* [https://pubmed.ncbi.nlm.nih.gov/31608304/](https://pubmed.ncbi.nlm.nih.gov/31608304/)
* [https://pubmed.ncbi.nlm.nih.gov/15158544/](https://pubmed.ncbi.nlm.nih.gov/15158544/)

Edit: added a summary section.

Because if you sweat out just water, salt would be a higher concentrate in your body. The ratio of salt and water in your body is important.

Edit: think about how high your sodium levels would be if you didn’t sweat it out.