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

Question

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

Answer 1

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.