On a hot day, the sweat on your face tastes salty, like sea water.  Moisture, evaporating from your hot skin, is the body's way of keeping cool.  But what about the salt?  If moisture is all it takes to cool down...

Why Do We Lose Salt As Well?

Sweat is produced in tiny coil-shaped chambers under the skin.  That chamber is connected to the surface by a narrow tube. Together, the chamber and the tube make up a single sweat gland.

When you start to sweat, the first thing that happens is that the walls of the coil-shaped chamber pump salt into the chamber.  As a result, the fluid inside the chamber becomes more concentrated with salt.

In order to balance out the new difference in salt concentration, water passes by osmosis into the chamber.  As the water pressure increases in the chamber, the salty water is forced out through the narrow passage to the surface of the skin.

Salt In, Salt Out

But that passage does more than simply carry the sweat to the surface.  To avoid losing any more salt than necessary, the walls of the passage take salt out of the sweat so it can be reused by the body.

So, when the sweat reaches the surface, it is less salty than when it was first produced in the sweat gland.  The salt, itself doesn't help keep you cool, but mixed with water to form sweat, the salt helps draw more water into the sweat gland.

Sweating And Efficiency

And the more you sweat, the more efficient the sweat glands get at removing the salt before the sweat leaves the body.

If you go to a hot climate, you'll find that after a few days, your sweat becomes less salty.  People who live in hot climates may sweat more, but they lose less salt.