As every thirsty person knows, swallowing your spit doesn't do the trick. But why is that? Saliva is about 98 percent water, so shouldn't it be hydrating?

First of all, let's think about what saliva is. "It's a very concentrated fluid that has proteins and enzymes. It's much more concentrated than water." [The 7 Biggest Mysteries of the Human Body]

Freshwater is much less concentrated with salts and other solutes than the fluids in the human body, and that's why it's so refreshing to drink when you're thirsty, he said. That's because of osmosis, when water flows toward higher concentrations. If you drink a less-concentrated substance, your body can absorb its fluids and become hydrated. But if you drink a liquid with a higher concentration than your body's, the water in your body will flow toward the substance, and make you dehydrated.

The fluids in many parts of the human body have the approximate concentration of the saline you pick up at the drug store — think of artificial teardrops or saline nasal sprays.

Saliva, however, is more concentrated than saline, so it would make you thirstier if you drank it, That's because it would cause the fluids in your body to flow toward the concentrated saliva, and not toward your dehydrated cells.

"In other words, saliva is not watery enough," 

If someone is severely dehydrated, health care workers will give him or her saline through an intravenous (IV) drip, especially if that person can't take liquids orally.

Read more at:
https://gist-splash.com/2018/04/11/why-doesnt-saliva-quench-your-thirst/