A saltiness detector
How Does Your Body Know You Need a Drink?
Thirst is not your mouth being dry. A patch of your brain is measuring how salty your blood has become.
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Step 1 of 5
Losing water makes your blood saltier.
The salt does not go anywhere when you sweat out water — so the same amount of salt ends up in less liquid, and the concentration rises.
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Step 2 of 5
A patch of brain sits in the blood and measures it.
Most of your brain is walled off from your bloodstream. A few small regions are deliberately not, so they can taste what the blood is actually like.
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Step 3 of 5
Two per cent is enough.
A rise of about 2% in saltiness starts the feeling. That is a small change, which is why thirst arrives long before you are in any real trouble.
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Step 4 of 5
Drinking stops the thirst before it fixes anything.
Water takes ten or fifteen minutes to reach your blood, but the thirst fades in a minute — because your throat and stomach count the mouthfuls and report ahead.
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Step 5 of 5
Salty food makes you thirsty for the same reason.
Eating salt raises the blood's saltiness directly, without you losing any water at all. The detector cannot tell the difference and asks for a drink either way.
Thirst starts at around 2% water loss. Serious trouble begins near 10%, so the alarm has a very generous head start.
The short version
Thirst is a small patch of brain measuring how salty your blood has become, and it switches off as soon as your throat reports water on the way.
Try it yourself
Notice how thirst fades within a minute of drinking, long before the water could possibly have reached your blood. Your throat is counting.
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