How the Body Knows It Is Thirsty

"My mouth keeps going dry, but drinking water only helps for the moment."

"I am told to drink plenty of water, but I do not much want to."

The two remarks sound alike, but to me they are different findings. It is because a dry mouth and craving water are not the same thing.

Thirst Is Not a Feeling but an Output

The signal that you are thirsty is not a mood. It is the result the body puts out after measuring something.

What it measures is two things. One is the concentration of the blood. When water is lost, the concentration of the things dissolved in the blood goes up. This concentration is called osmotic pressure. The other is volume. When the amount of blood going round falls, that too becomes a signal.

There Are Places in the Brain Where the Barrier Is Left Open

The brain does not let just anything in. There is a close barrier between the vessels and the brain, so most substances cannot cross. This is called the blood-brain barrier.

But there are a few places where that barrier is left open on purpose. Small areas at the lower front of the brain, attached to the wall of the third ventricle, are like this. The cells here are exposed directly to the blood going past, and when the blood thickens they sense that change themselves. Here lies the reason the barrier was left open. To measure the body's condition you have to meet it directly.

When these cells send a signal up, two things happen together. One is the feeling of being thirsty, and the other is that a hormone comes out which makes the kidneys throw away less water. It attends to the taking-in side and the putting-out side at once.

Why Thirst Settles the Moment You Drink

From here on is the part that is more interesting to me.

Drink water and thirst settles straight away. Yet for the water drunk to be absorbed in the gut and thin the blood takes several tens of minutes. Nothing has changed yet, and the thirst goes out first.

Signals coming up from the mouth and throat, and from the stomach, do that work. In animal experiments the mouth and throat measured the amount swallowed and the stomach side measured the concentration, and thirst had already gone out before the blood thinned.

Why was it made this way? Because if you waited until it was absorbed to stop it, you would already have drunk far too much. The body does not wait for the result — it looks at the amount that has come in and puts the thirst out in advance. It is at parts like this that I feel the body is very well made.

So This Medicine Asked the Two Apart

After coming to know this part, I came to look at the old records again. The old medical texts do not treat thirst as one lump. They set down the mouth being dry and craving water separately.

Now, holding the earlier account up against it, you can see why they separated them. A dry mouth may be a problem of little saliva coming out, and that problem is a different story from the water condition of the whole body. Craving water, on the other hand, means the measuring device has judged that there is not enough. The former is something that happened in the mouth, and the latter is a signal that came up from the whole body.

The reason the two have to be asked about together comes from here as well. Whether someone craves water yet stops soon after drinking, or keeps craving it however much they drink; whether they look for cold water or for hot — the condition the body is placed in differs along each of these branches.

I learned these questions from this medicine first, and came to know the name of the measuring device later. That was only the order of it; the place they point to was the same place.

What I Ask in the Consulting Room

So I do not ask only how much water you drink.

I ask whether you want to drink water, whether drinking makes you easier, whether it is only the mouth that is dry, whether you wake in the night to drink, whether you are drawn to cold water or to hot. Which branch it is, rather than how many glasses, is the information.

The account that you must drink more than a certain amount of water a day has spread widely. Only, the body already has a measuring device fitted to it, and making someone in whom that device is working perfectly well fill a number by force is not always a help. (A Test Looks at the Body of That Moment)

Thirst Like This Is Different, Though

If you suddenly crave a great deal of water, and urine increases, and weight falls, and the body is unusually run down, the story is a different one. Then you must have testing first. It may be a signal not of trouble with the measuring device but of something else going on.

It is also common for the mouth to go dry while taking medicines. If there are medicines you are taking, please let me know.

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We began with the words "my mouth is dry."

Inside those words two different things are mixed together. One is something happening in the mouth, and one is a judgement the body has put out after measuring its own condition.

So I do not write those words down as they stand but ask once more, dividing them. Depending on which of the two it is, the place to look changes. Where an old medical text set the two down separately, I watch the names of the devices worked out in our own time come in and take their seats one by one.

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References

• The structure that measures the body's concentration directly at places where there is no blood-brain barrier — Frontiers in Neuroscience, 2023
• Thirst goes out before the water reaches the blood — the signal from the gut to the brain — Nature, 2019
• The circuits that make thirst and salt appetite, taken as a whole — Neuron, 2024

Written by Dr. Heo Ji-young (PhD in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)