The brain science behind deciding to drink when you're thirsty is pretty complicated

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ARI SHAPIRO, HOST:

It feels so simple - when we are thirsty, we drink. But as NPR's Jon Hamilton reports, the brain science behind that decision is pretty complicated.

JON HAMILTON, BYLINE: Without water, most animals die pretty quickly. But Yuki Oka, a biology professor at Caltech, says the act of drinking water can also prove fatal.

YUKI OKA: The animal needs to minimize the ingestion time because that's the most vulnerable time.

HAMILTON: Take too long at the watering hole, and a predator may eat you. So animals and people have evolved the system to make sure we drink enough water, but not too much. It has a whole series of checkpoints. Oka says the first one is passed when water reaches your mouth.

OKA: That already gives your brain refreshing feeling.

HAMILTON: But then your brain needs to know how much you're drinking. And Oka says it can't wait until water gets absorbed, which can take 15 minutes or more.

OKA: So the body has an additional checkpoint, which is gulping.

HAMILTON: Swallowing acts like a meter, letting your brain know how much fluid is entering the body almost immediately. Still, Oka says, the brain takes a trust-but-verify approach to this information.

OKA: So there's a second checkpoint in the intestine to make sure that you drank water. That takes place after a couple of minutes.

HAMILTON: The intestine also moves water into the bloodstream, where there is yet another checkpoint, and that is the subject of Oka's latest study, which appears in the journal "Nature." His team used mice to study the vessels that carry blood from the intestine to the liver, and they found cells there that transmit messages up the vagus nerve to the brain when an animal has been hydrating. But here's where the system gets really complicated. These cells weren't sensing the water directly. Oka says they were responding to a hormone that's produced when the intestine contains more water, making the contents less salty.

OKA: So the change is now translated into hormonal signal to tell the brain that we are drinking water.

HAMILTON: Within a few minutes - why so complicated? Oka says it may be a way to make the system more sensitive to subtle changes. Christine Browning, a professor at Penn State College of Medicine, says it makes sense that the brain wants to know what's happening in the bloodstream.

CHRISTINE BROWNING: It's an obvious checkpoint to have. It's an obvious sensory point to look at whatever is then circulating.

HAMILTON: Browning says, of course, the brain has lots of other ways to monitor hydration.

BROWNING: There's then going to be changes in blood pressure and in plasma volume, and those can be sensed as well.

HAMILTON: The brain even has areas that directly monitor how watery our blood is, and all of these inputs affect how thirsty we feel. Browning says complexity and redundancy in the thirst system are to be expected given the life or death nature of our dependence on water.

BROWNING: That's great from a survival standpoint. It just makes it a little bit difficult when we're trying to figure out exactly the neural mechanisms involved in these apparently very simple responses.

HAMILTON: Browning says there's still lots to learn. Jon Hamilton, NPR News.

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