Why does drinking water feel so good when you're thirsty?

Imagine you're exercising outside on a hot day. You're drenched in sweat, and the feeling of thirst begins to overwhelm you. You take out your water bottle and swallow your first big gulp — and your body immediately fills with relief and elation.

"There's a hedonic reaction to it," Patricia Di Lorenzo, a professor emeritus of psychology at Binghamton University in New York, told Live Science. "When you're really thirsty and you drink water, it just tastes so good."

But why does drinking water feel so pleasurable when you're thirsty?

We get thirsty when we exercise heavily, because as we sweat, our blood volume decreases. Most areas of the brain are separated by the blood-brain barrier, a layer of cells that prevents harmful toxins and pathogens from infecting the brain. But certain parts of the brain fall outside this barrier, allowing for rapid detection of changes in our blood. When we lose blood volume from exercising or eating salty foods, neurons in these parts of the brain send a signal to trigger the feeling of thirst.

"This rapid response is very important for survival," Yuki Oka, a biology professor at Caltech, told Live Science. "If it takes so long, then you might get dehydrated."

Related: How much water do you really need to drink?

Three parts of the brain process thirst: the subfornical organ (SFO), the organum vasculosum lamina terminalis (OVLT) and the median preoptic nucleus (MnPO). Both the SFO and OVLT are located outside the blood-brain barrier. In a 2018 study in mice, Oka uncovered that while all three areas have neurons that drive drinking when those nerve cells are excited, the MnPO is in the middle of this process. It transmits thirst signals from the SFO and OVLT to other parts of the brain to prompt drinking.