thirst

Thirst is the conscious urge to drink — the behavior side of water balance. While hormones can tell the kidney to save water, only drinking can actually add water back, so thirst is the body’s way of recruiting you into the job.

Thirst is generated in the hypothalamus and nearby brain regions. The strongest trigger is rising blood osmolality, detected by osmoreceptors: even a small increase in saltiness makes you feel thirsty. A second trigger is a drop in blood volume or pressure, which acts partly through angiotensin II, the same hormone that raises blood pressure also stimulating the brain to seek water. Together these match drinking to the body’s actual deficit.

Thirst works hand in hand with vasopressin: osmoreceptors drive both at once, one telling you to drink and the other telling the kidney to conserve. The thirst sensor can become unreliable in old age, blunting the urge and putting elderly people at higher risk of dehydration, and rare damage to the thirst centers can cause dangerous failure to drink despite very concentrated blood.