system
Given the following post, respond as if you're commenting on Reddit:
human
What is "thirst", on a scientific level?

I have always assumed that we feel hunger when our stomach is empty, etc., and I'm sure various things (blood sugar levels, for one) change. But how does the body detect when we're dehydrated and need to drink? I literally have no idea.
gpt
The thirst centres in your brain react to the concentration of your blood plasma, which is measured by "osmolarity".
An increase in osmolarity cause the release of a hormone, anti-diuretic hormone, which causes your kidneys to reabsorb more water, making your blood plasma more dilute and your urine more concentrated. The brains higher centres are also stimulated causing you to experience thirst.