Thirst

At a fundamental level, organisms require a certain amount of water and salt to perform chemical functions. If the balance between water and salt is out of whack (that balance different for every species, and to a smaller degree between individual members of a species), the organism’s alarm systems will signal and spark some kind of impulse meant to rebalance things.

This is a pretty important failsafe, as cells are water-permeable, and if the fluids that surround our cells don’t contain the right balance of water and salt, our cells will change. As they change, our tissues, our membranes, our brains and muscles and circulatory systems can all start malfunctioning, in some cases triggering cascades that lead to permanent damage or death.

This alarm system is different from organism to organism, but in complex mammals like humans, our brains (the really primitive neural structures, like those that cause us to breathe and allow us to maintain our balance) are the ones that pick up on our bodies’ signals, monitoring this homeostasis and compelling us to seek out water or salt, fast, if we’re at risk of falling too far in either direction.

We don’t know all the specifics about how this works, as we’ve only recently been able to assess the brain, mid-operation, with any resolution, and this system (like most biological systems, especially those involving the brain) is incredibly complex and interconnected with other systems.

But what seems to happen when we don’t have enough water in our bodies is that a couple of organs, the vascular organ of lamina terminalis and the subfornical organ (both of which are tucked into the brain and serve as connections between the brain and our blood), sample our blood, check to see if it contains the right allotments of all the stuff we need to function optimally, and then tell the brain if something is missing.

The brain then responds to that data (which in the case of thirst is based on the proportion of salt, and then the lack or overabundance of water is inferred from that salt ratio) with signals that we need to drink some water or eat something; a food craving or a dry mouth, for instance.

Interestingly, though, there’s a delay of about 30–60 minutes between the brain getting this data and when the data was generated. So the brain has to look at what’s happening, make predictions about what happens next, and then look at the data from other sensors (like the mouth and tongue and throat) to see if it seems like the body it equalizes is consuming what it’s meant to be consuming.

This is why, despite that delay, the indications of thirst can go away almost immediately if we consume water: the brain understands that we’re doing what we need to do to rebalance our body’s composition, and is removing those thirst-indicators, those alarm bells, as appropriate.

The flip-side of thirst, the desire for salt, is primarily moderated by our taste receptors and reward pathways. A bit of something salty will taste good, if we’re not already overwhelmed with salt, but if we eat too much of it, or if we take a bite of something that’s been over-salted, it will not taste good—it may even taste horrible or cause us to vomit.

All of which is fascinating in part because our understanding of this sequence of events and these interconnected systems is a lot richer than it was even ten or twenty years ago, but also because we don’t fully understand how these computations and guesses are accomplished, and what other systems might be fundamental to this functionality, but which we don’t yet know to watch and study.