How does the brain immediately know when one's thirst has been quenched?

Question

The question struck me the other day when I drank a glass of water. I understand that there are at least two conditions under which the brain signals thirst: extracellular thirst, when there's not enough fluid in the blood, and intracellular thirst, when the cells need more water to lower the concentration of osmolites.

However, it takes time for water to enter the bloodstream after it has been ingested, and it spends at least a little while in the stomach.

Why does the sensation of thirst disappear despite the water having not yet made it into my bloodstream?

Answer 1

There are two different aspects to or drivers of thirst, osmolality and hypovolemia, with plasma osmolality elevation being the more potent stimulus of thirst. As stated by @arboviral (and supported by his link), not a lot is known about the mechanisms of immediate thirst satiety. Much more is known about the mechanisms causing thirst, and the mechanisms of return to normal serum osmolality over 120-180 minutes after rehydration than those immediately satisfying it.

Most of the research on this is old (pre-2000). In one often-cited very small study ¹, swallowing and esophageal reflexes (or as the authors termed it, “oropharyngeal metering”) had a significant effect on relief of thirst, that is,

the act of swallowing helps to regulate the volume of fluid ingested by providing an integrated signal proportional to the cumulative ingested volume.²

Study participants were all mildly dehydrated. One (control) group was allowed to drink ad lib. Another was allowed to drink ad lib but as they did so, the fluid was simultaneous extracted from the stomach by a nasogastric (ng) tube already in place. Interestingly, the difference in the amount of fluid ingested between these two groups (N.B. ad lib) was only 15% (the latter group drank a bit more.) The third group was given fluid by ng tube only.

The very act of swallowing fluid boluses induced satiety. However, plasma arginine vasopressin levels (involved in the regulation of fluid intake) dropped rapidly during the first 5 minutes of drinking in both ad lib groups, but not in those given fluids via ng tube, so it is not only mechanical receptors that are involved in quenching of thirst.

This confirmed an older study which demonstrated the importance of the rapid drop in plasma vasopressin in hydration before major changes in plasma osmolality occurred.³

Given the lack of extensive research and the relative immediacy of thirst suppression, The most likely explanation is a combination of sensory modalities and reflexes (mucosal wetness, temperature of rehydration fluid, mechanicoreceptors in the oropharynx, esophagus, etc.), and some rapid hormonal changes - e.g. in AVP - are responsible for limiting the amount we drink when thirsty.

_{1 Regulation of fluid intake in dehydrated humans: Role of oropharyngeal stimulation
2 Influence of age on thirst and fluid intake
3 Acute suppression of plasma vasopressin and thirst after drinking in hypernatremic humans}

Answer 2

Satiety is divided into pre-absorption and post-absorption - the mechanism you're asking about is preabsorption satiety, which as you point out occurs long before the water can be absorbed into the bloodstream and appears to be remarkably accurate. The receptors and underlying mechanisms of pre-absorption satiety are not fully known but are thought likely to include moistening of the mucosa of the mouth and stretch receptors of the stomach and intestine.

I'll try to fill this out later but there's a longer description with diagrams on p309 of Schmidt & Thews (2013) "Human Physiology" (published by Springer).

Answer 3

I'd say the answer is easy.

1. Since the whole body is one body all you need is the change of of water saturation in a part of you to determine / extrapolate how much water has been ingested.

2. We all came from the water and are mostly made out of water. So it's not like water is a foreign substance and we needs special receptors in order to determine water content.

3. We are water. And the system has evolved over time to be built in. I.e. the ones with a broken system have died of dehydration and anybody else survived.

4. Of course you might be looking for one singular mechanism that says stop, that's enough. But it's a mixture of body temperature, what you have eaten, how much energy you have, etc. (water requires energy in order to be heated to your body temperature)

5. That reminds me that apparently in hot regions you are supposed to drink tea or warm water because that allows you to drink more / be better hydrated. Based on that I'd guess it's just a temperature gradient? I.e. the water cools you down, and when it becomes too cold in your insides, you stop until you are hot enough again.