Speaker 1: My Welcome to Stuff to Blow Your Mind, the production
Speaker 1: of My Heart Radio. Hey, welcome to Stuff to Blow
Speaker 1: Your Mind. My name is Robert Lamb and I'm Joe McCormick,
Speaker 1: and we're back with part two of our series on thirst.
Speaker 1: What are you drinking right now? Absolutely nothing? Oh no, wait,
Speaker 1: I do have I have water here at my desk.
Speaker 1: I'm always drinking water. I'm a water guy. You know,
Speaker 1: some people are really into tea. I almost never drink tea.
Speaker 1: I go coffee water. That that's pretty much it. Uh,
Speaker 1: during the daytime hours at least. Oh yeah, well, I'm
Speaker 1: currently drinking tea. I generally go coffee, uh, te tea, water, tea,
Speaker 1: and then water and then then when the evening comes,
Speaker 1: you know, maybe I'll have something something else to drink.
Speaker 1: But uh yeah, right now it is tea, which is
Speaker 1: essentially water, but with extra things added to it. You know,
Speaker 1: I some times get confused about what tea means because
Speaker 1: I think of tea as being a specific kind of plant, like,
Speaker 1: isn't there a t tree? Again, I don't know much
Speaker 1: about tea, so I think about it in that way.
Speaker 1: But then you can make a tea out of like
Speaker 1: anything basically that you steep in water. Right, people make
Speaker 1: mushroom teas, people with all kinds of just oh, we
Speaker 1: have and plant stuff in water, and that's tea. Also yeah,
Speaker 1: like I mean, there's there's property and that's generally what
Speaker 1: I'm when I'm drinking. I'm drinking, you know, a dark
Speaker 1: tea or maybe a green tea. But also sometimes I'll
Speaker 1: have a tumeric tea, which is uh like like tumeric
Speaker 1: and ginger and a little coconut oil and a little
Speaker 1: honey hot water. But it doesn't actually contain tea, so
Speaker 1: it's not completely accurate to call it such because it's
Speaker 1: sort of how not all band aids are technically band
Speaker 1: aids or xerox it's or whatever. Has this been a
Speaker 1: terrible intro? Maybe we can cut my tea thing? No, No,
Speaker 1: this is a This is an intro that the people
Speaker 1: can relate to. People can thoughts on this. It's always
Speaker 1: good to start off asking what a word means. So
Speaker 1: I turns out I know nothing about T. I have
Speaker 1: no little There you go. We need to come back
Speaker 1: and do an episode or a series of episodes on T.
Speaker 1: It's a fascinating subject, fascinating history. Okay, well, we're going
Speaker 1: to jump back into some of the science that we
Speaker 1: were talking about in the last episode, where I was
Speaker 1: reviewing an interesting paper that I had read summarizing the
Speaker 1: the recent state of research on the biology of thirst.
Speaker 1: That was it was a paper published in Current Biology.
Speaker 1: We had to break off in the middle of talking
Speaker 1: about it. But but before we get back into that, Rob,
Speaker 1: I think you wanted to talk about the taste of water,
Speaker 1: which I think is in itself a very interesting subject. Yeah,
Speaker 1: it's it's a strange one because we probably don't think
Speaker 1: about it enough, but but we often talk about it,
Speaker 1: especially when we when we travel from one place to another. Um.
Speaker 1: We if you look back on I think back on
Speaker 1: water that you've consumed in the past, may have specific
Speaker 1: memories of different waters. Um Like, I remember when my
Speaker 1: family lived in the country and we had well water,
Speaker 1: and it had a it had a distinctive taste, and
Speaker 1: I don't I can't say that I loved it, But
Speaker 1: if I were to encounter the exact same flavor profile again,
Speaker 1: it would probably make me nostalgic, you know, like it's
Speaker 1: it's it's a definite flavor that is tied to a
Speaker 1: definite place. And time warning that this is a little crude,
Speaker 1: but I don't know of how else to explain it.
Speaker 1: You've ever been somewhere that had farty water, Um, well,
Speaker 1: it's kind of sulfury water, kind of kind of like
Speaker 1: water from the That's kind of what this was like. Yeah,
Speaker 1: it was that had kind of a a fire and
Speaker 1: brimstone kind of flavor to it. Okay, that's a more
Speaker 1: elegant way to put it. Well, the denizens of Hell
Speaker 1: are not insulted. They're flattered that you would you would
Speaker 1: compare this foul drinking water to flattens. But um, you know,
Speaker 1: but then sometimes I don't know, there's there's also beech water,
Speaker 1: you know, Like I I anytime I'm I'm in Florida
Speaker 1: and I'm having Florida water, like it has its own
Speaker 1: it tends to have its own bouquet, its own flavor
Speaker 1: profile that at the time I'm not enjoying. But then
Speaker 1: after I'm back and drinking tastier water, I'm kind of
Speaker 1: kind of long for it, you know, because like this
Speaker 1: is the water of of of the vacation that I
Speaker 1: am no longer on that sort of thing. I remember
Speaker 1: when I was in like a middle school. I went
Speaker 1: to New Mexico and we went somewhere there where the
Speaker 1: water was very I don't know, the tap water was
Speaker 1: almost kind of frothy somehow. It was like a kind
Speaker 1: of kind of white and cloudy, and I remember thinking
Speaker 1: it had a strange taste. I don't know what to
Speaker 1: connect that too, if it was supposed to be like
Speaker 1: that or not. Yeah, So I guess one thing we
Speaker 1: can definitely establish here is that there there certainly seems
Speaker 1: to there is an objective difference in in drinking waters
Speaker 1: from one place to another. Uh, you know, Atlanta water
Speaker 1: versus Florida water, that sort of thing. But there's also
Speaker 1: this wide variety in people just interpret the taste of water. Um.
Speaker 1: You know, I think we we often think of water
Speaker 1: as being neutral or tasteless. Um. If you're mixing a cocktail,
Speaker 1: for example, you don't want to shake it too long
Speaker 1: with the crushed ice because you will what will happen,
Speaker 1: You will water it down. Water in this case is
Speaker 1: the antithesis of an interesting flavor profile. Well. But the
Speaker 1: other half of that is sometimes people screw up making
Speaker 1: a drink because they don't shake it with ice at
Speaker 1: all when they're supposed to, and they don't understand that
Speaker 1: shaking with ice not only cools the drink, but it
Speaker 1: also adds a certain amount of water to the drink,
Speaker 1: which is an important ingredient. Yes, absolutely so, you're supposed
Speaker 1: to shake a cocktail and you don't. You end up
Speaker 1: with a drink that's usually too strong or too sweet. Yeah,
Speaker 1: there's a balance to be maintained there, much like the
Speaker 1: balance of of water in the human body that we
Speaker 1: discussed in the last episode. Many of you out there
Speaker 1: have probably met someone who claims to not like the
Speaker 1: taste of water. Sometimes they'll these individuals will will use
Speaker 1: flavored water or other beverages instead, and we'll tend to
Speaker 1: shy away from just drinking straight water. Um. But I
Speaker 1: don't know if you're like me, you know, perhaps you've
Speaker 1: noticed times when a glass of ice water is just
Speaker 1: super satisfying. Um, you know, particularly on a hot day,
Speaker 1: like a just a super cold water, lots of ice
Speaker 1: in it. Um, I would I would tend to. I
Speaker 1: found myself wanting to categorize that as delicious, though at
Speaker 1: the same time feeling weird for thinking that because it's
Speaker 1: like it's water, it I can't say it's delicious. It
Speaker 1: has no flavor, right, that's what we're we believe. Another
Speaker 1: another one I really like is the first sip of
Speaker 1: cold water that I have after I have brushed my teeth. Now,
Speaker 1: after I've brushed my teeth and rinsed and spit, I'm
Speaker 1: not drinking um water that way, but yeah, like, after
Speaker 1: I've brushed my teeth, I've walked into another room, I
Speaker 1: have a sip of water, super cool and refreshing, agreed,
Speaker 1: much better than the sip of orange juice after you
Speaker 1: brush your teeth, which is I don't know if you've
Speaker 1: ever tried that. It's it's fame disgusting. Oh this is
Speaker 1: this is always the worst. Really getting into the weeds here.
Speaker 1: But um, that that feeling when you you were you're
Speaker 1: busy in the morning, you go brush your teeth, You
Speaker 1: come back and you find your coffee cup and it
Speaker 1: still has half a cup of coffee in it. Heartbreaking
Speaker 1: because you know you can't drink it. Now, if you drink,
Speaker 1: you can drink it, you can you can throw it back,
Speaker 1: but it's gonna taste awful. I wonder if anybody's ever
Speaker 1: done a controlled study of how long you have to
Speaker 1: wait after brushing your teeth before those those horrible flavor
Speaker 1: interactions fade away. I don't think it's too long, it's
Speaker 1: I mean, if I had to venture to guess, I'd
Speaker 1: say it couldn't be more than like twenty minutes. Right. Yeah. Now,
Speaker 1: of course, a lot of this, some of this is
Speaker 1: gonna very culture to culture. In some cultures, it's not
Speaker 1: it's not considered advisable to drink cold water. You want
Speaker 1: to drink hot water. Uh, So you know, there's it's gonna,
Speaker 1: it's gonna, it's gonna vary. There's a lot of cultural
Speaker 1: stuff going on here as well. Oh, I've never heard
Speaker 1: of that. What's what's an example of of a culture
Speaker 1: that favors hot water drinking. There's a connection if Maray services,
Speaker 1: a connection to traditional Chinese medicine here where um uh,
Speaker 1: the idea is that it's better for your health to
Speaker 1: drink hot water as opposed to cold water. But I
Speaker 1: don't remember any of the deeper details of regarding it all. Right, Well,
Speaker 1: to get us back on track, you had mentioned that
Speaker 1: maybe maybe it was wrong to say that tap water,
Speaker 1: you know, cold water coming in on a hot day
Speaker 1: was delicious because water itself doesn't really have a taste.
Speaker 1: And I think this is a common understanding, but is
Speaker 1: that true? Like it does it have a taste? Does
Speaker 1: it not have a taste? Does it have a taste.
Speaker 1: We're just so used to that we can't taste it anymore. Yeah,
Speaker 1: we'll like to We'll go back to Aristotle. Aristotle was
Speaker 1: certainly kind of in the no taste camp, stating that
Speaker 1: it was for the most part tasteless. Uh. The idea
Speaker 1: that water is it made liver flavors, but it it's
Speaker 1: in and of itself, does not have flavor. But a
Speaker 1: lot of a lot of work has gone into this
Speaker 1: this question of Uh, some some interesting studies. UM. I
Speaker 1: was reading an article by Berlin Game at All titled
Speaker 1: Understanding the Basics of Tap Water Taste, publishing the American
Speaker 1: Water Works Association Journal in two thousand seven. UM, and Uh.
Speaker 1: They point out that the basically you get the water
Speaker 1: is going to contain negatively and positively charged ions as minerals.
Speaker 1: You're gonna find that in your tap water, and that
Speaker 1: can positively and negatively affect taste. This is especially key,
Speaker 1: they point out as far as water regulations and reverse
Speaker 1: os moses go, which is to say, without without getting
Speaker 1: into all the chemistry, there are certainly differences in water
Speaker 1: flavor due to different minerals and other elements in the water. Now,
Speaker 1: another interesting thing about about tap water, especially uh, generally
Speaker 1: when you're drinking tap water, UH, the drinking water probably
Speaker 1: contains calcium, magnesium, sodium um. And according to Azila at
Speaker 1: all In in an article in the Journal of General
Speaker 1: Internal Medicine from two thousand one titled Comparison of Mineral
Speaker 1: content of tap water and bottled waters, the mineral content
Speaker 1: of drinking water UH like this ultimately maybe an important
Speaker 1: source of daily recommended mineral dosages. UH, certainly in the
Speaker 1: United States, which was the the area they were looking
Speaker 1: at here um and UH. And so basically, why we
Speaker 1: might think of water as being this thing that we
Speaker 1: would will, let's just purify the heck out of it. Uh,
Speaker 1: just give me the purest water possible. The purest water
Speaker 1: possible isn't necessarily going to be the healthiest or the
Speaker 1: tastiest UM, which I thought that was an interesting distinction.
Speaker 1: Like when when we when we purchased bottled water, we're
Speaker 1: not going and by and buying the distilled water usually,
Speaker 1: I mean, maybe maybe that's what you're doing. But for
Speaker 1: the most part, people are are are not buying distilled
Speaker 1: water for drinking purposes, right. But distilled water, even though
Speaker 1: it is probably the purest form of water you can get,
Speaker 1: is not necessarily better for any reason, not for not
Speaker 1: for health or for experience of drinking. Now, in terms
Speaker 1: of just trying to get down to the question, though, well,
Speaker 1: what what does it taste like? Okay, you know we're
Speaker 1: saying it can can go a little in this direction,
Speaker 1: a little in that direction. We have these mineral components, uh,
Speaker 1: you know, adding to the the flavor. But but what
Speaker 1: is the taste itself? And I think one of the
Speaker 1: interesting things about that question is that it forces you
Speaker 1: to take a step back and think about taste itself. Um.
Speaker 1: You know, taste involves both sensations on the tongue and
Speaker 1: olfactory information as well. So it's not only it's not
Speaker 1: only tasting with the mouth, it's smelling, uh whatever you
Speaker 1: know it is that you're consuming as well. Um and uh.
Speaker 1: And certainly we do taste contents in the water, though
Speaker 1: under ideal circumstances that not that's not going to push
Speaker 1: you hard in any direction. But but obviously if you
Speaker 1: dilute some sugar and a glass of the universal solvent
Speaker 1: and drink it, you'll find it sweet saltwater taste salty.
Speaker 1: Your sense of taste is still weighing in on the water.
Speaker 1: But in terms of the this kind of brings us
Speaker 1: back to the same question. Does the water itself have
Speaker 1: a taste or is it just the vehicle for these
Speaker 1: various flavors, be it salt or sugar, or you know,
Speaker 1: a slight hint of of magnesium, that sort of thing.
Speaker 1: So one idea proposed in the sixties and seventies by
Speaker 1: psychologist Linda barda Shut was that the taste of water
Speaker 1: is more of an after taste of prior eating and drinking.
Speaker 1: And this too, I guess it is more in line
Speaker 1: with the idea of water as the vehicle flavor, but
Speaker 1: something that is in and of itself flavorless. So the
Speaker 1: idea would be that, like when you take a drink
Speaker 1: of water, you are somehow re experiencing flavors of foods
Speaker 1: you may have eaten most recently. Yeah, and this this
Speaker 1: will become important. And again we have to realize that
Speaker 1: when you take a sip of water, you were not
Speaker 1: introducing water no matter how sterile your water is, your
Speaker 1: mouth is is not sterile. You know your your mouth
Speaker 1: is going to even if you if you've just brush
Speaker 1: your teeth, great, but that you're going to have some
Speaker 1: semblance of of the of that experience in your mouth.
Speaker 1: If you're eating dinner, you know there's going to be
Speaker 1: the hint of food or drink as well. So, yeah,
Speaker 1: you're not introducing the water into a neutral place. If
Speaker 1: nothing else you your saliva is present. Emily Underwood wrote
Speaker 1: in an excellent short seventeen piece for the American Association
Speaker 1: for the Advancement of Science on this topic, the Flavor
Speaker 1: of Water, um and uh. At one point she's a
Speaker 1: citing Zachary Knight, oh, who is one of the three
Speaker 1: authors of the research summary thirst in Current Biology that
Speaker 1: I was referring to in the last episode, and I'll
Speaker 1: be talking about again in a few minutes. Yeah, So
Speaker 1: Underwood points to um, something that that we discussed in
Speaker 1: the last episode that you know, ultimately, when we're getting
Speaker 1: into taste, when we're getting into water detection in the mouth,
Speaker 1: the molecular and cellular mechanisms here are are not that
Speaker 1: well understood. And then she also touches on some of
Speaker 1: nights UM research regarding the thirst trigger, Uh, you know
Speaker 1: where where that is seems to locate in the brain
Speaker 1: and then ultimately what's where is the trigger in the
Speaker 1: in the rest of the body. But we'll come back
Speaker 1: to that in a bit. Yeah, but I guess there
Speaker 1: are somewhat separate questions for can you detect water in
Speaker 1: your mouth versus can you taste water? I mean those
Speaker 1: are slightly slightly different, right, yeah, well, but but then again,
Speaker 1: when you start thinking about like what what taste is,
Speaker 1: maybe they're not that different, like it's but but it
Speaker 1: basically comes down to, yeah, it's not about interpreting these
Speaker 1: minerals or sweetness or saltiness, but like the just the
Speaker 1: basic signal of it is water. It is in my mouth,
Speaker 1: you know, um, And then ultimately being able to tell
Speaker 1: if something is not water, um, like or if it
Speaker 1: or if the water is you know, too far in
Speaker 1: a particular direction, like oh it is it is actually
Speaker 1: oil and water and not just water, Um, I wanted water.
Speaker 1: That's sort of question. So researchers have been trying to
Speaker 1: isolate water sensing taste receptor. It sells on the tongue
Speaker 1: um of a particular note here is the paper the
Speaker 1: segular Mechanism for Water Detection in the Mammalian Taste system
Speaker 1: by Zocchi at All published in Nature Neuroscience in UM.
Speaker 1: This was a team I believe from the California Institute
Speaker 1: of Technology in Pasadena, and they were working with mice
Speaker 1: UH and this is this is one of those the
Speaker 1: studies that I mean, it's already an interesting area, like
Speaker 1: using mice to figure out how we're tasting, but then
Speaker 1: it goes in ultimately strange directions that are pretty fascinating.
Speaker 1: So they were they were using different varieties of mice
Speaker 1: with specific types of taste receptor cells genetically knocked out
Speaker 1: in order to try and isolate which ones were seemingly
Speaker 1: useful in tasting water. And they found that the acid
Speaker 1: sensing sour taste receptor cells seemed to be the most involved.
Speaker 1: Mice with those knocked out took far longer distinguishing oily
Speaker 1: water from clean water. UM. In the words of the study,
Speaker 1: this quote compromise discrimination between water and non aqueous fluids.
Speaker 1: So these cells seem like they may they may well
Speaker 1: be very much involved in UH in the process. Interesting, Okay,
Speaker 1: So if we are actually tasting water, it may rely
Speaker 1: more on the cells that normally taste sourness or acidity
Speaker 1: than other taste receptor cells, or at least in mice,
Speaker 1: and if if the same held true in humans, that
Speaker 1: would be the case for us. But this is where
Speaker 1: the mouse experiment from this team gets weird. Um. So
Speaker 1: to further test this out, they bread mice that could
Speaker 1: taste light with these acids sensing t rcs UH taste
Speaker 1: receptor cells. They train them to drink water from a pigot,
Speaker 1: and they replace that'spigot with an optic fiber cable, so
Speaker 1: they they they apparently treated the mice in this case,
Speaker 1: they apparently treated the light as if they were tasting water,
Speaker 1: but they didn't stop. They kept drinking the light long
Speaker 1: after UH they would have normally stopped drinking water. So
Speaker 1: the acid sensing t rcs might be involved in triggering drinking,
Speaker 1: but they might not be involved in stopping you from drinking,
Speaker 1: like saying, okay, well that's that's all the water I
Speaker 1: need to drink right now, um and um. And I'm
Speaker 1: not sure there may be other complications there. I mean,
Speaker 1: once you you have an animal drinking light and not
Speaker 1: actual water. Um, it just seems to me like there
Speaker 1: might there might be other things going on there on
Speaker 1: in a very physical basis. So this would be taste
Speaker 1: receptor cells in the mouth, but they're like optically sensitive cells.
Speaker 1: Am I right about this? It's literally like drinking light
Speaker 1: with their mouth, like letting light shine into the oral cavity.
Speaker 1: Yes it uh, I mean it sounds insane and you
Speaker 1: say it out loud, but but yeah, there's there's actually
Speaker 1: a YouTube video about it from Science Magazine titled these
Speaker 1: mice are drinking light. H So if anyone needs to
Speaker 1: actually see what we're talking about here, pull that up.
Speaker 1: And yeah, it just it's it's a this, this black
Speaker 1: mouse going up to this, um, this little receptacle, and
Speaker 1: it appears to be drinking, but there's blue light flooding
Speaker 1: out of the hole, okay, and it's got light sensitive
Speaker 1: cells in its mouth. So the mouse's brain is reacting
Speaker 1: as if by shining light into its mouth it was
Speaker 1: swallowing water. Yes, that's crazy, man, Yeah it is. This
Speaker 1: is this is it's it's it's mind blowing on several
Speaker 1: levels here. Um. So ultimately this this all might be
Speaker 1: related to changes in pH level. When in a normal situation,
Speaker 1: when you know, a mouse or any mammal supposedly is
Speaker 1: drinking something that is not light. When it it tries
Speaker 1: out the water, saliva is washing around in the mouth,
Speaker 1: and the removal of saliva might be key to sensing water. Again,
Speaker 1: coming back to what we said earlier about uh, you know,
Speaker 1: water is not entering into a neutral environment when it
Speaker 1: goes in your mouth. It's going to interact with at
Speaker 1: least saliva, and in doing so that might change the
Speaker 1: pH level which triggers these um these t r c
Speaker 1: s on the tongue. Um. That seems to be the
Speaker 1: basic framework we're potentially looking at here. Oh that's interesting.
Speaker 1: So at least according to the idea here, part of
Speaker 1: the sensation of of drinking water might be the water's
Speaker 1: ability to wash natural saliva out of the mouth. Right, So,
Speaker 1: of course standard caveat that more researches is required, And
Speaker 1: then we still don't we still don't have this one, Kno,
Speaker 1: But it's yeah, it's it. Ultimately, it makes you rethink
Speaker 1: again what taste actually is and consider there there may
Speaker 1: be precise triggers in place for detecting water and distinguish
Speaker 1: it from other liquids, and then also letting you know
Speaker 1: that you have had enough water. Like all these things
Speaker 1: that we just take for granted, we think of them
Speaker 1: as we think of them as choices that we make,
Speaker 1: like I have to, I have, I am, I've decided
Speaker 1: now I must have water. I have decided now that
Speaker 1: I don't need any more water. But but these are
Speaker 1: all tied into the you know, this intricate biological system.
Speaker 1: Thank thank Alright, well, Rob, if you're ready, I wanted
Speaker 1: to discuss some more of the things I was reading
Speaker 1: in that article and Current Biology that was about the
Speaker 1: biology of thirst. Again. This was by David Leeb, Christopher Zimmerman,
Speaker 1: and Zachary Knight. Let's do it all right now. We
Speaker 1: already talked about some of the conditions that will trigger
Speaker 1: thirst and water seeking behavior in animals such as rodents
Speaker 1: and humans. Uh uh. Commonly, one thing would be a
Speaker 1: decrease in water volume in the body, but another thing
Speaker 1: would be an increase in what's called blood OSMO reality
Speaker 1: O S M O L A L I T Y.
Speaker 1: I spell it because that word will keep coming up
Speaker 1: in this discussion. OSM reality is the concentration of substances
Speaker 1: dissolved in the water content of the body, so the
Speaker 1: blood osmolality will go up. Say, if you consume salt
Speaker 1: or consume other kinds of salts of various minerals, whether
Speaker 1: that's sodium or magnesium or whatever, there are lots of
Speaker 1: things dissolved in the blood, and is the concentration of
Speaker 1: those things dissolved in the blood goes up. That's that's
Speaker 1: known as increasing OSMO reality. And as we discussed in
Speaker 1: the last episode, your your awsom reality really needs to
Speaker 1: be pretty tightly constrained within an ideal range, or it
Speaker 1: can start leading to system wide problems with cellular function
Speaker 1: because cells need a pretty tightly controlled electrochemical gradient on
Speaker 1: each side of their their membranes in order to control
Speaker 1: the passage of ions in and out of the cell
Speaker 1: to you know, take in things, the cell needs to
Speaker 1: release waste products and so forth. So uh So, in
Speaker 1: order for your body to work right, it needs to
Speaker 1: have the right level of the right concentration of things
Speaker 1: like salts dissolved in its water. But you also have
Speaker 1: to keep your your body, your your body's water volume
Speaker 1: at the right level, uh in order to maintain ideal
Speaker 1: blood pressure. Because that blood's gotta flow, and if if
Speaker 1: you suddenly are to remove a lot of liquid from
Speaker 1: the body, suddenly the heart has to pump harder and
Speaker 1: harder to get the red blood cells to do all
Speaker 1: the different parts of your body. So keeping up keeping
Speaker 1: up the right amount of water in the body and
Speaker 1: the correct concentration of substances dissolved in that water is crucial.
Speaker 1: And that dichotomy we talked about last time actually breaks
Speaker 1: down into two distinct types of dehydration that the authors
Speaker 1: talk about. And these two types of dehydration actually lead
Speaker 1: to different behavioral reactions in animals. So you can have
Speaker 1: dehydration within cells or dehydration between cells. A loss of
Speaker 1: water from within cells, known as intracellular dehydration, is usually
Speaker 1: caused by high blood osmolality, so the introduction of salts
Speaker 1: or other things like that into the body. This draws
Speaker 1: water out of cells by osmosis and causes the cells
Speaker 1: to shrink, which certainly doesn't sound good. Standard behavioral response
Speaker 1: to to uh that kind of thing. That intracellular dehydration
Speaker 1: is thirst. You want water, so you go get it.
Speaker 1: Loss of water from between cells is known as extracellular dehydration,
Speaker 1: and this usually is caused by a loss of total
Speaker 1: blood volume, for example by bleeding. You know, if you
Speaker 1: cut yourself and lose a bunch of liquid out of
Speaker 1: your arm or something, you will lose total blood content
Speaker 1: without changing the body's osmolality. You know, you think about
Speaker 1: it that way, like you're the liquid is going down,
Speaker 1: but you're not changing how salty the liquid that's left is.
Speaker 1: So in order to recover from that condition, you will
Speaker 1: actually need both water and salt to replenish the lost volume.
Speaker 1: Just drinking water alone would leave your osmolality too low.
Speaker 1: So the behavioral response to a loss of total water
Speaker 1: content or content from between cells is usually thirst plus
Speaker 1: what the author is called salt appetite. You want water
Speaker 1: and salt at the same time. But interestingly, many things
Speaker 1: that happened to the body cause both types of dehydration
Speaker 1: at the same time. Uh. Then the example they give
Speaker 1: is sweating. This is very common, right, you know. You
Speaker 1: you go out in the sun and you sweat. Well,
Speaker 1: sweat is not only a loss of blood volume. Uh,
Speaker 1: sweat is salty, but sweat is actually less salty than
Speaker 1: your blood. So if your body is losing liquid that
Speaker 1: is less salty than its water content overall, like sweat
Speaker 1: is the salt content of what's left behind and is
Speaker 1: therefore increasing. Does that make sense, So the extra salt
Speaker 1: left over inside you when you sweat causes an increase
Speaker 1: in blood osmlality, again triggering a thirst for water. It's
Speaker 1: kind of counterintuitive because if you ever do taste sweat it,
Speaker 1: you know it tastes salty, so you would think it
Speaker 1: would feel like you're losing salt, but you're actually gaining
Speaker 1: salt is in relationship to the amount of water left
Speaker 1: in you. Yeah, I think we've touched that on this
Speaker 1: on the show before that that sometimes we give too
Speaker 1: much credit to sweat in its ability to remove things
Speaker 1: from our body, certainly when some arguments for the removal
Speaker 1: of impurities through sweat, for example, Oh yeah, yeah, right
Speaker 1: with people who think that you can like cleanse all
Speaker 1: the toxins by sweating or something, I mean, sweating is great. No,
Speaker 1: no knocking on sweat and uh and if memory serves
Speaker 1: like I mean, there is some toxin removal, but not
Speaker 1: not anywhere near as good as say, good old fashioned urination.
Speaker 1: I mean, that's that's right, when we have urination exactly.
Speaker 1: So I guess from here we go on to talk
Speaker 1: about how the brain senses and monitors osmolality. Uh. This
Speaker 1: is what the authors say is quote, probably the most
Speaker 1: important homeostatic signal for drinking in everyday life is the
Speaker 1: brain's ability to monitor osmolality. That's what's going to be
Speaker 1: causing you to go drink water. Um. And the authors
Speaker 1: point out some interesting things, one of which is that
Speaker 1: when blood osmolality and blood volume are both threatened at
Speaker 1: the same time, for example, if they both increase above
Speaker 1: the ideal range at the same time, the body places
Speaker 1: a higher priority on defending the the ideal osmolality than
Speaker 1: it does on defending volume. So examples of this would be,
Speaker 1: you know, hypernatremia having too much sodium, or hyperglycemia having
Speaker 1: too much glucose in the blood. Whatever it is. The
Speaker 1: state of having too high of an osm reality, which
Speaker 1: they call hyper tonicity, is probably more threatening, more of
Speaker 1: a danger overall than than having not quite the right
Speaker 1: amount of water volume in your body. But anyway, well,
Speaker 1: you know, so what takes care of this whole job, Well,
Speaker 1: you've got some physical structures in your brain that sort
Speaker 1: of that dip their finger into the soup to to
Speaker 1: to taste it for salt and uh and and let
Speaker 1: you know what's going on. Both of them are small,
Speaker 1: they reside in the fore brain, and they are known
Speaker 1: as first of all, the sub formical organ or SFO,
Speaker 1: and then the organum vasculosum of the lamina terminalis or
Speaker 1: ov LT. Now you've probably heard before of something called
Speaker 1: the blood brain barrier. This is a system of border
Speaker 1: cells that prevent things that happen to be floating around
Speaker 1: in the blood from passing non selectively into the brain.
Speaker 1: So you know, the brain does need blood. It has
Speaker 1: to receive oxygen and other nutrients from blood flow, but
Speaker 1: the brain has to also protect itself against totally unregulated
Speaker 1: exchange with the blood. And there may be multiple reasons
Speaker 1: for this, but one of the main ones I've seen
Speaker 1: is that this prevents blood borne pathogens from infecting brain tissue,
Speaker 1: so that seems important. So in regions where it's active,
Speaker 1: the blood brain barrier only allows selective passage of certain
Speaker 1: types of material from the blood into the neurons. But
Speaker 1: if you're part of the brain that needs to get
Speaker 1: raw data about the contents of the blood moment by moment,
Speaker 1: apparently it won't do to be hiding behind this protective
Speaker 1: fence of cells. So, according to the authors here, the
Speaker 1: SFO and the o v l T, though they are
Speaker 1: in the brain, are located outside the blood brain barrier,
Speaker 1: so they can sort of taste the blood river unfiltered,
Speaker 1: undiluted to get a raw sense of what's going on.
Speaker 1: To read directly from the paper here quote, it is
Speaker 1: thought that these SFO and ov LT neurons monitor the
Speaker 1: blood osmolality directly, possibly via stretch sensitive ion channels embedded
Speaker 1: in their plasma membranes that detect changes in cell volume
Speaker 1: following intracellular dehydration. However, the identity of the specific ion
Speaker 1: channel or other protein responsible for OSMO sensing by these
Speaker 1: neurons is unknown. Furthermore, the possibility cannot be excluded that
Speaker 1: other cell types such as galia play an important role
Speaker 1: in OSMO sensation. So here they they identify, they put
Speaker 1: the flag up for One more thing that hasn't fully
Speaker 1: been figured out in in the science of thirst is
Speaker 1: what is the direct molecular mechanism that the neurons in
Speaker 1: these brain regions use to detect changes in blood osmolality.
Speaker 1: It maybe these stretched sensitive cells that you know, get
Speaker 1: dehydrated and then send information based on that to to
Speaker 1: the brain regions that then filter out to to other
Speaker 1: brain regions from there. But we don't know for sure
Speaker 1: anyway that is sensing intracellular dehydration or increases in blood osmolality.
Speaker 1: But what about that other kind extra cell or dehydration
Speaker 1: where the blood volume actually decreases overall, such as after bleeding. Well,
Speaker 1: decreases in blood volume are known as hypovolemia, like low
Speaker 1: volume hypovolemia UH, and they correspond with a decrease in
Speaker 1: blood pressure hypotension UH and the body's reactions to hypotension
Speaker 1: take places sort of a complex chain of mechanisms involving
Speaker 1: multiple multiple organs and several forms of a hormone, very
Speaker 1: important hormone called angiotenson. You might see this abbreviated sometimes
Speaker 1: with like capital letters A in G I and then
Speaker 1: maybe A in G I I for Angiotenson one and
Speaker 1: angiotensin two. Apparently the most crucial form of the hormone
Speaker 1: is Angiotenson two, which causes things like the narrowing of
Speaker 1: blood vessels. So if you shrink blood vessels and make
Speaker 1: them smaller, that helps keep blood pressure up when volume
Speaker 1: is low uh, and it also leads to water reuptake
Speaker 1: by the kidneys. The kidneys are like holding fast to
Speaker 1: the water content rather than just squandering it as they
Speaker 1: might if you had plenty of water in your body.
Speaker 1: And there's evidence that the presence of angiotenson to this
Speaker 1: hormone also causes a thirst drive to be generated in
Speaker 1: the brain, primarily involving the SFO or the sub formical organ. Interestingly,
Speaker 1: in rodents, if you just give them straight Angiotenson two,
Speaker 1: it causes what the authors call profound water consumption, just
Speaker 1: like you know, voluminous drinking of water. But in humans,
Speaker 1: apparently Angiotenson two levels quote do not correlate well with
Speaker 1: the perception of thirst, and infusions of physiological levels of
Speaker 1: Angiotenson to do not stimulate drinking. Interesting, so, perhaps while
Speaker 1: this hormone does things to increase blood pressure, like constrict
Speaker 1: blood vessels and cause water reuptake by the kidneys, it
Speaker 1: does not make us thirsty for water like it us
Speaker 1: and other animals for some reason the author's right quote Well,
Speaker 1: this suggests that Angiotenson too might be less important for
Speaker 1: regulation of drinking and humans. Interpretation of these negative results
Speaker 1: is complicated by the fact that peripheral infusion of angiotenson
Speaker 1: too rapidly increases blood pressure, which can then feed back
Speaker 1: to counteract any effects of angiotenson too on thirst. So
Speaker 1: maybe they're just complicated balancing interactions here that when you know,
Speaker 1: if you directly infuse this hormone, it increases blood pressure,
Speaker 1: which has other downstream effects which which counteract the onset
Speaker 1: of thirst. Okay, I mean, I guess just it just
Speaker 1: goes to show, you know, one of the factors of
Speaker 1: experiments with with with mice and other non human mammals
Speaker 1: is that we have a whole lot in common with them,
Speaker 1: but not everything is going to um is going to
Speaker 1: apply to us one to one, Right, A lot does,
Speaker 1: but not everything. Okay, But anyway, I guess what we're
Speaker 1: trying to uh figure out here is building a bridge
Speaker 1: from our existing knowledge about these brain regions that play
Speaker 1: a role in monitoring the body's osmolality and water content
Speaker 1: and UH and regular in these hormones that helped regulate
Speaker 1: the body's physiologic response to dehydration, and how that connects
Speaker 1: to the actual behavior and the drives that we since
Speaker 1: when we get thirsty and go get water. Um. So
Speaker 1: there there are sort of system wide homeostatic responses when
Speaker 1: when we get dehydrated again, the s f O and
Speaker 1: the O v l T together with another region in
Speaker 1: between them, the media and pre optic nucleus or m
Speaker 1: n p O. These three regions together comprise a hub
Speaker 1: called the lamina terminalys. This is sort of the brain's
Speaker 1: fluid control center. What is fairly well understood is the
Speaker 1: the autonomic and neuroendocrine pathways by which the body responds
Speaker 1: to dehydration. You know, all these things we've been talking about,
Speaker 1: the the angiotenson to the constriction of blood vessels to
Speaker 1: increase blood pressure, water reuptake by the kidneys, the release
Speaker 1: of other hormones, and not just angiotenson, but things like
Speaker 1: vasa pressin and oxytocin. But what is less well understood
Speaker 1: are the mechanisms leading to the generation of thirst as
Speaker 1: a motivation state. But we know some things that are
Speaker 1: very interesting. So to come back to something you actually
Speaker 1: mentioned earlier in the episode, Rob, the brains regulation of
Speaker 1: water drinking is not based only on the current osmolality
Speaker 1: of the blood. So it's not just these brain regions
Speaker 1: that you know. Dip a finger in and see how salty.
Speaker 1: The soup is the brain also appears to change our
Speaker 1: motivation to drink water before changes actually show up in
Speaker 1: the blood. Their behavior changes that occur in anticipation of
Speaker 1: changes in osmolality. So what would this mean in plain English? Okay,
Speaker 1: so you're out in the hot sun wrestling alligators, or
Speaker 1: you know, you're working up a sweat, whate it is
Speaker 1: you're doing, and you come in, you get in the shade,
Speaker 1: and you drink a nice glass of water. When you
Speaker 1: do that, there is actually a delay on the order
Speaker 1: of tens of minutes. It might be ten, twenty or
Speaker 1: even more minutes before the water that you just drank
Speaker 1: is fully absorbed by the digestive system and added into
Speaker 1: the blood. However, it might just take you a few
Speaker 1: seconds to drink a glass of water and then decide
Speaker 1: whether you're going to drink more. So, if it's taking
Speaker 1: your body tens of minutes to fully incorporate the water
Speaker 1: you've just consumed and for that to you know, show
Speaker 1: up in a blood as molality test, how come you
Speaker 1: don't just keep drinking water constantly until that happens. You know,
Speaker 1: you don't, even if you're thirsty, you don't usually drink
Speaker 1: a glass of water and then just fill up another one,
Speaker 1: and another and another and another for you know, fifteen
Speaker 1: minutes or something. If your level of thirst were only
Speaker 1: based on your blood oz molality, you might do that.
Speaker 1: You might kill yourself just drinking gallons of water while
Speaker 1: you're waiting for your fluid monitoring system to register the changes. Right, yeah, yeah,
Speaker 1: I mean, I mean it needs to be again. It
Speaker 1: comes back to just what a fine balance it is.
Speaker 1: And uh and and therefore you need you need different
Speaker 1: sensors in different places in the human mechanism here, right,
Speaker 1: So the sensation of your thirst being quenched by drinking
Speaker 1: water must be created by a different process altogether the
Speaker 1: author's right quote. Thirst is not quenched by the reverse
Speaker 1: of the process that generates it. Instead, the brain appears
Speaker 1: to somehow detect the intake of liquid. It's usually thought
Speaker 1: that this happens somewhere in the orofaranks in the middle
Speaker 1: part of the throat, and then somehow adjusts the feeling
Speaker 1: of thirst in anticipation of the coming changes in blood
Speaker 1: os molality. So it looks like what happens is that
Speaker 1: your hydrated you drink water. The swallowing of water is
Speaker 1: somehow sensed in the throat, and then these sensations are
Speaker 1: transmitted to the sub formical organ, the SFO, and then
Speaker 1: from there they inhibit thirst generating pathways. And so this
Speaker 1: raises interesting questions how exactly do we sense water intake
Speaker 1: in the throat? This was not well understood at the
Speaker 1: time this paper was written. I actually have come across
Speaker 1: a couple of studies in the years since that we
Speaker 1: can talk about as we go on. We might get
Speaker 1: more into those in the next part of the series.
Speaker 1: Um But at the time, some of the ideas out
Speaker 1: there were, well, maybe it has something to do with temperature.
Speaker 1: Apparently cooling of the throat triggers water intake signals, and
Speaker 1: evidence for this claim would be that research has found
Speaker 1: that cold liquids inhibit thirst faster than warm liquids. I
Speaker 1: haven't tried it myself, but that sounds correct to me.
Speaker 1: Interesting and it would make sense water usually tends to
Speaker 1: cool the mouth in the throat, and that this cooling
Speaker 1: may be used as a rough signal that water is
Speaker 1: coming in. Also, it seems that cooling of the mouth
Speaker 1: on its own, even if it's not water, just making
Speaker 1: the mouth colder can somehow reduce thirst and reduce activity
Speaker 1: of the SFO. Interesting, so you would potentially be able
Speaker 1: to use just like a cooling breath UH technique to
Speaker 1: to to inhibit thirst possibly if you know, if the
Speaker 1: temperature explanation has anything going for it. But there may
Speaker 1: be other things as well. There may be other ways
Speaker 1: of sensing water in the oral cavity, maybe some stuff
Speaker 1: more along the lines of what you were talking about earlier.
Speaker 1: Things could have to do with UH taste sensors that
Speaker 1: that somehow to detect the presence of water through acid
Speaker 1: sensing taste receptors or something, or there could be there
Speaker 1: may be some limited evidence that stretch receptors and osma
Speaker 1: sensors in the stomach might also detect water intake before
Speaker 1: full absorption. But again, at the time of this summary
Speaker 1: in this was just not fully understood. I guess one
Speaker 1: way of looking at it would be that it's it's
Speaker 1: not necessarily like one trigger or one sensor that is
Speaker 1: that is playing a crucial role here. It's more of
Speaker 1: a whole suite of things, that is UM that is
Speaker 1: generating this UM this this understanding of how much water
Speaker 1: has entered the system. Right, But whatever the exact mechanism is.
Speaker 1: It's definitely anticipatory in nature. It's definitely changing your behavior
Speaker 1: before the thing that your behavior is supposed to fix
Speaker 1: has actually changed yet. Wow, Like when you're done drinking
Speaker 1: the glass of water, you're still dehydrated. You're going to
Speaker 1: be dehydrated for another ten to fifteen minutes at least.
Speaker 1: It's it's the kind of excellence in supply chain management
Speaker 1: that technology companies are chasing after. You know, the idea
Speaker 1: that that they want to anticipate the need um and
Speaker 1: and then you know, be able to uh to to
Speaker 1: alter the supply chain um uh, you know, at moment
Speaker 1: to moment to make sure that the need is met
Speaker 1: without wasting water or product or whatever the case may be. Yeah,
Speaker 1: I like it. Okay, there's more anticipation to come. How
Speaker 1: about anticipatory regulation of thirst from eating from food? Because
Speaker 1: when we eat food, thirst appears to be generated in
Speaker 1: anticipation of coming changes to blood osmalality. So why would
Speaker 1: eating food make us need water? Well, first of all,
Speaker 1: fluids are used in digestion. You think about when you
Speaker 1: eat food, you generate saliva you uh, you know, not
Speaker 1: to be gross, but there's a lot of lubrication that
Speaker 1: needs to happen like swallowing requires some water. Oh, I
Speaker 1: mean you have to. You have to to to chew
Speaker 1: up the food and form the bolus that will then
Speaker 1: travel down the throat. I mean, I think we've we've
Speaker 1: covered that on the show before, Like, think about it today.
Speaker 1: Your next meal really focused on everything that's going on
Speaker 1: in your mouth. It's a it's a beautiful chorus of
Speaker 1: disassembly and then rhasson into the right sort of package
Speaker 1: to then make the journey down to the stomach. It's
Speaker 1: really it's like it's beautiful and it's horrible. You can't
Speaker 1: think about it while you're eating, or at least I
Speaker 1: find I can't just think. I mean, have you ever
Speaker 1: thought of more appetizing idea that you're like mouth and
Speaker 1: throat are lubricating the ball of food that you're smashing up.
Speaker 1: But anyway, so that's one thing. But then the other
Speaker 1: half of it is that eating usually increases blood osmalality
Speaker 1: by adding salt and other things to the body. Yes,
Speaker 1: so much salt. So much so the body appears to
Speaker 1: have an anticipatory response to eating that is generating thirst
Speaker 1: before those changes even register. And this thirst that comes
Speaker 1: from eating is known as pran deal thirst. It seems
Speaker 1: to occur before changes in blood osmalality come into effect,
Speaker 1: so UH a lot of animals are observed to drink
Speaker 1: at the same time that they eat, if they're able.
Speaker 1: The mechanism of this anticipatory new real pathways still not
Speaker 1: fully understood, but if pran deal thirst is not sated
Speaker 1: I mentioned this earlier, sometimes animal brains tend to react
Speaker 1: by reducing appetite until water is consumed. UH. This is
Speaker 1: known as dehydration induced stand orexia, but in general dehydration
Speaker 1: will cause animals to restrict their food intake, with some exceptions,
Speaker 1: of course, because some animals, you know, they get their
Speaker 1: water entirely from food and so forth. But dehydration induced
Speaker 1: stand orexia tends to I thought this was interesting reduce
Speaker 1: meal size but not meal number, so UH it might
Speaker 1: not affect how often an animal is willing to eat,
Speaker 1: but how much they eat when they do eat, meaning
Speaker 1: that it probably works by causing an animal to terminate
Speaker 1: feeding behaviors earlier than they would normally each time it
Speaker 1: it has a meal now at the at the end
Speaker 1: of their right up here, the authors acknowledge that UH
Speaker 1: acknowledge some exceptions to the stuff they're talking about. There.
Speaker 1: These generalizations tend to be true for humans and rodents
Speaker 1: and some other animals, but of course they're very different
Speaker 1: ecological niches that will cause variation to these generalizations. For example,
Speaker 1: a lot of grazing herbivores do not seem to experience
Speaker 1: pran deal thirst or or thirst related to eating, you know,
Speaker 1: because they eat all day, but they only drink drink
Speaker 1: water a few times a day. And then they're very
Speaker 1: different kinds of animals like amphibians that don't technically drink
Speaker 1: at all, like many amphibians just absorb water through their skin.
Speaker 1: We've talked about amphibians before in the show and about
Speaker 1: how the you know, they're how delicate their place in
Speaker 1: the environment can be. And this is this is part
Speaker 1: of it. It's like if if I'm in the environment,
Speaker 1: i am drinking it, I am breathing it. Uh, and
Speaker 1: my skin is the is the barrier through which all
Speaker 1: this takes place. Yeah, very different relationship to one's environment,
Speaker 1: very very different kind of chemist of being for the
Speaker 1: for the amphibian, I recall it's been a while, but
Speaker 1: I recall reading a funny article about terrestrial toads that
Speaker 1: would primarily do water absorption through their pelvic regions, so
Speaker 1: they sort of like like thrust their lower bodies and
Speaker 1: bellies into any surface that's wet while they're while they're
Speaker 1: hanging out in water absorption mode. All right, we're gonna
Speaker 1: go ahead and close out this episode, but we will
Speaker 1: be back for a third thirst episode. We have a
Speaker 1: lot of additional information we want to get to. Don't worry,
Speaker 1: there will be parasites in it. Uh. So it should
Speaker 1: be uh, it should be a grotesque a good time.
Speaker 1: And of course in the meantime, right in, let us
Speaker 1: know your thoughts on water. And all of you are
Speaker 1: water drinkers. You consume water one way or another or another.
Speaker 1: And I imagine you have some thoughts on all of this.
Speaker 1: Do you do you love drinking water? Do you hate
Speaker 1: drinking water? Um? Is there? What are your thoughts on
Speaker 1: the consumption of cold water versus hot water? Um or
Speaker 1: maybe prefer the lukewarm wa water. I mean, I guess
Speaker 1: they're factors we didn't even get into. You know, like
Speaker 1: sometimes cold water can be a sensitive on the teeth
Speaker 1: and gums. Uh, the same can be said for for
Speaker 1: hot water as well, So I don't know, perhaps you
Speaker 1: have thoughts on that. Have you never drunk water? Have you?
Speaker 1: Are you a creature that only absorbs water from chicken nuggets? Oh,
Speaker 1: the chicken chicken nugget water? Yes? Well, anyway, right, and
Speaker 1: let us know it's all fair game. In the meantime,
Speaker 1: as usual core episodes of Stuff to Blow Your Mind
Speaker 1: Tuesdays and Thursdays and the Stuff to Blow your Mind
Speaker 1: podcast feed We have a listener mail on Monday, Artifact
Speaker 1: on Wednesday, and on Friday we do Weird how Cinement
Speaker 1: at our time, just that most serious matters aside and
Speaker 1: just talk about a strange film. Huge thanks as always
Speaker 1: to our excellent audio producer Seth Nicholas Johnson. If you
Speaker 1: would like to get in touch with us with feedback
Speaker 1: on this episode or any other, to suggest a topic
Speaker 1: for the future, just to say hello, you can email
Speaker 1: us at contact at stuff to Blow your Mind dot com.
Speaker 1: Stuff to Blow Your Mind is production of I Heart Radio.
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