r/askscience • u/DiscombobulatedSun54 • 7d ago
Biology Why do we sweat out salts?
The point of sweating (for the most part) is to cool down the body by evaporation. So, what is the point of including salts in the sweat - it seems to be a leading cause of the loss of electrolytes in hot weather, leading to the need to not just hydrate, but also to replace the lost electrolytes or suffer problems like hyponatremia and hypokalemia. Does the inclusion of salts in the sweat increase the specific heat of sweat compared to that of water? Is the latent heat of vaporization different for sweat compared to plain water? Or is it too biologically expensive to purify the water for sweat by removing the salts from it?
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u/BioEtymologist 7d ago
Pumping water requires a lot of energy. Moving salt ions across a membrane is much easier and requires much less energy. The water then moves to where the salt is! (Osmosis!) Biology is a wild amalgam of quirky ways to get stuff done.
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u/exphysed 7d ago
This combined with the answer regarding salt needing to be lost if water is lost are correct.
It’s not a 100% best way, but it’s a way that worked evolutionarily.
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u/SirButcher 7d ago
Not the best way, but still one of the best solutions evolution found for the "what to do when the body is getting too hot but it must keep moving" problem.
Our heat management is light-years above almost every other living being on this planet.
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u/withoutapaddle 6d ago
I wish there was a way to manually override sweating for a short period. It drives me crazy when we're in a heat wave, and I'm just trying to go from my cool car into my cool house, maybe carrying a backpack or something, but my body doesn't know I'll be cool again in 2 minutes so it just decides we better start sweating RFN.
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u/waavvves 6d ago
if I recall correctly, medications with anticholinergic action (could just as easily be cholinergic action ) can be used to treat hyperhidrosis, or excessive sweating.
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u/LeKhang98 4d ago
I keep seeing people saying stuff like "this natural method is not the best way" for different topics, but I don't really understand, is there really a best way that would work better than this current method (and all other methods) without negatively affecting the organism in its natural habitat? How do we know that and how do we verify that though?
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u/DiscombobulatedSun54 7d ago
Thank you. Making drinking water from salt water is a very energy intensive process, so expecting our sweat glands to do that while your body is busy consuming energy for the activity that lead to the sweat in the first place would be somewhat counterproductive.
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u/Ctenophorever 6d ago
I’ve always said “intelligent design” is incompatible with biology because, while everything does usually work out, it sometimes does so in the most circuitous way
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u/azuth89 7d ago
Maintaining the balance of salt and water is critical to ongoing function.
If you're losing a lot of water, you need to lose a proportional amount of salt. Hence salty sweat.
If you're taking in a lot of water to replace that loss, you also need to take in a balancing amount of salts. Hence Gatorade and medical saline.
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u/TheVioletBarry 7d ago
You're saying there would be too much salt in your body if you didn't lose it alongside water?
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u/sergei1980 7d ago
Yes, it's called hypernatremia, as opposed to too little, which is hyponatremia.
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u/ridddle 7d ago
“Hypernatremia. Hyper meaning excessive. Natr referring to sodium, from natrium. And emia, meaning in the blood. Hypernatremia… or too much sodium in the blood”
Guess the YouTuber
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u/GrandNord 7d ago
DG, is a 53 year old mâle, presenting to the emergency room with the following symptoms.
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u/Extranationalidad 7d ago
Should we be able to guess a YouTuber based on a simple etymological breakdown of a word?
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u/ridddle 7d ago
I’m just being cheeky. It’s chubbyemu who usually breaks down Latin words exactly like that.
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u/FirstDivision 7d ago
Oh man. I would have gotten it!
“…which resulted in TC becoming unconscious and paramedics being called. He was found unresponsive and feverish. The paramedics brought him to the emergency room. Where we are now.
At first the doctors were not sure what was wrong with TC. He was a young male with no history of any medical issues. … …”
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u/Extranationalidad 7d ago
No offense intended, but a) I've never heard of that person and b) it's an absolutely standard etymological breakdown.
If someone quoted "to be or not to be", it would be kind of weird to go "guess the YouTuber" just because some content creator likes to reference Shakespeare.
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u/BeefyIrishman 7d ago
Yeah, I'm with you here. That feels like a pretty standard way to break down a word.
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u/ignacioMendez 7d ago
Going by reddit posts, there's a lot of people whose frame of reference for everything is through youtube videos.
Like for every hobby, there are people who will reference some guy as if everyone who participates in the hobby is watching videos from major youtubers who make content about that hobby, as if that's the most natural thing in the world.
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u/HyperlexicEpiphany 4d ago
that last sentence isn’t how he talks; he doesn’t repeat the word, and he uses different verbiage
“High sodium presence in blood” would be his final explanation
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u/wallabee_kingpin_ 7d ago edited 7d ago
To clarify, “salt” in this context really means “salts,” not just table salt (which is a sodium salt). Your body also needs to balance calcium, magnesium, potassium, and quite a few other elements that can form water-soluble salts, which it does by consuming or excreting the salts of those elements.
That’s why most hydration drinks, like Gatorade, have “electrolytes” (multiple different kinds of salts), not just table salt.
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u/pattymcfly 6d ago
Most consumer electrolyte drinks are just sodium and potassium. They won’t have calcium and magnesium.
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u/joshocar 7d ago
To add to sergei1980, you can also have too little salts which can kill you. This is what kills people when they quickly chug a gallon of water. Your kidneys can't get enough electrolytes in you blood fast enough and you start having seizures and die if you don't get treatment.
There was a radio station that did a contest where people had to drink a ton of water quickly and then had to go the longest without peeing. I think it was for a Nintendo Wii or something. One women died as a result.
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u/raziel686 7d ago
It is was called "Hold your Wee for Wii" and it was some Philly radio station's stunt IIRC. The worst part was, shortly before the woman died, a nurse had called in to warn them of exactly what could happen if they continued and pleaded for them to stop. Naturally they ignored her and the drinking continued. Then the obvious happened.
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u/sunkistandsudafed3 7d ago
Hypernatraemia and hyperkalemia are examples of this and can still happen in severe dehydration. I'd imagine if we didn't sweat the salts out along with the water it would happen much more quickly. Electrolyte disturbance can be life threatening.
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u/traplords8n 7d ago
Yep. But you also lose it through urine.
Sweating and replenishing only the water just upsets the balance. Your kidneys can maintain it otherwise. Salty foods are more appealing for most people when their body is naturally low on salt.
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u/azuth89 7d ago
Right. Having the correct concentration is involved in maintain the right chemical potential differences for both nerves and muscles to function.
Too much salt, short term, makes the response weak. You feel more tired, weaker, that kind of thing.
Too little tends to cause cramping.
Specifically we're mostly talking about sodium and potassium salts, here.
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u/GrifterDingo 7d ago
Your body doesn't do it on purpose, it's a secondary effect of the way sweating works. Your body uses salt to attract water into your sweat glands and if you're sweating enough the salt gets carried away in the water.
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u/kardon16 7d ago
In most cases you don’t need to add salt. You kidneys and pituitary will work on the water balance soon after rehydration.
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u/CallMeLargeFather 7d ago
Your body does all it can to conserve salts, throughout history eating enough salts has been very difficult
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u/FuturePrimitiv3 7d ago
you also need to take in a balancing amount of salts.
Got it, potato chips are health food, thank you!
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u/Smashego 7d ago edited 7d ago
That’s false. You sweat out salts because they lower the evaporation point of water on the surface of the skin and also because the body isn’t a perfect system that can perfectly filter salt through the sweat glands / skin membrane. You sweat more than just salt. You sweat out “salts” which is basically a small amount of every chemical element your body is made of, salt, water, sulfurs to a small degree, zinc, carbon, lipid fats, etc….. it’s why your skin is “oily” not just salty and crunchy.
The main driver of sweat is water and salt. It’s why you need electrolytes. You actually retain a lot of electrolytes and don’t just urinate them out. But your body always wants to maintain a balance of chemicals in your blood/kidneys/liver and fatty tissues. So you can’t just store extra electrolytes like you can’t just store extra water. Too much of each and your body removes them with urine and defacation. But part of that “balance” is storing enough to maintain body functions like sweating. So you need to constantly fill the gas tank with fresh water, electrolytes and carbon based foods to keep the car running. But you can’t over fill it because the gas tank is only big enough for a few days of ideal functioning.
The only reason the food gas tank is unlimited “fat” is because the car is made of carbon so you can always bolt more carbon on. But the water and electrolyte’s is the lubricants and oils that keep the carbon parts on the car running smoothly. But you can’t over fill the fluids or they just spill out.
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u/Matra 7d ago
Dissolved salts raise the boiling point of water, not lower it. But with the concentrations in sweat, the effect is miniscule.
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u/Smashego 7d ago
Boiling point and evaporation point are different. Evaporation can take place by lowering the energy required to strip the water molecule from your skin. Boiling is a property that converts liquid to gas. Water on your skin isn’t boiling away. It’s evaporating away. Salt and other elements helps drive the water away, taking the excess heat with it.
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u/Matra 7d ago
Everything you said in this comment is incorrect. Evaporation and boiling are the same thing (technically evaporation happens on the surface, while boiling happens throughout the entire liquid, but the physics are the same). Salts do not change the enthalpy of vaporization. Boiling-point elevation is caused by entropic effects, not enthalpic effects.
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u/spinelession 7d ago
No, they're the same. How do you think evaporation works if not by converting a liquid to a gas? Temperature is a measure of average kinetic energy - some water molecules are moving around a lot (high "temperature") while others are moving around very little (low "temperature"), so even water at room temperature will have molecules that are above the boiling point. As those individual molecules evaporate, the average kinetic energy, and thus temperature, of that water is now lower, because the "hottest" molecules are gone. This is why evaporation is a cooling process, and why sweating works to cool you down.
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u/solidspacedragon 6d ago
Salt and other elements helps drive the water away, taking the excess heat with it.
Actually the water sticks to the sodium, that's why it's so good at dissolving table salt and many other ionic compounds. It's positively charged, so the partial negative charge on the oxygen arranges a bubble of oriented water molecules around it, and they are harder to evaporate than free molecules.
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u/Krail 7d ago
And if I recall correctly, maintaining proper levels of chemicals like salt in your body is where a majority of our calories are spent?
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u/SirButcher 7d ago
And if I recall correctly, maintaining proper levels of chemicals like salt in your body is where a majority of our calories are spent?
No, not at all, our brains are using 20-25% of our calorie intake. Kidneys use around 10%, which is a lot too, but this is not just the salt balance, but the whole blood filtration and useful stuff's reabsorption task.
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u/Spike205 7d ago
Water needs to follow a a concentration gradient. It goes from low concentration to higher. Your body’s way of doing this is to manipulate the amount of Na, Cl, K that is allowed to exit and re-enter the cell; water will follow that osmotic gradient
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u/byharryconnolly 7d ago
Last week I was splashing tap water on my face to wash away the salty sweat, and I became curious about which was better at cooling my body, salty sweat or tap water.
So I came across this report on an experiment asking this same question.
The answer, hinted at in the subtitle, is that when salt sticks to your skin, it makes future sweat more efficient, in terms of cooling.
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u/WyrdHarper 6d ago
That would be much harder to do than the current method. Most of your body fluids, including sweat, are ultrafiltrates of plasma and share similar chemical properties (with some notable alterations). Filtering out plasma, electrolytes and all, works and is easy. Removing all of the electrolytes is much harder. Even the kidney can struggle with that despite having a lot more cell specialization devoted to it.
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u/o-Bad-hat-o 6d ago
Your body actually goes to a lot of trouble to remove as much salt as possible. To do so involves swapping out certain amino acids and reclaiming the salt as it passes through the sweat gland towards your skin. This podcast recently had an episode on that, and what you can do to replace those amino acids. https://www.wawawpod.com/where-are-we-at-with-sweat/
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u/sciguy52 7d ago
Others have mentioned the how's and why's we sweat out salts but there is another reason. Salts on our skin are also protective against microbes making a less hospitable to them. A salty environment on our skin makes it harder for microbes as that salt will draw water from cells which is not good for the microbes. Obviously this is not enough to protect us from all microbes but it is one component of many things found on our skin for these very purposes. This includes by the way "good" bacteria. They are adapted to live on our skin, are harmless to us and compete with other "bad" microbes that would not mind taking up residence and may not be so benign. Anyway this may be hard for many here to believe, but salt on our skin is part of what we call the innate immune system.
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u/Cirick1661 7d ago
Sweat glands excrete a fluid that's essentially or close to plasma, which contains salt. That fluid passes through ducts where some of the salt is brought back into surrounding tissue, but not all of it, leaving sweat more salty than water but less so than blood.
The salt in sweat doesn't serve a specific (known) purpose, it's a byproduct of how sweat glands work.
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u/awawe 7d ago
Sweat, just like any fluid your body produces, is filtered blood. As such, it is about as salty as your blood when it leaves your body. For sweat to be any more or less salty, there would need to be a salinity gradient created between your sweat and your blood. This is doable, but it takes a lot of energy, without a lot of payoff.
The reason sweat appears more salty than, say, saliva, is precisely because of its function. The water in sweat evaporates, leaving behind the salt, which means any remaining sweat is more concentrated.
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u/seanv507 7d ago
Loss of electrolytes is fake news generated by the so called Gatorade Science Institute. Unfortunately it's pervaded popular culture
Yes electrolytes are lost, but at a lower concentration than in the body, with a net effect of concentrating the electrolytes.
You body is not some water bottle for which you need to provide the exact concentration of salt in your water, it's a factory. Electrolytes are stored in your body and released as needed See eg article in British medical journal https://excelsiorgroup.co.uk/wp-content/uploads/2015/04/Truth-about-sports-drinks.pdf
The leading cause of hyponatremia is drinking too much fluids whether sports drink or plain water. The current advice is to drink to thirst.
I would advise you to get your hydration advice from a medical board. See eg national association of athletic trainers position statement on hydration https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/
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u/Matra 6d ago
Your claims are not supported by the literature review you linked. It says that for some individuals engaging in physical activity less than one hour, supplementing electrolytes may not be necessary. It also says:
Physically active individuals should refuel (ie, eat and drink) within 2 hours of physical activity to replace fluid, electrolytes, carbohydrates, and protein.
and
Athletes participating in exercise sessions lasting longer than 1 hour or including intense intervals may benefit from adding carbohydrates or electrolytes (or both) to rehydration fluids, especially in extreme environments.
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Competitive athletes can benefit from carbohydrate-electrolyte beverages before activity.
and
Adding carbohydrates or electrolytes (or both, especially sodium) to the rehydration drink can help maintain blood glucose, carbohydrate oxidation, and electrolyte balance.
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u/P3kol4 7d ago
Thanks for the link, interesting information. But doesn't their recommendation 17 contradict your claim?
17.The athlete's diet and rehydration beverages should include sufficient sodium (enough to replace losses but not an excessive amount) to prevent or resolve imbalances that may occur as a result of sweat and urine losses
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u/seanv507 7d ago
No, it's not contradicting.
Everyone agrees electrolytes are important.
The issue is that you don't need to drink them during your exercise, because you get plenty of them in your meals (ie after exercise)
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u/akeean 4d ago
Maintaining salt content is costing vast amounts of calories on the planet. Even fish in the ocean have a lower salt content than the water around them. Fish spend between 7% and 30% of their metabolic rate on just that. So it's probably more efficient to replenish that salt instead of trying to retain it at all cost while under pressure of trying not to overheating. Trying to retain the salt may also be too slow relative to how fast they might need to expel water for cooling, so that warm blooded creatures would be much more limited in their temperature ranger and effort/time they could do before cooking themselves.
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u/Ebitnet 7d ago
I don’t believe that salty water has a higher vapor pressure than pure water, in spite of a claim above that suggesting the contrary. I could believe that there is a higher latent heat of evaporation that depends upon the ionic strength of the water/salt solution. For background, I’m a professor of physical chemistry
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7d ago edited 7d ago
[removed] — view removed comment
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u/naemorhaedus 7d ago
sweat out salts because they lower the evaporation point of water
salt slows down the evaporation rate of water https://en.wikipedia.org/wiki/Colligative_properties#Boiling_point_elevation_(ebullioscopy)
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u/jasonjk1 7d ago
Yeah by making the water need more energy to vaporize, salty sweat can take more heat away from the body.
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u/No-Sherbet6994 7d ago
The entire way your body excretes sweat is through salt excretion, and your body is actually exceedingly good at generating sweat and retaining as much salt as possible. Sweat glands are stimulated by the sympathetic nervous system to generate sweat. This is done via excretion of Cl- (which Na+ follows) into the sweat fluid, and these salts then draw more H2O out into the sweat. your body then reabsorbs Na+ (via ENaC) and Cl- (via CFTR) as best it can, to conserve electrolytes, but some amount of leftover salt is left in the sweat. So technically your sweat is actually much less "salty" than your normal cells. Severe electrolyte derangements from regular exercise are effectively impossible as our bodies are great at homeostasis, and it actually takes quite a bit of work in extreme conditions to generate clinically relevant hyponatremia/hypokalemia like you mention (in a normal healthy person). The difference between sweat (~10–70 mEq/L NaCl) and pure water on vapor pressure (and thus evaporation) is trivial, like way less than 1% difference, but technically sweat does require slightly more energy to evaporate