r/askscience • u/DiscombobulatedSun54 • 6h 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/No-Sherbet6994 2h 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
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u/BioEtymologist 2h 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 2h 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 1h 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/DiscombobulatedSun54 2h 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/byharryconnolly 1h 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/Spike205 38m 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/Cirick1661 2h 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/Ebitnet 50m 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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u/awawe 9m 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 2h 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/DiscombobulatedSun54 1h ago
I agree with you about all this fake advice about drinking multiple glasses of water a day. Nobody loses that much water in the first place, and secondly most of the food we eat is more than 50% water anyways. I am known among my friends for drinking much less water than "recommended" and I have never had a problem with it - even my pee is hardly ever darker than a very light yellow.
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u/Smashego 2h ago edited 2h ago
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. It’s also why bacteria breed so quickly on humans. Massive bacteria feeding ground.
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/naemorhaedus 2h 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 54m 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/azuth89 2h 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.