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Why can't humans drink sea water?

Why can't humans drink sea water?


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Why can't humans drink sea water?

It would seem to be a huge evolutionary advantage for an animal to be able to drink sea water rather than have to rely on fresh water, and it's provably not impossible due to the existence of sea mammals that must filter the sea water somehow.

Could someone explain to me any likely reasons that this is the case?


For humans, our blood and other tissues have a salinity which is less than that of sea water. The osmolarity of blood is 275-299mOsm/L, while the osmolarity of seawater is ~1000 mOsm/L.

The major reason we need to drink and excrete (sweat, breath and urine) water is to rid the system of waste- i.e. water soluble chemicals or heat we do not need. The kidneys do a lot of this work and they need a reasonable osmotic potential to help them.

The osmotic potential is the tendency of molecules, for instance salts, to diffuse from one reservoir of liquid to another. The kidneys burn metabolic energy (food) to create a slightly higher concentration of waste chemicals into the bladder/urine. Other organs will fail or also work to maintain a local salt concentration (e.g. any nerve cell which depends on Na+/K+ concentrations to transmit their signals. But the kidneys are the gatekeepers of salinity in the body. They work hardest (and can get injured) when there is more salt in the water we drink. If you drink sea water you will quickly find your kidneys overworked and not really functioning.

That's why salt water will tend to make you thirsty (if you can drink any at all). Most people will gag when they drink even a little sea water, much less a glassful.

Sea mammals actually eat their fresh water. They rely on the fluid they get from fish to keep their salt balance. Its true that some animals like fish and molluscs etc have adapted other means of keeping their salt balance in check. Its just also true that its a lot of work to keep this ability and when animals move away from the oceans, they quickly lose that ability.


Why shouldn’t people drink seawater?

Actually, the concentration of salt in the human body is about a quarter of what it is in sea water. Drinking a lot of seawater will raise the concentration of salt in your blood and can make you more dehydrated than drinking nothing at all. Let’s say you go down to the beach, fill up a big glass with sea water, and drink it down. The salty water goes into your stomach. Then it enters your bloodstream – and increases the salt concentration in your blood. The job of your kidneys is to filter your blood – to get rid of toxins and wastes, including too much salt – and to expel it through your urine. But human kidneys have to excrete a certain amount of water when they expel wastes.

That’s why you can end up more dehydrated if you drink seawater. Your kidneys have to get rid of that extra load of salt – and that takes extra water. So if you’re stranded at sea it’s better to hope for rain than to drink seawater. Or catch and eat a fish – that’ll give you a less salty source of fluids.


Why Can't Humans Drink Salt Water?

The problem with drinking seawater is that for land mammals like us, drinking salt water actually causes dehydration. There are a few land mammals that are known to occassionally drink seawater, like otters and sea lions, but even marine mammals like whales and dolphins don't rely on water from the ocean to stay hydrated internally.

The issue, according to the US Geological Survey, is that in order to maintain a healthy balance of salt in our blood, any excess salt needs to be filtered out by our kidneys and converted into urine.

Since our body can't produce urine that is saltier than our blood, and typical ocean water has three times the salt of our blood, our kidneys would actually need to produce a greater volume of urine than the volume of water we took in by drinking seawater.

To get that water, the kidneys would pull freshwater from other available sources in our bodies, like our cells, and this can lead to dehydration and death in pretty short order.

A humpback whale straining krill &mdash also probably drinking seawater. We don't really know. | Source: NOAA/Flickr


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Why does drinking salt water make you thirsty?

Shouldn't the water hydrate you more than the salt dehydrates you?

The salt concentration is higher in the salt water than in your bodily fluids, and it'll wind up pulling more water away to get rid of the excess than youɽ get from drinking it. You can really only hydrate yourself with fluids with a lower concentration of salt than say what your bodily fluids have on average. This is why you hear stories of people surviving at sea while drinking nothing but sea turtle blood or coconut water, for example, but not by drinking sea water.

Salt water fish have developed means for adapting to this higher salt content, in that that they can match their body fluids to the surrounding salinity or passing excess salt through their gills, and they're constantly drinking to stay hydrated. The only means we have to get rid of excess salt is through urinating, sweating, or vomiting, but we aren't adapted to be able to handle that much salt and still remain hydrated. The whole gradient thing comes into play here, since the water in your cells will travel out to try and dilute the salt. The body will try and flush out this excess of salt, either by urinating it out, sweating it out, or throwing it back up again, but all of these things take a lot of water, which needs to be replenished. This makes you thirsty, or at least sets up the biological feedback mechanism for such. So, the entire point of having ingested the salt water was defeated -- and yes, you do get some hydration from the water, but not enough to offset the amount lost.


Don't drink seawater


Ocean Waves: Photo courtesy of Wikipedia.

When you are swimming in the ocean, you might want to be careful not to drink the water. First of all, that salty seawater doesn’t taste too good. But also, scientists just discovered that there are way more bacteria swimming in the ocean than they previously thought.

According to a study recently published in Proceedings of the National Academy of Sciences, one liter of seawater can be home to over 20,000 different species of bacteria. This is about 20 to 100 times greater than previous estimates. Scientists now estimate that there could be between five and ten million types of bacteria living in the ocean.

The discovery was made possible by a new technique known as “454 tag sequencing” that allows for the quick identification of organisms. This technique allows them to identify thousands of kinds of unusual bacteria, which may have gone unnoticed in other research.

Dr Mitchell Sogin, from the Marine Biological Laboratory's Josephine Bay Paul Center for Comparative and Molecular Biology and Evolution, told BBC News that this biodiversity discovery “really points to our lack of knowledge and how much more there is to learn." There really is much to learn about the world around us.

So, next time you're swimming at the beach and you accidently take a big gulp of seawater, just think of the wide diversity of microorganisms you just swallowed.


Why don't we just boil seawater to get freshwater? I've wondered about this for years.

If you can't drink seawater because of the salt, why can't you just boil the water? And the salt would be left behind, right?

2 4

You can do this, and we do. It's call desalination. The process you describe is called distillation desalination, and historically was the only way to turn salt water into drinking water. However, this is getting less and less common these days. Now it is mainly done by "reverse osmosis" where pressure is applied to sea water to drive it through a special filter that separates the salt from the water.

The reason these technologies are not more widely used is because they are expensive. Obviously distillation desalination requires you to boil water, when we're talking gigalitres of water a year, this means a lot of electricity is needed. Reverse osmosis isn't cheap either. You have to pump the water to develop pressure, and the reverse osmosis membranes are always getting fouled and damaged. Roughly speaking, the highest efficiency desalination plants make water at about 10x the price of rain water collection. That is why desalination is somewhat rare (though more common than a lot of people think) and is only used in large amounts in very dry places. Australia, for instance, is extremely dependent on desalination for drinking water, and the large desalination plant in the world operates in Saudi Arabia.

EDIT: I'm having lots of complaints from Australian. If your city's backup supply of water is desalination, you are dependent on it. Australia has some of the highest desalination capacity per capita in the world. The are huge plants in three states. I never said they supply your daily drinking water.


Not all water sources are contaminated to a dangerous level

Many people believe that the water sources in the wild are all contaminated to a large degree, but they are not all that bad in reality. I mean, they may be contaminated to some extent, but they are not so full of germs that it will be a problem for anyone who drinks them, whether they are animals or even humans.

Moreover, most mammals have a finely developed sense of smell, which helps them determine to a certain extent whether a water body is bacteriologically active. When they encounter such a body, they often simply pass on the idea of drinking from it.

Not all water bodies are contaminated enough to kill you. (Photo Credit: pxhere.com)


Can You Make Seawater Drinkable?

As you may already known, human cannot drink seawater as an alternative to fresh water. Seawater is full of salt that typically dehydrates the human body.

But what if we could make seawater safer to drink? It turns out that we can and the process is called desalination. Desalination is a process that removes dissolved minerals (including but not limited to salt) from seawater, brackish water, or treated wastewater. A number of technologies have been developed for desalination, including reverse osmosis (RO), distillation, electrodialysis, and vacuum freezing. So why is seawater such an attractive water resource that we go through all those costly process to make it safe to drink?

Here are a few reasons. Seawater provides an unlimited, reliable water supply for coastal populations worldwide brackish water is a plentiful, relatively drought-proof water resource for inland populations and reduces dependency on imported water. And, of all the Earth's water, 97 percent is salt water, only 1 percent is fresh water available for humans to drink, and 2 percent is frozen. Of the more than 7,500 desalination plants in operation worldwide, 60% are located in the Middle East. The world's largest plant in Saudi Arabia produces 128 MGD of desalted water. In contrast, 12% of the world's capacity is produced in the Americas, with most of the plants located in the Caribbean and Florida.

To date, only a limited number of desalination plants have been built along the California coast, primarily because the cost of desalination is generally higher than the costs of other water supply alternatives available in California (e.g., water transfers and groundwater pumping). However, as drought conditions occur and concern over water availability increases, desalination projects are being proposed at numerous locations in the state. Desalination costs are decreasing as technology improves and more plants are built. Today there are more than 15,000 desalination plants in 120 countries. The desalination market is forecast to grow more than $70 billion in the next 20 years. About half of the world's desalted water is produced with heat to distill fresh water from seawater.

Bottle Water Contaminant

The distillation process mimics the natural water cycle in that salt water is heated, producing water vapor that is in turn condensed to form fresh water. One such treatment process is called Multi-Stage Flash Distillation. Another desalination treatment process with the most expanded use is membrane-based reverse osmosis. In this process, pressure is applied to the water, which allows water to flow through a membrane, leaving the ions, salts, and other dissolved solids and nonvolatile organics behind.

last paragraph here Read Next: Salt Content Levels -Sea vs Fresh Water


Salt dehydrates, so the more saltwater you drink, the more water your body loses. As you drink seawater, the water already present in your body is rerouted to help your body dilute the excess of salt, according to the Marine Knowledge website 3. As result, other bodily functions start to suffer because of the lack of water in your system. Dehydration causes an increase in thirst, but it also causes you to urinate more often, as your body is trying to get rid of the excess of salt 4. More urine and less water worsens the dehydration problem 4.

Does Drinking a Lot of Water Help You Lose Weight?

Drinking saltwater will cause a number of early side effects as you get dehydrated. Dry mouth and rapid heartbeat are followed by low blood pressure, headaches and dizziness. Lethargy and confusion will eventually start to set in. Depending on how much sea water you had to drink and whether you’re now drinking fresh water to eliminate the effects, you can also experience blood in the stool or vomit, loss of appetite and unconsciousness 3,

  • Drinking saltwater will cause a number of early side effects as you get dehydrated.
  • Depending on how much sea water you had to drink and whether you’re now drinking fresh water to eliminate the effects, you can also experience blood in the stool or vomit, loss of appetite and unconsciousness 3,


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