Salt draws water out of cells by the process of osmosis. Osmosis is the process by which a liquid moves through a semipermeable membrane from a solution of low concentration to a solution of high concentration. If you rub salt on meat, for example, the salt outside the meat has more ions that the water in the cells inside the meat. Nature likes balance, so the water inside the meat will flow to the salt until the number of ions is balanced. This is why salt draws out water. Sugar has the same effect. If you put a raisin in water, it will swell up because the water flows into the raisin.
If we spend a long time in the sea, nothing will happen because our skin is waterproof. However, if we drink saltwater, it can be very damaging. Seawater is 3.5% salt by weight. The saltwater goes into your stomach and your intestines, and the high salt content draws the water out of the cells that come into contact with it through osmosis. This continues until the saltwater is diluted and the two solutions are equal. Obviously, having the water drawn out of your cells is not good. They collapse and die. If you are stranded in a boat, drinking seawater will kill you faster than not drinking anything. This is the reason why eating salty food makes you thirsty. It is drawing the water out of your cells and your brain is telling you to drink water to fix the balance. If you cannot replenish the water, you will die.
Our kidneys are also not able to filter out the salt. Too much salt raises the levels of sodium in our blood and it raises blood pressure. Our kidneys have evolved to use a delicate balance of sodium and potassium to pull the water out of the blood. Too much sodium messes this up.
So, why can fish drink saltwater and not die? The reason is that they drink far more than we do, and their kidneys can purge the excess salt from their bodies.
Saltwater has the exact same effect on fish as it does on us. The salt draws the water out of the fish’s cells and starts to dehydrate the fish. The fish combat this by constantly taking in water through their gills. They are then able to desalinate the water so that it can replenish the water that is being drawn out of their cells. There are two parts of their body that help with the desalinization. The first are special cells in their gills that pump excess salt out of the water. The second are their kidneys which filter the salt out of their bloodstream. They then urinate this salt back into the sea. This means that saltwater fish drink and urinate almost constantly.
If you were to put a saltwater fish in freshwater, they would accumulate so much water in their cells that they would bloat up and die. They have evolved a balance where they replace the water as it is drawn out of them by the salt.
There are some fish that can live in saltwater and freshwater. These fish are called euryhaline and they can adapt themselves to different environments. Salmon are a good example. Salmon are born in freshwater, they then spend most of their lives in the ocean, before returning to freshwater to lay their eggs. They have to be able to survive in both types of water. They do this in two ways. Firstly, they change their behavior. When in saltwater, they have to drink a lot to stay hydrated. When in freshwater, the salmon change their behavior and actually drink less. The second thing are two proteins in their gills. They have active transport and passive transport proteins. All saltwater fish have these to move salt out of their gills, but salmon can use energy to reverse the process. This lets them talk salts in from the water when they are in freshwater and flush it out when they are in saltwater. Because these changes take a little while to bring about, salmon usually have a few days in an area that is halfway between the ocean and the freshwater so that they can adapt. Incredible.
So, saltwater fish can drink saltwater because they constantly drink to replenish the lost fluid and they urinate out all of the salt that their kidneys remove. We cannot do this, so saltwater will pull out the liquid from our cells and kill us unless we can get a drink of freshwater. And this is what I learned today.
Photo by Jiří Mikoláš: https://www.pexels.com/photo/tropical-clownfish-near-seaweed-in-aquarium-4593110/
Sources:
https://animals.mom.com/how-do-saltwater-fish-deal-with-osmosis-12201571.html
https://www.amnh.org/exhibitions/water-h2o-life/life-in-water/surviving-in-salt-water
https://www.quora.com/Why-does-salt-dry-out-cells
https://www.sun.ac.za/english/Lists/news/DispForm.aspx?ID=7172
https://en.wikipedia.org/wiki/Euryhaline