Do Fish Drink Water?


All living things on Earth need water, because
the chemical reactions that make life possible happen in an aqueous solution. Which means us land animals have to drink
it to keep it inside of us all the time. But… what about fish? It seems like since they’re always in the
aqueous solution of whatever body of water they’re swimming around in, they wouldn’t
need to actually drink the stuff. But it turns out that some fish actually do
need to drink. And others do not. It depends on the fish, and what kind of neighborhood
it lives in. Water, you should know first of all, is really
into finding a balance. If you have a membrane that water can pass
through, and there are different concentrations of salt on each side, then the water will
always flow toward the saltier side of the membrane, until the concentrations on both
sides are the same. That means that freshwater fish have it easy. Since the concentration of salt in their bodies
is higher than the concentration in the water, that fresh water just flows right in, mainly
through their gills, and into their bloodstream. They also swallow some water when they eat
— I mean, it’s kind of unavoidable — but they don’t need to actively gulp down water. But, in order to retain that balance of concentrations,
the fish’s tissues still need to have some salt in them. And that’s where chloride cells come in. These are special cells in the gills that
produce large amounts of an enzyme that controls the flow of dissolved salts — like sodium
and potassium — across cell membranes. So, in freshwater fish, these chloride cells
work hard to bring just the right amount of salt into the fish’s bloodstream. Now, saltwater fish have the opposite problem. For most of them, the concentration of salt
in the water is higher than the concentration in their blood. That means that as they pass water over their
gills to breathe, the higher concentration of salt outside of their body is constantly
sucking water out of them. So these fish have to drink a lot, which they
just gulp down by the mouthful. But since the only water around is salty,
they have to filter out the salt to make it safe for them to absorb. A lot of that salt is filtered out in the
fish’s kidneys. But these fish have chloride cells in their
gills, too. It’s just that, in their case, those cells are constantly pumping salts out
of the bloodstream and into the saltier water. So they may be surrounded by water, but no
matter where they live, fish are always looking to find that balance. Thanks for asking. If you have a quick question
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