How Do Fish Breathe Underwater?

Eliza Clark
October 5, 2009

You never know when certain inevitable kid questions will strike. You’ve taken your child on countless visits to the aquarium. So why is today the day your four-year-old is suddenly struck with an urgent need to understand how fish breathe underwater?

Perhaps she’s just taken with a particularly cunning little angelfish swimming around in the tank. Or perhaps she’s suddenly noticed that the diver cleaning out the tank is wearing a big oxygen tank, in contrast to the fish who are not. Or maybe she is being visited by a too-vivid memory of a recent swimming lesson in which she very acutely experienced her own inability to breathe underwater.

Whatever the mental association and timing, it’s one of those questions that is bound to come up. And when it does…you’ll need an answer!

A good way to start is by talking about how humans breathe. Ask your child: how do you breathe? In and out. Yes, we breathe air in, our lungs absorb the oxygen, filter out the rest, and then we breathe out what we don’t need.

Next question: do you think there might be oxygen in water just like there is in the air? Well yes, there must be, because fish need it to live just like we do. They just have a different way of filtering. Instead of lungs, they have gills.

At this point, it’s time to take another look at that fish in the tank. Do you see those slits on the side of its head? Those are the gills, the fish’s lungs. And see the fish opening their mouths as they swim? They are pushing water through the gills, which are made of tiny capillaries (blood vessels) that can absorb the oxygen and filter out everything else.

And there you have it! Fish can only breathe underwater, and people can only breathe air (excepting, of course, Aquaman – but that will have to be a story for another day).

From the Parents

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