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How Do Fish Breathe Underwater? - Lesson for Kids

Instructor: Elizabeth Foster

Elizabeth has been involved with tutoring since high school and has a B.A. in Classics.

Learn how fish use their gills to breathe underwater. Find out what's so special about their gills and why they stop working if a fish is removed from the water.

How Fish Breathe Underwater

Here's a brain teaser: what do you have in common with whales and dolphins, but not with salmon or tuna fish?

The answer: it's how you breathe. Not everything that lives in the ocean can breathe underwater. Whales and dolphins have to hold their breath, just like you do. But fish like salmon and tuna can breathe underwater. In this lesson, you'll learn how they do it.

Gills

Fish can breathe underwater by using their gills. By using gills, fish can get oxygen out of the water, just like you get oxygen out of the air.

Gill Arches & Filaments

Fish gills look a little like a row of tiny hairbrushes with the bristles turned towards the outside of the fish's body. The handle and back of the 'hairbrush' are the gill arches, which are stiff, boney supports. All fish have at least three gill arches on each side, but some fish have up to seven. Attached to the gill arches are gill filaments, which are like the bristles of the hairbrush. The filaments are bendable and wave around in the water.

Fish gills: the white part is the gill arch, and the pink things are the gill filaments.
Fish gills

How Gills Work

When you breathe air into your lungs, tiny blood vessels in the lungs take up oxygen from the air and send it to the rest of your body. Fish gills work the same way. The fish takes in water through its mouth, just like you take in air through your mouth. The air passes over its gills, and blood vessels in the gill arches and filaments take up the oxygen and send it to the rest of the fish's body. Then the water passes out through the other side of the gills.

Salmon gills
Salmon gills

Availability of Oxygen

Fish actually have a tougher job breathing than you do in one way: there's less oxygen in water than there is in the air. For example, if you used your lungs to breathe underwater, you'd have to take 7.5 breaths every single second just to get enough oxygen. That's impossible: try it for yourself and see.

Because water doesn't have a lot of oxygen, fish gills are really good at grabbing all the oxygen they can get. For example, they have a lot more blood vessels than human lungs. Unfortunately, that also prevents fish from breathing on land.

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