Nature · For ages 7–11
How Fish Breathe for kids, explained simply
Fish breathe using organs called gills instead of lungs. A fish takes water in through its mouth, passes it over the gills, and pushes it out through gill slits on the sides of its head. The gills are packed with tiny blood vessels that pull dissolved oxygen from the water and release carbon dioxide — the same exchange that happens in our lungs, just underwater.
The big ideas
Gills are the underwater version of lungs
Just as our lungs pull oxygen from air, gills pull oxygen from water. Water contains dissolved oxygen — and gills are full of blood vessels that are thin enough to let that oxygen pass straight through into the fish’s bloodstream, while releasing carbon dioxide the other way.
Water flows in one direction
Most fish breathe very efficiently by keeping water moving in one direction: in through the mouth, out through the gill slits. This one-way flow means fresh, oxygen-rich water is constantly passing over the gills — much more efficient than breathing air in and out of the same opening the way we do.
Some fish can breathe air too
A few fish, like lungfish and mudskippers, can also breathe air using a simple lung or by absorbing oxygen through their skin. This was an important step in the story of life: the ancestors of all land animals were fish that slowly developed the ability to breathe on land.
A quick quiz
1. What do fish use to breathe instead of lungs?
Choices: Gills · Their skin only · A blowhole
Answer: Gills. Fish use gills — organs packed with tiny blood vessels that pull dissolved oxygen out of water as it flows past. Lungs work with air; gills work with water.
2. Which direction does water flow through a fish’s gills?
Choices: Back and forth, in the same way we breathe · In through the mouth and out through the gill slits in one direction · Directly in through the gill slits
Answer: In through the mouth and out through the gill slits in one direction. Water enters through the mouth and exits through the gill slits on the sides of the head — a one-way flow that keeps fresh, oxygen-rich water moving over the gills continuously.
3. What gas do fish take in through their gills, and what do they release?
Choices: They take in carbon dioxide and release oxygen · They take in oxygen and release carbon dioxide · They take in nitrogen and release water
Answer: They take in oxygen and release carbon dioxide. Gills absorb dissolved oxygen from the water into the blood, and release carbon dioxide back into the water — the same gas exchange that happens in our lungs, just from water rather than air.
For parents: helping your child think about how fish breathe
Fish breathing is a beautiful entry to one of biology’s most satisfying ideas: the same problem — getting oxygen into the blood — solved in different ways by different animals. Before explaining gills, ask your child: "We breathe air, but fish live underwater. How do you think they get the oxygen they need?" Let them reason through it. The key insight is that water contains dissolved oxygen — it’s not just H₂O, it has tiny amounts of gas mixed in — and gills are perfectly built to extract it. A useful comparison is holding your hand under running water versus dipping it in a still bowl: the moving water constantly brings something fresh. That’s what the one-way flow through gills achieves. The evolutionary bridge is worth mentioning for older children in the 9–11 range: some fish can breathe air, and hundreds of millions of years ago fish ancestors were the first animals to crawl onto land and breathe it. Every land animal alive today — including us — descends from fish that made that transition. That puts "fish breathing" in a completely different light: it’s not just about fish, it’s about where we all came from. Ask your child to explain, in their own words, how a gill does the same job as a lung — just in a different medium.
Frequently asked questions
How do fish breathe underwater?
Fish breathe through gills, which are packed with tiny blood vessels. Water enters through the mouth, flows over the gills, and exits through gill slits. The gills absorb dissolved oxygen from the water into the blood and release carbon dioxide the other way.
Is there oxygen in water for fish to breathe?
Yes — water contains small amounts of dissolved oxygen gas, not just the oxygen atoms bonded in H₂O molecules. Gills are built to extract that dissolved oxygen, which is why fish suffocate in water that has been depleted of oxygen, such as a badly polluted pond.
Can any fish breathe air?
A few can. Lungfish have a simple lung alongside their gills and can breathe air when their ponds dry out. Mudskippers can absorb some oxygen through their damp skin on land. These fish give us a glimpse of how the ancestors of all land animals first began to breathe air.
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