How Fish Breathe Underwater
Fish breathe underwater by using gills to pull dissolved oxygen out of the water, instead of using lungs to take oxygen from the air. Although water looks like it has no air in it, small amounts of oxygen are dissolved between the water molecules, and this is what a fish depends on to stay alive.
To breathe, a fish keeps water flowing in one direction: it takes water in through its mouth, passes it over the feathery red gills on each side of its head, and pushes it out through the gill flaps. As the water flows over the gills, oxygen passes from the water into the fish's blood, while the waste gas carbon dioxide passes out of the blood into the water — a swap that keeps the fish's body working.
Key ideas a learner will grasp include: oxygen can dissolve in water, gills do the job that lungs do in humans, water must keep moving in one direction over the gills, and gills only work when wet — which is why a fish taken out of water gasps and cannot breathe air.
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Meet a fishSCIENCE • SEA LIFE How do fish breathe underwater? 🐠 You need air to breathe. If you stay underwater too long, you run out of air. But a fish can live underwater its whole life and never come up for air! Let's discover the fish's amazing secret: gills. Tap Next to dive in! 🤿
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Air hides in waterSecret 1: There is air inside water 💨 The part of air we need to breathe is called oxygen. Here's the surprise: tiny bits of oxygen are floating dissolved inside the water too — you just can't see them! Where do you think fish get their oxygen from? Tap the correct answer. ☁️ From the sky above the sea 💧 From oxygen dissolved in the water 🍚 From the food they eat Pick one — it's okay to try again!
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Meet the gillsSecret 2: Gills are the fish's "lungs" 🌬️ Instead of lungs, a fish has gills — feathery red parts on each side of its head, hidden under a flap. Tap each glowing part to learn what it does! 👆 Tap the mouth, the gill flap, or the red feathery gills.
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Water flows throughSecret 3: Water flows in one door, out another 🚪 A fish keeps water moving over its gills. Water goes in the mouth ➜ over the gills ➜ out the gill flap. Tap the button to help the fish pump water! mouth in flap out Oxygen collected: 0 of 5 gulps 💧 Pump water!
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The swap inside gillsSecret 4: A tiny swap happens 🔄 Inside the red gills, blood does a clever swap with the water: Oxygen moves into the blood 👍 | waste gas carbon dioxide moves out into the water 👋 Drag each token into the right box. Which goes into the fish, and which comes out? ⬅️ INTO the blood OUT to water ➡️ 🟢 Oxygen ⚪ Carbon dioxide 🟢 Fresh oxygen Drag a token onto a box.
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Why fish gasp on landSecret 5: Why a fish can't breathe air 😮 Gills only work when they are wet. Out of water, the feathery gills stick together and dry out — so they can't grab oxygen, even though there's lots of air around! Sort these animals: does it breathe with gills or with lungs? Tap to sort. 🐟 Fish 🐶 Dog 🦈 Shark 🧒 You 🐬 Dolphin Tap an animal, then it will ask gills or lungs. 🌊 Gills 🫁 Lungs
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You did it!🐟🎉 You cracked the fish's secret! Here's how a fish breathes underwater: 💧 There is oxygen dissolved inside the water. 🌸 A fish uses gills (not lungs) to grab that oxygen. 🚪 Water flows in the mouth, over the gills, out the flap. 🔄 Oxygen goes into the blood; carbon dioxide goes out. 🌊 Gills only work when wet — that's why fish gasp on land. Fun fact: Sharks must keep swimming so fresh water keeps flowing over their gills — some would suffocate if they stopped! 🦈 Well done, young scientist! Now you'll never look at a fish tank the same way again. 🐠✨
Frequently asked questions
- How do fish breathe underwater without lungs?
- Fish use gills instead of lungs. As water flows over the gills, oxygen that is dissolved in the water passes into the fish's blood, so the fish never needs to take a breath of air.
- Is there really oxygen inside water?
- Yes. Tiny amounts of oxygen are dissolved between the water molecules. It is far less oxygen than in the air, but it is enough for a fish's gills to collect.
- Why does a fish die when it is taken out of water?
- Gills only work when they are wet. Out of water the thin, feathery gill parts stick together and dry out, so they can no longer grab oxygen — even though there is plenty of oxygen in the air around the fish.
- How does water move through a fish's gills?
- The water travels in one direction: it enters through the mouth, flows over the gills, and leaves through the gill flaps on each side of the head. This one-way flow keeps fresh, oxygen-carrying water passing over the gills.
- What is the difference between how fish and humans breathe?
- Humans use lungs to take oxygen from the air, while fish use gills to take dissolved oxygen from water. Both are collecting oxygen and getting rid of carbon dioxide, just from different places.
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