How do fish breathe underwater?

1. A fish opens its mouth to take in water, then closes it and pushes the water back. The water flows out through openings behind the gill covers.
2. Under each gill cover are the gills: rows of soft, red, feathery parts. They're red because they're full of blood.
3. Each gill filament is covered in tiny, thin plates. Water flows between the plates, and blood flows inside them, just a hair's width away.
4. Oxygen moves from the water, where there's more of it, into the blood, where there's less. At the same time, carbon dioxide moves out of the blood into the water.
5. The fish's heart pumps blood through the gills to pick up oxygen. Then the bright red blood travels all around the body, giving the muscles the oxygen they need.
6. You have lungs, which take oxygen from air. Air goes in and out the same way. Fish gills take oxygen from water, which flows one way. Out of water, gills flop together and stop working.

Tap each part. The lens shows a close-up of a gill filament.

• Mouth. Water comes in through the mouth. When the fish closes its mouth, it pushes the water back over the gills.
• Gill cover (operculum). The gill cover is a bony flap that protects the soft gills. It opens to let water flow out.
• Gill arch. Gill arches are curved bones that hold up the gills, like the spine of a comb.
• Gill filaments. Each arch has two rows of soft, red filaments. Having so many gives a big surface for oxygen to pass through.
• Lamellae. Lamellae are tiny, thin plates on each filament. Blood flows inside them, very close to the water, so oxygen can get in easily.
• Heart. A fish's heart pumps blood forward to the gills, where it picks up oxygen. Then the blood flows on around the body.
• Blood vessels. Blood vessels carry oxygen-rich blood from the gills to the muscles, and bring used blood back to the heart.
• Swim bladder. The swim bladder is a bag of gas that helps the fish float at the right depth. It isn't for breathing.

Change how fast the fish pumps water and how fresh the water is. Watch oxygen move into the blood and the blood turn bright red.

Experiment: What happens to the fish's blood when it pumps more water over its gills?

1. Start with cold, fresh water.
2. Set the pumping very slow and look at the blood.
3. Then pump fast. How red does the blood get?

What we found: Pumping more water brings more oxygen past the gills, so more goes into the blood. Warm, still water holds less oxygen, so the fish has to work harder.

Turn on JavaScript to try it yourself.

Sort each animal and fact: does it go with gills or with lungs?

Every animal needs oxygen to turn its food into energy. You get oxygen from the air with your lungs. Fish get oxygen from the water with their gills. The oxygen fish use isn't the oxygen inside the water molecules: it's oxygen gas dissolved in the water, like the fizz in a soda.

Water has much less oxygen in it than air does, so fish have to move a lot of water over their gills. They gulp water in through their mouths and push it out through openings behind their gill covers. Some sharks keep swimming with their mouths open to keep water flowing.

Gills are made of lots of thin, red filaments covered in even thinner plates. Blood flows inside the plates, just a tiny distance from the water. Oxygen moves from the water, where there is more of it, into the blood, where there is less. Carbon dioxide moves the other way.

If a fish is taken out of water, its soft gills collapse and stick together, so they can't take in oxygen even though air has plenty. In the same way, your lungs can't take oxygen from water. Each breathing system is built for its own world.

Key words

Gills

Organs that take oxygen from water.

Diffusion

When something spreads from where there's a lot of it to where there's less.

Lamellae

Thin plates on gill filaments where gas exchange happens.

Countercurrent exchange

When two flows go opposite ways so more can pass between them.

Operculum

The bony gill cover of a bony fish.

Check your thinking

1. Where do fish get their oxygen from?

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   Oxygen dissolved in the water. A little oxygen gas is mixed into water, and gills take it out.

2. Why are gills red?

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   They are full of blood. Lots of blood flows close to the surface of the gills, which makes them red.

3. Why can't a fish breathe out of water, even though air has more oxygen?

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   Its gills collapse and stick together in air. Without water to hold them apart, the thin gill plates stick together and lose their surface.

4. Which animal lives in the sea but breathes air with lungs?

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   A whale. Whales are mammals with lungs, so they swim up to breathe through their blowhole.

5. How does water move over a fish's gills?

   Show the answer

   In through the mouth and out behind the gill covers, one way. Water flows one way: in the mouth, over the gills and out.