How Fish Breathe Underwater

Breathing in a world of water

Every animal needs oxygen to live. You breathe oxygen from the air using your lungs. But fish live underwater, where there is no air to gulp. So how do they breathe? The answer is one of nature's neatest inventions: gills.

There is oxygen hiding in water

Here is the first surprise. Water is not just water — there is a small amount of oxygen mixed into it, far too little for us to use, but enough for a fish. This is called dissolved oxygen. Tiny water plants and the surface of streams and seas add this oxygen to the water all the time.

Fish cannot breathe the oxygen that is joined inside the water molecules (the H₂O itself). They breathe the loose oxygen dissolved between the water — the same oxygen that bubbles into a fast-flowing stream.

What are gills?

A fish's gills are tucked just behind its head, usually under a flap on each side. Gills are made of rows of thin, feathery flaps that are bright red because they are packed with tiny blood vessels.

The flaps are thin and spread out for a reason: this gives a huge surface for oxygen to pass through. The more surface the water touches, the more oxygen the fish can grab.

The journey of a breath

Watch a fish in a tank and you will see its mouth opening and closing. It is breathing! Here is what happens:

1. The fish opens its mouth and takes in a gulp of water.
2. It shuts its mouth and pushes the water back over the gills.
3. As the water flows past the thin gill flaps, oxygen passes into the blood, and waste gas (carbon dioxide) passes out into the water.
4. The used water flows out through the gill flaps at the sides.

Why push water in one direction? Water always flows the same way — in the mouth, out the gills. This one-way flow means fresh, oxygen-rich water keeps passing over the gills, so the fish never runs out of oxygen.

Why a fish can't breathe air

It seems strange: air is full of oxygen, far more than water has. So why does a fish suffocate on land?

Out of water, the wet gill flaps collapse and stick together, like wet pages of a book pressed flat. Once they are stuck, water (and air) cannot flow between them, so they can no longer take in oxygen. The fish is surrounded by oxygen it simply cannot reach. That is why we should always return a caught fish to the water quickly.

Some clever exceptions

A few fish have found extra tricks:

• Lungfish can gulp air using a simple lung-like organ, so they survive when their ponds dry up.
• Sharks and tuna must keep swimming so water keeps rushing over their gills — they cannot pump it while still.

These special cases show how, over millions of years, animals adapt to where they live. You can read more about that in habitats and adaptation, and meet many more sea creatures in oceans and sea life.

Observe and investigate

1. Watch the breathing: at an aquarium, pet shop or pond, watch a fish's mouth and the flaps behind its head. The opening and closing is the fish breathing. Try counting how many breaths it takes in a minute.
2. See dissolved oxygen: pour cold water into a clear glass and let it sit. Look for tiny bubbles forming on the inside — that is gas, including oxygen, coming out of the water. Warm water lets go of its oxygen faster, which is why warm, still ponds are harder for fish.
3. Compare to you: put your hand on your chest and feel yourself breathe with lungs. A fish does the same job a completely different way, using gills and water. Both get the oxygen the body needs.

Gills are why fish rule the rivers and seas — they turned water itself into something an animal could breathe.