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How Do Fish Breathe in Water?

Last updated date: 04th Mar 2024
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An Overview of How Fish Breathe in Water

Every animal species is unique and made different. There might be some similarities but the distinct features make them different. A fish can breathe in water easily but cannot breathe outside. Why? There are many animals that hold their breath for several minutes but cannot breathe in water. Why? Let us find out the answers to these questions.

What is a Fish?

A fish is an animal that lives in water. It has scales all over its body except the head. Some fishes have bigger scales whereas some have minute scales protecting their bodies. A fish has a tail and fins to swim underwater and maintain balance.

How Does a Fish Breathe?

A fish is an aquatic animal. It means it cannot come out of the water for a long period. We know that all animals need to breathe. Hence, there must be a special set of organs that assist fish to breathe underwater. Let us find out what it is.


Fishes have gills. It is a breathing organ that acts as the lungs of fish. It is made of feather-like structures stacked together. A fish intakes water through its mouth and passes it through its feathery gills. These structures have a huge supply of blood vessels making them appear red.

Gills in a Fish

Gills in a Fish

The blood flowing in the vessels can easily absorb oxygen and let carbon dioxide flow out. This is how the main breathing organ of a fish works. This organ is often protected by a bony structure called the operculum. Most bony fishes have such structures to protect the gills from injuries.

How do Gills Work?

As mentioned earlier, gills are made of feather-like structures. A fish has a pair of gills on each side at the base of its head. These gills are connected to the fish’s mouth through a duct.

The mechanism of exchange of gases between the gills and water is diffusion. Oxygen is present at a higher level in water than in the blood of a fish. Hence, its tendency is to get into the blood vessels of the gills crossing the thin membrane. It happens naturally. This is how a fish breathes through its gills.

A fish not only breathes in oxygen through its gills but it also expels carbon dioxide produced in its body due to respiration. The same physical phenomenon happens. The amount of carbon dioxide in the blood is more than in water. Hence, this gas crosses the thin membrane and is expelled from the bloodstream to the water.

Why do Whales and Dolphins Breathe outside Water?

Whales and dolphins are not fish. They are mammals and they don’t have the gills to breathe underwater. They have to reach the water surface to use their lungs to breathe in air and hold it for several minutes. Sharks, on the other hand, are fish with gills. They can breathe underwater and can remain submerged as long as they want.

Trivia of Breathing System

  • Earthworms and frogs can breathe through skins.

  • Frogs can breathe in air and underwater. They have lungs to breathe in air and skin to breathe in water.

  • An octopus has the breathing organ of fish.

  • Mangrove rivulus is a fish species that can breathe air. This fish can also walk using its fins. It is an amphibious fish.

Mangrove Rivulus Breathing Air

Mangrove Rivulus Breathing Air

Tips for Parents

Explain what a breathing system is first and why animals need to breathe. Proceed to describe the process of fish breathing through gills and how the gases are exchanged. Use pictures to describe the process and make it easier to understand for the kids.

FAQs on How Do Fish Breathe in Water?

1. Which underwater fish can breathe in air and water?

Lungfish can breathe in air and water. Koi have been witnessed to come to the surface and breathe air even if they have gills. They do it when dissolved air in water is reduced.

2. Why do animals breathe?

Animals breathe to get oxygen from the air in order to burn food and generate energy to run their physiological functions.

3. Why do fish breathe?

Fish breathe air to get oxygen like all animals. They also need oxygen to burn food and make energy.