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How do fish breathe underwater?

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Quick answer

Fish breathe underwater by using gills to extract dissolved oxygen from water. Water enters the mouth, passes over thin gill filaments rich in blood vessels, and oxygen diffuses into the blood while carbon dioxide diffuses out. The oxygenated blood then delivers oxygen to the fish's cells, allowing it to survive without breathing air.

Fish breathe underwater by extracting dissolved oxygen from water using gills. Water flows in through the mouth and out over the gills, where oxygen enters the blood and carbon dioxide is released.

Step by step

  1. The fish opens its mouth and takes in water containing dissolved oxygen.
  2. Water flows over the gill filaments, which are covered in tiny blood vessels.
  3. Oxygen from the water diffuses into the blood, while carbon dioxide diffuses out.
  4. The oxygen-rich blood is pumped by the heart to the rest of the body.
  5. The water, now low in oxygen and high in carbon dioxide, exits through the gill slits.

Key things to understand

  1. Fish pull water into their mouths and push it out through gill slits, keeping a continuous flow over the gills.
  2. Each gill contains thin, fleshy filaments packed with tiny blood vessels called capillaries.
  3. As water passes over the filaments, oxygen dissolved in the water diffuses into the blood.
  4. At the same time, carbon dioxide from the blood diffuses out into the water.
  5. The oxygen-rich blood then circulates through the fish's body to supply its cells.

Labelled diagram

  1. Mouth

    Water enters here, carrying dissolved oxygen from the environment.

  2. Gill filaments

    Thin, feathery structures where gas exchange takes place.

  3. Capillaries

    Tiny blood vessels that absorb oxygen and release carbon dioxide.

  4. Gill slits

    Water exits here after oxygen has been removed.

  5. Blood circulation

    Oxygen-rich blood is pumped to the body's cells.

How a fish breathes underwater

Real-world examples

  • A goldfish in a home aquarium continuously opens and closes its mouth to push water over its gills.
  • Some sharks, such as the great white, must keep swimming to force water over their gills, a method called ram ventilation.
  • In the Amazon River, the arapaima fish gulps air at the surface because the water is low in oxygen.
  • Trout in fast-flowing mountain streams rely on the constant current to bring oxygen-rich water to their gills.

Common mistake

✗ Myth: Fish breathe oxygen from water molecules (H2O).

✓ Fact: Fish actually extract dissolved oxygen gas (O2) that is mixed in the water, not the oxygen bound in water molecules. The gills absorb this free oxygen, leaving the water molecules intact.

People also ask

How do fish breathe?
Fish breathe by taking in water through their mouths and passing it over their gills. The gills contain many thin filaments with blood vessels that absorb dissolved oxygen from the water. The oxygen enters the bloodstream, and carbon dioxide is released back into the water. The water then exits through the gill slits.
Why do fish need oxygen?
Like all animals, fish need oxygen to release energy from food through cellular respiration. Without oxygen, their cells cannot produce enough energy to survive. Oxygen is carried by the blood to every cell in the body, where it helps convert nutrients into usable energy.
What happens if fish don't get enough oxygen?
If fish do not get enough oxygen, they become lethargic and may gasp at the water's surface. Prolonged oxygen shortage can lead to suffocation and death. In aquariums, poor water quality or overcrowding can reduce oxygen levels, harming the fish.
Can fish breathe air?
Most fish cannot breathe air because their gills collapse out of water and cannot extract oxygen from the air. However, some fish like lungfish and bettas have special organs that let them gulp air at the surface.
Why do fish die when taken out of water?
When removed from water, the gill filaments stick together and dry out, so they cannot take in oxygen. Without a constant flow of water, the fish suffocates even though air has plenty of oxygen.
Do fish sleep?
Fish do rest, but they do not sleep in the same way mammals do. Many fish become inactive and reduce their metabolism, often hiding in safe spots, but they must keep water flowing over their gills to breathe.
How do fish get oxygen in frozen lakes?
In winter, lakes do not freeze solid from top to bottom. The water below the ice still contains dissolved oxygen, and fish slow down their metabolism to survive until the ice melts.

Part of the guide: Nature & Earth: Living Things and Rocks →