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Ask A Scientist

How does a fish’s gills take oxygen out of the water and why can’t they breathe oxygen through the air?

Asked by: Matthew King
School: Seton Catholic at All Saints
Grade: 5
Teacher: Matthew Martinkovic
Hobbies/Interests: Football and sports 
Career Interest: Science Teacher

Answer from Douglas W. Green, EdD

Adjunct Lecturer, Binghamton University

Research Area: Leadership, Learning Theory and Social Media
Family: Daughter Lena, age 28, who is an animator for Nickelodeon in New York City
Interests/hobbies: Playing my banjo, biking, golf and reading
Web page address: http://www.drdouggreen.com    

Like all animals, fish need oxygen. They use oxygen dissolved in the water where they swim. In place of the lungs that land animals use, fish have specialized organs called gills in their jaws. These are amazing organs indeed as the concentration of oxygen in water is only about 10 milligrams per liter. This is ten parts per million! Compare this to the air we breath that contains 20% oxygen, or 200,000 parts per million.

Fish gills are remarkable, but the conditions under which they function are pretty specific. They are rather delicate, and have a tremendous surface area, which is why they work so well. There are many folds of tissue called filaments in each gill. The filaments are filled with small blood vessels called capillaries where oxygen exchange takes place. At the same time fish give off carbon dioxide as a waste product, via their gills.

Gill filaments are supported by surrounding water as it flows in the fishes mouth and out the gill slits at the back of the head. Water is about 800 times more dense than air. Without this support from water the gill filaments would stick together and most of the surface area would be unable to absorb oxygen.

Remarkably, there are some types of fish that breath air and use their gills to take oxygen out of water. So whenever you talk about fish, you have to pay attention to what species of fish you are talking about.

To better understand how water supports gills and keeps them separated for better oxygen transfer, try to see if you can float the next time you are in a pool. This is easier to do if you fill your lungs as much air as possible and keep just your face out of the water. The water supporting your body does the same thing for gill filaments. After you try floating in water, try floating in air. Unless you have help from a blimp or a hot air balloon you won't have much luck, as the air is nowhere near dense enough.

If you have pet fish, you probably have a pump that bubbles air into the water. This is to replace the oxygen that fish remove from the water via their amazing gills. To see many pictures of fish gills, go to Google.Com, click on images, and search for types of fish gills.

Last Updated: 9/18/13