How Fish Breathe

Fish live underwater, but they need oxygen to survive, just like people do. Instead of breathing air with lungs, fish use special parts called gills. Gills are found on the sides of a fish's head. When a fish opens its mouth, water flows in and passes over the gills. The gills take the oxygen from the water and send it into the fish's blood. At the same time, gills help remove carbon dioxide, which is a waste gas the fish needs to get rid of.

Gills have thin layers of tissue with lots of tiny blood vessels. These blood vessels collect oxygen as water passes by. Fish need to keep moving or pump water through their mouths to keep fresh water flowing over their gills. Some fish, like tuna, must swim all the time to breathe, while others can stay still and move water with their mouth and gill covers.

Fish also have a special organ called a swim bladder. The swim bladder is a small, air-filled sac inside the fish's body. It helps fish control their buoyancy, which means their ability to float or sink in the water. When a fish wants to float higher, it adds more gas to its swim bladder. If it wants to sink, it lets out some gas. This allows fish to stay at the right level in the water without using too much energy.

The way fish breathe and float helps them survive in lakes, rivers, and oceans. Aquariums use special pumps to add oxygen to the water for pet fish, because fish need oxygen-rich water to stay healthy.

Interesting Fact: Some fish, like lungfish, can breathe air with lungs and survive out of water for a short time!