Explain breathing in fish?

Fish breathe by taking in water through their mouth, passing it over gills where oxygen is absorbed and carbon dioxide is released.

Fish live in water, which contains dissolved oxygen. When a fish opens its mouth, water flows in and then out through gill slits. Gills have thin filaments with many blood capillaries. Oxygen diffuses from water into blood, and carbon dioxide moves from blood into water. This process is called counter-current exchange, where blood and water flow in opposite directions to maximize oxygen uptake.

The absorbed oxygen circulates through the fish’s body. Because oxygen levels in water are low, fish must keep water constantly moving over gills. Some species like lungfish have adapted to breathe air during dry periods.

Key steps

1. Water enters mouth.
2. Passes over gills.
3. Oxygen diffuses into blood.
4. Carbon dioxide diffuses out.
5. Deoxygenated water exits through gill openings.