How can fish breathe underwater?

Fish are able to breathe underwater because they have special organs called gills. Gills are bars of tissue at the side of the fish's head. They carry masses of finger-like projections that contain tiny blood vessels. Water enters the fish's mouth and flows over the gills. The gill filaments take in dissolved oxygen from the water and pass it into the fish's blood. In this way the gills have the same function as the lungs of air-breathing animals.

Many fish that live in the water containing little oxygen are ble to breathe air. They usually have to do this in foul water, when bacterial decay reduces oxygen levels. Some fish have a special breathing organ called a labyrinth, and they rise to the surface every few minutes to breathe air into this organ. Many catfish breathe air in similar way -- they swallow it and it passes into their gut where oxygen is absorbed through the thin lining of the gut wall.