The respiratory organ in a fish is called as gills which is covered by gill covers. The fish has gills on both sides of its head. Fish live in water containing dissolved oxygen. During the process of respiration, fish takes in the water through its mouth and passes it over the gills. The gills extract the oxygen, dissolved in the water, and send it to all the parts of the fish through blood. The carbon dioxide produced during respiration is carried by the blood to the gills where it is expelled into the surrounding water.

Figure to show the process of breathing in fish