How do fish breathe in the water?

As the fish forces water past its gills tiny blood vessels lining the gills  absorb oxygen out of the air that is dissolved in the water.

Just as  our lungs take oxygen from the air.  Fishes take oxygen from the water.  But fishes breathe with gills instead of lungs. By watching a fish in an aquarium,  You can see it open and close  its mouth constantly as though drinking in gulps of water.

Each time the fish opens its mouth a mouthful of water is drawn in. Then the fish closes its mouth and the water is forced back out through the gill openings, which  are the slits located on the side of the fish’s head. The gills are very  thin tissues.  They are bright red because they contain many tiny blood vessels.

As the water passes over  the gills the oxygen dissolved in the water is absorbed  into  the tiny blood vessels and is carried by the blood to other parts of the fish’s body. (Fishes have nostrils which they use only for smelling.)

Fish live wherever there is water, except in  very salty  water, such as the dead sea and  the great salt lake of Utah, or in water polluted by man. In such water fish cannot find enough oxygen to breathe. – Dick Rogers