by Miriam Baker

Created on: June 24, 2010

Fish have gills to allow them to take in oxygen from the water around them. Gills allow a fish to ‘breathe’ underwater, just as other animals have lungs to allow them to breathe air on land. Unlike mammals such as dolphins and whales, fish do not need to surface for air; this is because their gills are very effective, enabling them to absorb all the oxygen they need from the water.

The gills are located just behind the mouth on either side of the fish. They have the appearance of slits, and are covered by a flexible bony flap called the operculum. When the fish opens its mouth, the operculum closes, drawing fresh water in towards the gills. The water passes through gill rakers; these are structures which act as filters to remove food and dirt from the water before it passes through to the gills. The gills themselves are made up of extremely delicate membranes which contain many networks of capillaries, tiny veins which are close to the surface to make the absorption of oxygen easier. As the water passes over these membranes, the oxygen exchange takes place: oxygen passes into the blood while carbon dioxide is removed from it. The de-oxygenated water then leaves the fish’s body through the operculum, and the newly-oxygenated blood begins to circulate around the fish.

However, not all fish have such effective pumping systems. Some fish, such as tuna and sharks, have to swim constantly with their mouths half-open to ensure a steady supply of fresh water through the gills. Bottom-dwelling fish such as rays have an enlarged opening behind their eyes which allows them to suck water in over their gills; this enables them to take in oxygen even when they are partially buried in the sand.

The process of absorbing oxygen through the gills is very similar to the human respiratory system, in which the body filters the fresh air before it passes to the lungs, and disposes of ‘used’, de-oxygenated air by exhaling, or breathing out. Land animals cannot have gills; gills would collapse when taken out of water because their delicate structure is held in place by the buoyancy of the water that surrounds them. As well as being efficient surfaces for exchanging oxygen, gills provide efficient surfaces for exchanging water. A land animal would quickly lose a lot of water through exchange at the gills, causing it to dehydrate and die.

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