Gill is an organ in fish and other animals that absorbs oxygen from the water. Gills replace the oxygen with carbon dioxide, an exchange called respiration (see Respiration). Many tiny water animals can absorb all the oxygen they need directly through the surface of their bodies. Most larger animals that live underwater need gills.
Gills perform much the same function as lungs. In both organs, gases are exchanged through diffusion. Diffusion is the natural movement of molecules from areas of high concentration to areas of low concentration. Blood entering the gills is high in carbon dioxide and low in oxygen, compared with the water. As water passes over the gills, diffusion causes carbon dioxide to pass out of the blood and oxygen to enter it.
Most fish have four pairs of gills. The gills are enclosed in a gill chamber on each side of the head. To get oxygen, fish gulp water through the mouth. The water passes into the gill chambers and flows over the gills. A flap of bone called a gill cover protects the gills of bony fish. Sharks and rays do not have gill covers. Their gills open into gill slits visible on the outside of the body.
Each individual gill contains a rigid gill arch. The gill arch supports two rows of filaments. The filaments are ropy tissues that carry blood. They are covered in sheetlike membranes called lamellae, where gas exchange occurs. The lamellae give the filaments a relatively large surface area for gas exchange. Efficient exchange is vital because water contains about 20 times less dissolved oxygen than is available in air.
In addition to fish, such mollusks as sea snails, clams, and squids have gills. Crustaceans, such as shrimp and lobsters, also have gills. Amphibians may have gills for at least part of their life cycle. As larvae (young), for example, salamanders have feathery gills outside their bodies, just behind the head. The body absorbs these gills when the animal matures and begins to live on land.
See also Fish (Respiratory system).
