GILLS | DON STEVE BLOG
March 28, 2024

Fish and tadpoles are aquatic vertebrates. They possess gills for gaseous exchange.

 

Gills of fish

Fish has four gills on each side of the head. Each gill has two rows of soft gill filaments. The filaments are tiny, long and numerous structures which are well supplied with blood vessels. In bony fish such as Tilapia, the four gills of each side are covered with a bony plate called operculum. The filaments are attached to a curved bony structure called gill arch. The inner sides of the gill arch are short bony structures called gill rakers, which are used for feeding rather than gaseous exchange. They also prevent food substances from entering the gills from the pharynx.

 

Breathing mechanism in Tilapia

When the mouth is opened, the floor of the mouth is lowered and at the same time the two opercula are closed, water is drawn into the mouth to occupy the depression created by the lowering of the floor of the mouth. When the mouth is closed, the floor of the mouth is raised and at the same time the two opercula are opened, water containing dissolved oxygen passes through the gills. The dissolved oxygen is then absorbed by the blood capillaries and carried into the general blood circulation.

 

During cellular respiration, the carbon (IV) oxide produced is carried by blood vessels to the gills where it diffuses into the water.

Note that in the fish, the nostrils are not used for breathing in and out as in other vertebrates.

 

Gaseous exchange through external gills

Tadpoles (of frogs and toads) and salamanders (all belonging to the phylum Amphibia) have external gills for gaseous exchange. The external gills are well supplied with blood vessels. Oxygen dissolved in water is absorbed by diffusion through the external gills and carbon (IV) oxide diffuses from the blood vessels of the gills into the water.

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