Gills mediate the gas exchange in fish. These organs, located on the sides of the head, are made up of gill filaments, feathery structures that provide a large surface for gas exchange.
The filaments are arranged in rows in the gill arches, and each filament has lamellae, discs that contain capillaries. Blood enters and leaves the gills through these small blood vessels.
Although gills are restricted to a small section of the body, the immense respiratory surface created by the gill filaments provides the whole animal with an efficient gas exchange.
The surrounding water keeps the gills wet.