Most fish have gills on both sides of their heads.
Gills are tissues composed of feathery structures called gill filaments that provide a large surface area for gas exchange.
Because water contains very little dissolved oxygen, a large surface area is essential for gas exchange in aquatic organisms.
In the gill arch, the filaments in fish gills are organized in rows.
Each filament comprises lamellae, which are capillary-supplied discs.
These tiny blood vessels carry blood into and out of the gills.
Although fish gills occupy only a tiny portion of their body, the extensive respiratory surface produced by the filaments provides the entire organism with oxygen.
Fish inhale oxygen-rich water through their mouths.
When water flows over the gill filaments, the blood within the capillary network takes up the dissolved oxygen.
The circulatory system then transports oxygen to all tissues of the body and, finally, to the cells while absorbing carbon dioxide expelled from the body via the gills.
It exits the fish's body after the water moves past the gills through openings in the sides of the throat or the operculum, a flap covers and protects the fish's gills in bony fish.
Many fish, including lampreys and sharks, have multiple gill openings. Rohu is a bony fish with one gill opening on each side.
