Britannica Sources used in this answer

How does water move through a fish’s mouth and over its gills?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

To breathe, a fish continuously takes water in through its mouth and directs it backward into the mouth cavity. From there, the water flows over gill filaments, which are fleshy, branched structures supported by gill arches and filled with blood vessels.123

As water passes over these filaments, a gas exchange occurs: oxygen diffuses from the water into the blood, while carbon dioxide is released from the blood into the water. This process is highly efficient due to countercurrent exchange, where blood in the gill capillaries flows in the opposite direction of the water, ensuring that the blood is always exposed to water with a higher oxygen concentration. Finally, the water is squirted out through the gills, which are protected by a gill cover in many species or by skin flaps in others.321