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How do fishes extract oxygen from water with their gills?

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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.

Fishes extract oxygen from water using gills, which are specialized structures filled with blood vessels that provide a large surface area for gas exchange.12

The process generally involves the following steps:
* Water Intake: Water is drawn into the mouth and forced over the gill filaments, which are supported by gill arches.12
* Gas Exchange: As water flows over the filaments, oxygen diffuses into the blood within the gill capillaries, while carbon dioxide is released into the water.1
* Countercurrent Exchange: To maximize efficiency, blood in the gill capillaries flows in the opposite direction to the water. This ensures that the blood constantly encounters water with a higher oxygen concentration, allowing oxygen to diffuse into the bloodstream along the entire length of the gill filament.13

Different species employ various mechanisms to maintain this flow, such as active pumping or relying on forward motion through the water. Because gills are specifically adapted for extracting oxygen from water, they are generally ineffective at absorbing oxygen from the air, which is why most fish cannot survive on dry land.32