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Why are gill filaments filled with many small blood vessels?

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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.

Gill filaments are filled with many small blood vessels to maximize the efficiency of gas exchange between the water and the fish's circulatory system.1

This feathery, branched structure provides a large surface area that brings the blood very close to the surrounding water. This design allows oxygen to diffuse easily into the blood while simultaneously allowing carbon dioxide to be released into the water.12

Furthermore, the arrangement of these capillaries facilitates a process known as countercurrent exchange, where blood flows in the opposite direction of the water. Because the blood constantly encounters water with a higher oxygen concentration, oxygen is able to diffuse into the bloodstream along the entire length of the gill filament, significantly improving the fish's ability to breathe underwater.1