Gills are branching organs located on the side of fish heads that have many, many small blood vessels called capillaries. As the fish opens its mouth, water runs over the gills, and blood in the capillaries picks up oxygen that's dissolved in the water.

How do gills help a fish to breathe?

In order to remove oxygen from the water, they rely on special organs called "gills." ... A fish breathes by taking water into its mouth and forcing it out through the gill passages. As water passes over the thin walls of the gills, dissolved oxygen moves into the blood and travels to the fish's cells.

What function do gills perform?

Gills are evaginated respiratory surfaces used for breathing in water. Gills are present in all amphibian larvae and in some aquatic salamanders. They are typically highly branched structures.