What is the role of gills in a fish?

The gills are the site of gas exchange in a fish, much like the alveoli & lungs are the site of gas exchange in humans. It works on a similar principle - except the oxygen is being carried in water rather than air. At the gills there is a set up where the blood of the fish flows in one direction while the water flows in the other. As the flow past each other the oxygen from the water diffuses into the blood where it is then carried around the fishes circulatory system to respiring cells

Gills are the equivalent of "lungs" in a fish. It allows him to breath in the water. Gills are most commonly located on each side of a fish's head and helps him extract the oxygen contained in the water and release carbon dioxide.

Gills are the site of gas exchange in bony fish. They are specialised organs made up of arches, filaments and hill lamellae. The gill lamellae increases the surface area for gas exchange as water is passed over the gills via the fish. Oxygen is exchanged between the water and the capillaries within the full lamellae. Due to the countercurrent flow a concentration gradient for oxygen is maintained across the length of the gill, leading to the exchange of oxygen