How does a fish breathe?

Fish, like all animals, need oxygen to survive. They absorb oxygen from water using gills, specialized organs located behind their mouths. Gills have a large surface area to efficiently absorb oxygen from water, which contains less oxygen than air. As water flows over the gills, oxygen enters the fish's bloodstream, while carbon dioxide is expelled. This process is similar to how human lungs function, allowing fish to live underwater.

Why do fish need to extract oxygen from water instead of air?

Describe the process that occurs when water flows over a fish's gills.