Fish respire through gills, which enable them to extract oxygen from water. Gills are specialized organs located on either side of a fish's head, consisting of thin filaments that increase the surface area for gas exchange. When water flows over the gill membranes, oxygen diffuses into the fish's bloodstream while carbon dioxide, a waste product of metabolism, is expelled into the surrounding water. This process allows fish to survive in aquatic environments, where oxygen availability can vary. Gills are a critical adaptation that distinguishes fish from respiratory systems found in terrestrial animals.