Gills are the primary structures that enable fish to breathe underwater. Located on either side of a fish's head, gills extract oxygen from water as it flows over them and allow carbon dioxide to be released. Fish breathe by taking in water through their mouths, which then passes through the gill membranes. This process of respiration is efficient in aquatic environments, where oxygen levels can vary. Different species of fish have adapted their gill structures to suit their specific habitats, optimizing their ability to extract oxygen. The efficiency of gills plays a crucial role in the survival and physiology of fish throughout various aquatic ecosystems.