The main respiratory organ in fish is the gill. Fish have gills located on either side of their heads, which are specialized structures that extract oxygen from water. As water flows over the gill membranes, oxygen dissolved in the water diffuses into the fish's bloodstream, while carbon dioxide is expelled. This process allows fish to breathe underwater effectively. Gills are highly efficient, featuring a large surface area created by gill filaments and lamellae, enhancing their ability to absorb oxygen. The health of fish and their gills is crucial for their survival, impacting their overall respiratory and physiological functions.