Gills are specialized organs in fish that perform the critical function of gas exchange. They allow fish to extract oxygen from water as it flows over them while simultaneously expelling carbon dioxide. Gills are richly supplied with blood vessels that maximize oxygen absorption, enabling fish to respire efficiently in aquatic environments. This adaptation is crucial for their survival, allowing them to thrive underwater where oxygen concentration can be lower than in the air. Without gills, fish would be unable to breathe and thus would not be able to sustain their metabolic needs.