What is the main function of gills in fish?

Gills are specialized respiratory organs in fish that primarily function to facilitate the exchange of gases between the water and the fish's bloodstream. When water flows over the gill membranes, dissolved oxygen is absorbed into the blood while carbon dioxide, a waste product of metabolism, is expelled. This efficient process allows fish to extract the necessary oxygen from the water, enabling them to breathe and sustain their metabolic processes while living underwater.

In contrast, other processes such as digestion, circulation, and excretion involve different systems and organs. Digestion occurs primarily in the digestive tract, circulation involves the heart and blood vessels to transport nutrients and oxygen throughout the body, and excretion involves the removal of waste products, typically handled by kidneys in most animals. Therefore, the role of gills is specifically tied to respiration, making it the correct function to highlight in the context of this question.