What are gills primarily responsible for?

Gills are primarily responsible for both oxygen absorption and carbon dioxide release, functioning as the primary respiratory structures in many aquatic animals, such as fish. Water flows over the gill membranes, where oxygen dissolved in the water diffuses into the bloodstream of the animal. This process occurs due to the concentration gradient where oxygen levels are higher in the water than in the blood.

Simultaneously, carbon dioxide, which is a waste product of cellular respiration, diffuses from the blood, where its concentration is higher, into the water, allowing for its release into the surrounding environment. This dual function is critical for maintaining gas exchange, ensuring that aquatic organisms can effectively obtain the oxygen they need while expelling carbon dioxide, thus supporting their metabolic processes and overall survival in water.

Other options focus solely on one function of the gills, which does not encompass their complete role in respiration. Therefore, the choice that includes both oxygen absorption and carbon dioxide release accurately reflects the comprehensive physiological function of gills.