Gills are part of which body system in fish?

• A. Digestive
• B. Circulatory
• C. Respiratory
• D. Nervous

Answer: (C)

Gills are the respiratory organs of fish, specialized for extracting dissolved oxygen from water and releasing carbon dioxide. They enable gas exchange by presenting a large, thin surface area rich in blood vessels, with water flowing over them to pick up oxygen and drop off CO2. In many fish there's a countercurrent exchange mechanism that maximizes oxygen transfer by moving water and blood in opposite directions. While the circulatory system works with the gills to carry oxygen-rich blood to the body and return deoxygenated blood to the gills, the primary function of the gills themselves is respiration, not digestion or nervous control.