What are gills?

What are gills?

• A. The digestive organs in fish.
• B. The respiratory organs that extract oxygen from water.
• C. The sensory organs for balance.
• D. The fins used for steering.

Answer: (B)

Gills are the respiratory organs that extract oxygen from water. In fish, blood flows through thin filaments with many lamellae, creating a large surface area for gas exchange. Water moves over the gills as the fish opens its mouth and operates its gill covers, and the blood and water flow in opposite directions to maximize oxygen uptake through countercurrent exchange. This setup lets dissolved oxygen diffuse into the blood and carbon dioxide diffuse out efficiently. If gills were digestive organs, they’d be involved in digestion; if they were for balance sensing, they’d be part of the inner ear or lateral line system; if they were fins, they’d help with steering and propulsion, not gas exchange.