Why do some animals actively pump water over their gills?

Some animals, like certain fish and amphibians, actively pump water over their gills to enhance respiration efficiency. This process helps to ensure a continuous flow of oxygen-rich water, allowing for more effective gas exchange as oxygen diffuses into the blood and carbon dioxide is expelled. Active pumping is particularly important in environments where water flow is limited, such as stagnant ponds or when animals are at rest. Additionally, it helps to optimize oxygen uptake in low-oxygen conditions.