Why do fish need a counter-current system?

Fish need a counter-current system in their gills to maximize the efficiency of oxygen exchange. This system allows blood and water to flow in opposite directions, maintaining a gradient that ensures oxygen diffuses from the water into the blood throughout the entire gill surface. As a result, fish can extract a higher percentage of oxygen from the water, which is crucial for their survival in often oxygen-poor aquatic environments. This adaptation enhances their respiratory efficiency, supporting their metabolic needs.