The advantage of the counter-current flow system in fish gaseous exchange

In the counter-current flow system, blood and water flow in the opposite direction along any gill lamellae. This means that as blood flows across the respiratory surface, it approaches water which has more and more oxygen available for exchange. The advantage of the counter-current flow system is that a stiff diffusion gradient is maintained throughout the gaseous exchange surface. Graphical representation of the counter-current flow system

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