The gill lamellae have a rich blood supply.The flow of water over the gill lamellae and the flow of blood through the gill lamellae are in opposite directions.This ensures the maximum possible gas exchange.Blood that is already loaded with oxygen meets water with a maximum concentration of oxygen so diffusion still takes place.Blood with little oxygen meets water with most but not all of its oxygen removed.Again diffusion still takes place.The countercurrent flow ensures that diffusion takes place over the entire length of the gill lamella. This is necessary as the concentration of oxygen in water is relatively low.In contrast, concurrent flow would achieve an equilibrium before the end of the gill lamellae and diffusion would stop.