How are fish are well adapted for gas exchange?

Gas exchange is important for fish as they need oxygen for respiration. Considering Fick’s law, in order to maximise the rate of gas exchange, fish have many gill filaments covered in lamellae, maximising the surface area available.In order to maximise the concentration gradient, the gills have a good blood supply to ensure that oxygenated blood is removed from the gills as quickly as possible. They also have a good water supply so that water with plenty of oxygen is always passing across the gills, helped by swimming quickly or gulping. The water flow is set up in a counter current flow against the blood, maximising the amount of oxygen the fish is able to take from the water. Counter current flow allows concentration gradients to be maintained as far as possible across the length of the exchange surface, as the two fluids either side have much less chance of reaching equilibrium, which increases the efficiency of the system.