How do freshwater fish manage to maintain their internal environment?

Freshwater fish maintain their internal environment by taking in water, excreting ions, and producing dilute urine. In a freshwater environment, the concentration of solutes in the water is lower than that of the fish's bodily fluids, leading to a tendency for water to flow into the fish's body through osmosis. To counterbalance this influx of water, freshwater fish actively excrete excess water by producing a large volume of dilute urine.

Additionally, they need to regulate their ion concentrations because they continuously lose ions due to diffusion into the surrounding water. To maintain ion balance, these fish actively transport ions from the environment into their bodies, ensuring that their internal conditions remain stable. Through this combination of water intake, ion excretion, and urine production, freshwater fish effectively regulate their internal osmotic balance and ensure their survival in a low-salinity environment.