How could sharks maintain these ion concentration differences at the levels found in freshwater fish?
Tissues of sharks are isotonic to seawater, but their concentrations of sodium ions, potassium ions, and chloride ions in cells and extracellular fluids are similar to those of freshwater fish. How could sharks maintain these ion concentration differences at the levels found in freshwater fish?
The nephron/kidney structure helps them keep their ion/water concentrations in a favorable quantity.