In Figure you have seen that the body fluids of freshwater animals are hyperosmotic to their aqueous surroundings. This results in two-kinds of osmotic problems;
Therefore, freshwater animals just prevent net gain of water and net loss of salts. Net gain of water is prevented by producing dilute urine. Freshwater fishes produce copious urine than the marine fishes. The useful salts are largely retained by reabsorption in the kidney tubules. The salts which are passed out in urine are replaced partly from the ingested food. Salts are also extracted from the hypoosmotic surroundings by active transport across the transporting epithelia. The transporting epithelia for example are found in the gills of fish and in the skin of amphibians. The active transport of NaCl in gills takes place against a concentration gradient in excess of 100 folds.