 In turn, they excrete only the minimum volume of urine necessary to rid of solutes that are not excreted by other routes, and the salt is mostly excreted via gills.This is why the composition of solutes in urine differs significantly from that of the blood plasma.Urine has a high concentration of divalent ions, such as Mg2+ and SO42− (U/P»1), as these ions are mostly excreted by the kidneys keeping their concentration in blood plasma from rising.