Urine Concentration and the Length of the Renal Papilla

ABSTRACT Countercurrent multiplier theory predicts that the maximum urineconcentration should be directly related to the length of the loopof Henle. However, those small rodents that achieve very high urineconcentrations have much shorter loops than large mammalsthat produce far lower concentrations. To resolve this contradiction, we propose that the high mass-specific metabolic rates of small mammalsmake their kidneys capable of the intense active transportrequired to achieve high urineconcentrations.