Urine Concentration and the Length of the Renal Papilla

ABSTRACT Countercurrent multiplier theory predicts that the maximum urine concentration should be directly related to the length of the loop of Henle. However, those small rodents that achieve very high urine concentrations have much shorter loops than large mammals that produce far lower concentrations. To resolve this contradiction, we propose that the high mass-specific metabolic rates of small mammals make their kidneys capable of the intense active transport required to achieve high urine concentrations.