Countercurrent multiplication by the thin loops of Henle

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Abstract
The contribution of the thin loops of Henle and the vasa recta to the production of the the hypertonicity of the mammalian renal medulla was investigated in rats using a modified version of the partial nephrectomy technique of Sakai et al. In the region between 2 and 4 mm from the tip of the papilla, no significant difference in osmolality was found between adjacent descending c limbs or between descending limbs and adjacent hairpin turns. In contrast, ascending limbs were found to have a significantly lower osmotic pressure than adjacent descending limbs (mean difference -117 milliosmols). A lower concentration of sodium accounted for most of this difference. Fluid from the descending limbs of the loops had a significantly higher osmolality than plasma from the descending vasa recta, but roughly equal to that of plasma from the ascending vasa recta. In this region of the medulla, blood entering through the descending vasa recta appears to lag in attaining osmotic equilibrium with its surroundings, a finding consistent with the behavior of a passive countercurrent exchanger. The differences between ascending and descending limbs of the loop of Henle, on the other hand, suggest that the thin segment functions as a countercurrent multiplier in the production of the hypertonic renal medulla.