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In a number of mammalian kidneys a close correlation was found between renal medullary thickness and ability to concentrate electrolytes in the urine, indicating that both outer and inner zone of the medulla act as a countercurrent multiplier system. Further evidence for this assumption was furnished through studies of the distribution of urea and electrolytes in different kidney types during antidiuresis. The beaver with 100% short-looped nephrons, the rabbit with 44% long-looped nephrons, and the desert rodent Psammomys with 100% long-looped nephrons were studied. In all three kidney types sodium and urea concentrations increased to approximately the same level in the outer zone of the medulla. In the rabbit and Psammomys kidneys a considerable increase in sodium concentration was found through the inner zone of the medulla. This can best be explained if we assume that the sodium pump, which has been demonstrated in the thick ascending limb of the loop of Henle, functions in a similar manner in the thin limb.
Schmidt‐Nielsen et al. (Thu,) studied this question.