Functional model of inner medulla of rabbit kidney based on its structural principle.

Abstract

The function of the inner medulla was analyzed on mathematical models with parameters of medullary structure. Three different theoretical assumptions were made on Henle's loops. The loops were regarded as an exchanger system of dif-fusion type, a counter-current multiplier system or a transport system with active sodium transport only on the ascending limb. The collecting tubule was treated as having active sodium transport and giving water to the interstitial tissue. When Henle's loops were treated as a sodium-depriving system, the structure of the inner medulla characterized by quantitative predominance of Henle's loops over the collecting tubule toward the outer medulla acted synergetically to raise interstitial sodium concentration toward the medullary apex. In regard to the effect, the assumption of Henle's loops as a sodium-supplying system was conflicting with the medullary structure. The counter-current multiplier hypothesis of Henle's loops was not supported, because it presupposed the loops as a sodium-supplying system. Conversely, the model with Henle's loops as an exchanger system of diffusion type was found to be the most reasonable simulation of the inner medulla. A hypothesis of intramedullary sodium circulation involving both outer and inner medullas was proposed for the mechanism of water absorp-tion from the collecting tubule or urinary concentration.