![]() The same cannot be said for the renal inner medulla, however. Thus renal physiologists have developed a good understanding of the process that concentrates solutes in the renal outer medulla. The key evidence was the demonstration that the thick ascending limb of Henle's loop is capable of a high rate of active NaCl transport out of the lumen, which results in luminal dilution owing to the low osmotic water permeability of this segment ( 3, 67). Their version of Hargitay and Kuhn's ( 22) countercurrent multiplier hypothesis has become generally accepted as the mode of solute accumulation in the renal outer medulla and is now supported by extensive experimental evidence (reviewed in Ref. In 1959, Kuhn and Ramel ( 43) proposed a model to explain concentration of solutes in the renal medulla based on countercurrent amplification of a small osmotic difference between the ascending limb and the descending limb of Henle's loop, resulting from active solute transport out of the ascending limb. Measurements were made by vapor pressure osmometry of slices from different levels of rabbit kidney after quick freezing. 1.Osmolality gradient in renal medullary tissue of antidiuretic rabbit. The high water permeability allows osmotic equilibration of urine with the medullary interstitial fluid.įig. ![]() The high medullary interstitial osmolality provides a driving force for osmotic water flow across the collecting ducts, which are rendered permeable to water through the action of vasopressin ( 33). The classic micropuncture studies of Gottschalk and Mylle ( 17) have established that the medullary hypertonicity is due to solute accumulation in all structures in the medulla, including loops of Henle, vasculature, and collecting ducts. 1, have demonstrated that the urinary concentrating process is associated with the generation of a corticomedullary osmolality gradient in the medullary tissue, oriented with the maximum osmolality in the deepest part of the inner medulla, i.e., at the papillary tip. A large number of studies, exemplified by the data shown in Fig. It does this by concentrating the solutes in the urine to osmolalities that markedly exceed the osmolality of plasma. In states of fluid deprivation or nonrenal water loss, the kidney can conserve water while maintaining excretion of solutes. These considerations set the stage for renewed experimental investigation of the urinary concentrating process and a new generation of mathematical models of the renal concentrating mechanism, which treat the inner medullary interstitium as a viscoelastic system rather than a purely hydraulic system. 4) We discuss the special properties of hyaluronan, a glycosaminoglycan that is the chief component of a gel-like renal inner medullary interstitial matrix, which may allow it to function as a mechano-osmotic transducer, converting energy from the contractions of the pelvic wall to an axial osmolality gradient in the medulla. ![]() 3) We summarize and expand on the Schmidt-Nielsen hypothesis that the contractions of the renal pelvocalyceal wall may provide an important energy source for concentration in the inner medulla. 2) We review the major hypotheses that have been proposed to explain the axial osmolality gradient in the interstitium of the renal inner medulla. 1) We summarize a theoretical basis for classifying all possible steady-state inner medullary countercurrent concentrating mechanisms based on mass balance principles. The purposes of this review are fourfold. Although the concentrating process in the renal outer medulla is well understood, the concentrating mechanism in the renal inner medulla remains an enigma.
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