The effects of collecting duct active NaCl reabsorption and inner medulla anatomy on renal concentrating mechanism

Recent measurements have pointed out two flaws in the three-dimensional model of A. S. Wexler, R. E. Kalaba, and D. J. Marsh [Am. J. Physiol. 260 (Renal Fluid Electrolyte Physiol. 29): F368-383, 1991]. First, the representation of the inner medulla incorporates an exaggerated radial separation between tubules, vessels, and collecting ducts; and, second, the hydraulic permeability in the upper portion of the inner medullary collecting ducts was erroneously set to zero. In the current work, we explore the role of collecting duct hydraulic permeability and anatomical heterogeneity via mathematical modeling. The model predicts concentrated urine for measured values of the hydraulic permeability and homogeneous lower inner medulla as long as net active NaCl reabsorption is incorporated in the upper inner medullary collecting duct epithelium. This new three-dimensional model results in two recycling paths. The upper portion of the inner medulla recycles NaCl, whereas the lower portion recycles urea. kidney; renal anatomy; inner medulla; water permeability; urine concentrating mechanism; mathematical model