OUTER MEDULLARY ANATOMY AND THE URINE CONCENTRATING MECHANISM

In earlier work, mathematical models of the urine concentration mechan ism were developed incorporating the features of renal anatomy. Howeve r, several anatomic observations showed inconsistencies in the modelin g representation of tile outer stripe (OS) anatomy. In this study, bas ed on observations from comparative anatomy and morphometric studies, we propose a new structural model of outer medullary anatomy, differen t from that previously presented [A. S. Wexler, R. E. Kalaba, and D. J . Marsh. Am. J. Physiol. 260 (Renal Fluid Electrolyte Physiol. 29): F3 68-F383, 1991]. The modifications include the following features of ra t outer medullary anatomy, for example, 1) in the OS, the limbs of lon g loops of Henle surround the descending and ascending vasa recta that develop into the vascular bundles in the inner stripe (IS), whereas t he limbs of short loops are close to the collecting ducts; and 2) the descending limbs of short loops shift from the tubular region in the O S to near the vascular bundle in the IS, whereas the limbs of long loo ps are situated away from the vascular bundles in the tubular region. Tile sensitivity of the concentrating process to the relative position of loops and vessels was investigated in the different medullary regi ons. With these modifications, the model predicts a more physiological , axial osmolarity gradient in both outer and inner medulla with membr ane parameters that are all in the range of measured physiological val ues, including the urea permeabilities of descending vasa recta report ed by Pallone and co-workers (T. L. Pallone, J. Work, R. L. Myers, and R. L. Jamison. J. Clin. Invest. 93: 212-222, 1994).