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REVIEW

Role of three-dimensional architecture in the urine concentrating mechanism of the rat renal inner medulla

¹Department of Physiology, College of Medicine, University of Arizona, Tucson, Arizona; and ²Department of Mathematics, Duke University, Durham, North Carolina

Submitted 15 April 2008 ; accepted in final form 19 May 2008

ABSTRACT

Recent studies of three-dimensional architecture of rat renal inner medulla (IM) and expression of membrane proteins associated with fluid and solute transport in nephrons and vasculature have revealed structural and transport properties that likely impact the IM urine concentrating mechanism. These studies have shown that 1) IM descending thin limbs (DTLs) have at least two or three functionally distinct subsegments; 2) most ascending thin limbs (ATLs) and about half the ascending vasa recta (AVR) are arranged among clusters of collecting ducts (CDs), which form the organizing motif through the first 3–3.5 mm of the IM, whereas other ATLs and AVR, along with aquaporin-1-positive DTLs and urea transporter B-positive descending vasa recta (DVR), are external to the CD clusters; 3) ATLs, AVR, CDs, and interstitial cells delimit interstitial microdomains within the CD clusters; and 4) many of the longest loops of Henle form bends that include subsegments that run transversely along CDs that lie in the terminal 500 µm of the papilla tip. Based on a more comprehensive understanding of three-dimensional IM architecture, we distinguish two distinct countercurrent systems in the first 3–3.5 mm of the IM (an intra-CD cluster system and an inter-CD cluster system) and a third countercurrent system in the final 1.5–2 mm. Spatial arrangements of loop of Henle subsegments and multiple countercurrent systems throughout four distinct axial IM zones, as well as our initial mathematical model, are consistent with a solute-separation, solute-mixing mechanism for concentrating urine in the IM.

kidney; countercurrent system; computer-assisted reconstruction; functional anatomy; NaCl transport; urea transport; mathematical model

This article has been cited by other articles:

				T. L. Pannabecker Loop of Henle interaction with interstitial nodal spaces in the renal inner medulla Am J Physiol Renal Physiol, December 1, 2008; 295(6): F1744 - F1751. [Abstract] [Full Text] [PDF]