Architecture of inner medullary descending and ascending vasa recta: Pathways for countercurrent exchange

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Abstract

Pathways and densities of descending vasa recta (DVR) and ascending vasa recta (AVR) in the outer zone of the inner medulla (IM) were evaluated to better understand medullary countercurrent exchange. Nearly all urea transporter B (UT-B)-positive DVR, those vessels exhibiting a continuous endothelium, descend with little or no branching exclusively through the intercluster region. All DVR have a terminal fenestrated (PV-1-positive) segment that partially overlaps with the UT-B-positive segment. This fenestrated segment descends a distance equal to ∼15% of the length of the connecting UT-B-positive segment before formation of the first branch. The onset of branching is indicative of vessel entry into the intracluster region. The number density of UT-B-positive DVR at 3,000 μm below the OM-IM boundary is ∼60% lower than the density at 400 μm below the OM-IM boundary, a result of DVR joining to fenestrated interconnecting vessels and an overall decline in UT-B expression. AVR that lie in the intercluster region (designated AVR2) lie distant from CDs and ascend to the OM-IM boundary with little or no branching. AVR2a represent a subcategory of AVR2 that abut DVR. The mean DVR length (combined UT-B- and PV-1-positive segments) nearly equals the mean AVR 2a length, implying a degree of overall equivalence in fluid and solute countercurrent exchange may exist. The AVR2/DVR ratio is ∼2:1, and the AVR2a/DVR ratio is ∼1:1; however, the AVR/DVR ratio determined for the full complement of fenestrated vessels is ∼4:1. The excess fenestrated vessels include vessels of the intracluster region (designated AVR1). Countercurrent exchange between vasa recta occurs predominantly in the intercluster region. This architecture supports previous functional estimates of capillary fluid uptake in the renal IM.

Original languageEnglish (US)
Pages (from-to)F265-F272
JournalAmerican Journal of Physiology - Renal Physiology
Volume299
Issue number1
DOIs
StatePublished - Jul 1 2010

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Keywords

  • Computer-assisted reconstruction
  • Concentrating mechanism
  • Countercurrent system
  • NaCl transport
  • Urea transport

ASJC Scopus subject areas

  • Physiology
  • Urology

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