Thomas L. Pallone, MD
University of Maryland at Baltimore
Division of Nephrology
Department of Medicine
Baltimore, MD 21201
The microcirculation of the renal medulla is arranged as a countercurrent exchanger. It is generally accepted that this facilitates efficient diffusional trapping of the principle solutes, NaCl and urea to favor establishment of corticomedullary gradient generated by Henle's loops. In contrast to the simplistic view of highly permeable microvessels immersed in corticomedullary solute gradients, a number of more subtle complexities are now understood. There are marked microanatomical differences between descending (DVR) and ascending vasa recta (AVR) that are reflected by similar differences in their hydraulic conductivity. DVR express the aquaporin 1 water channel across which NaCl and urea drive water flux from the lumen to the medullary interstitium in the hydropenic kidney. DVR endotheia also express a facilitated urea carrier that probably serves to efficiently trap urea between AVR and DVR lumen. DVR are surrounded by contractile perictyes and respond to a variety of hormones and autacoids by vasoconstriction and dilation. Apart from their role in urinary concentration, some evidence favors participation of vasa recta in the regulation of NaCl excretion through modulation of medullary perfusion.