Microcirculation : the official journal of the Microcirculatory Society, Inc
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The architecture of the renal medullary microcirculation is highly specialized. Consistent with their role in countercurrent exchange, the vessels (the vasa recta and the intervening capillaries) have high permeabilities to fluid and small hydrophilic solutes. The urea permeability of the continuous endothelium of the descending vasa recta (DVR) in the outer medulla is greatly enhanced by a urea transporter. ⋯ Because the AVR have high Lp's [10(-5) cm s-1 (cm H2O)-1] and are mechanically linked to surrounding structures, small increments of ISF pressures above the pressure within the AVR can drive significant volumes of fluid into AVR if ISF volume expands. The lower reflection coefficients to serum albumin of the AVR as compared with the DVR may be important in the clearance of interstitial plasma protein. Recent work on isolated DVR from the outer medulla has revealed that these vessels are capable of vasoconstriction and thus of regulating medullary blood flow.