1. We have investigated the hypothesis that ascending vasa recta (AVR) in the rat renal medulla are able to remain open when the external pressure is greater than the internal. 2. Individual vasa recta were cannulated in anaesthetized rats with Evans Blue albumin solution and then occluded downstream prior to the first branchpoint. When the intraluminal pressure was lowered, the lumina collapsed at a mean pressure of approximately -4.0 cmH2O for both AVR and descending vasa recta. 3. The studies were extended to include microvessels from rat spinotrapezius muscle and mesentery and frog mesentery; mean closing pressures were -3.2, -4.2 and -5.3 cmH2O, respectively. 4. Subatmospheric closing pressures may allow small differences in hydrostatic pressure alone to drive fluid uptake into the AVR.
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