Direct determination of vasa recta blood flow in the rat renal papilla. Circ Res 53: 401–413

SUMMARY. Blood flow in vasa recta capillaries of the exposed renal papilla of young antidiuretic rats (n = 18) was determined by an adaptation of the video-photometric technique of Intaglietta. The erythrocyte velocity and capillary diameter in vasa recta ( « = 97) were measured at the same location by means of fluorescence video microscopy, with fluorescein-labeled bovine 7-globulin as a plasma marker. A factor relating erythrocyte velocity to mean cross-sectional blood velocity was determined in vitro to permit the calculation of single vasa recta blood flows from the measured indices, erythrocyte velocity and capillary diameter. Mean blood flow in descending vasa recta was 8.83 ± 0.96 (SE) nl/min, significantly greater than that in ascending vasa recta, 4.82 ± 0.34 nl/min. The total numbers of ascending and descending vasa recta at the base of the exposed papilla were also determined. Over 1500 vasa recta were identified as ascending vasa recta or descending vasa recta in electron micrographs of three papillas. At this level in the papilla (2 mm from the tip), there were four ascending vasa recta for each descending vas rectum. From the total numbers of ascending vasa recta and descending vas rectum, single vessel blood flows were converted to total blood flow. Total blood outflow in all ascending vasa recta, 11.3 /xl/min, substantially exceeded total blood inflow in all descending vasa recta, 5.2 fil/min. The differenc