Fluid uptake by vasa recta was determined by two independent methods, videomicroscopy and the micropuncture technique, in the exposed papilla of nine antidiuretic rats to reconcile differences in values previously obtained by the two techniques. Erythrocyte velocity (Vrbc) and diameter (D) in descending vasa recta (DVR) (n = 22) and ascending vasa recta (AVR) (n = 31) near the "base" of the papilla were measured. Using a conversion function determined in vitro, Vrbc was transformed into mean blood velocity (Vblood). From D and Vblood, mean blood flow (Q) in DVR and AVR was calculated. In DVR, mean Vrbc, D, and Q were 1.06 +/- 0.01 mm/s, 16.3 +/- 0.4 micron, and 10.6 +/- 1.4 nl/min, respectively. In AVR, each corresponding value differed significantly, 0.47 +/- 0.06 mm/s (P less than 0.001), 19.8 +/- 0.8 micron (P less than 0.001), and 5.65 +/- 1.3 nl/min (P less than 0.025), respectively. Blood samples from DVR and AVR were obtained by micropuncture from the same location. Plasma protein concentration (g/dl) was 5.1 +/- 0.6 in DVR, 4.0 +/- 0.4 (P less than 0.05) in AVR, and 3.6 +/- 0.3 (P less than 0.025) in the renal vein. Assuming no net transcapillary loss of protein, total plasma outflow exceeded inflow by 29%, the excess representing fluid uptake; and to reconcile the blood flow and plasma protein concentrations found, functioning AVR should outnumber functioning DVR by a ratio of 2.1-2.4 to 1, depending on local hematocrit. Given the total number of AVR + DVR = 2,944 (at the base), capillary fluid uptake was calculated to range between 1.5 and 2.6 microliter/min.

Like other regional circulations, the medullary circulation supplies oxygen and other primary substrates to the medulla and removes carbon dioxide and other waste metabolites. It also acts as a countercurrent exchanger and simultaneously removes water reabsorbed from the renal tubule to preserve mass balance. Our present understanding of how the medulla serves both these functions at the same time is illustrated in Figure 3. Blood leaves the efferent arteriole with an elevated plasma protein concentration as a consequence of glomerular filtration, and flows down descending vasa recta within a vascular bundle. The increased interstitial osmotic-concentration coupled with a finite capillary reflection coefficient for small solutes causes additional water to be extracted so that at the termination of descending vasa recta, the plasma protein concentration exceeds that in the systemic circulation by approximately twofold. Solute, urea more than sodium chloride, also enters descending vasa recta. As blood flows through the interconnecting capillary plexus and up ascending vasa recta, transcapillary oncotic and osmotic pressure differences combine to cause capillary uptake of fluid. There is also simultaneous loss of urea such that the medullary trapping of urea is very effective. Countercurrent exchange of sodium chloride, however, appears to be less efficient and as a consequence, not only water but sodium chloride is removed from the medulla. Antidiuretic hormone reduces medullary blood flow, both directly by its vasoconstrictor (V1-receptor mediated) effect and indirectly by its antidiuretic (V2-receptor mediated) effects. Prostaglandins are able to enhance medullary blood flow by counteracting vasoconstrictive influences.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of arginine vasopressin (AVP) in the regulation of renal medullary blood flow is uncertain. To determine if AVP has a direct vasoconstrictive action on vasa recta, the effect of AVP on erythrocyte velocity (VRC), diameter, and blood flow (QvR) in descending vasa recta (DVR) and ascending vasa recta (AVR) was studied in the exposed renal papilla of four groups of chronically water diuretic rats using fluorescence videomicroscopy. There were three periods: control (period 1), experimental (period 2), and recovery (period 3). In periods 1 and 3, all groups received hypotonic saline. In

The microcirculation of the renal papilla was investigated in 32 vasa recta of Wistar rats. Using fluorescence microscopy in combination with a high-sensitivity television system we measured the velocity and flux of fluorescent-tagged erythrocytes in descending (DVR) and ascending vasa recta (AVR). After staining the plasma with fluorescent high molecular weight dextran we determined the diameters of DVR and AVR. Red cell flux (Qrbc) was determined from the ratio of the frequency of fluorescent-tagged red cells detected per unit time (fFITC) to the number of fluorescent-tagged red cells per nanoliter packed red cells (NFITC). From red cell velocity (Vrbc) and vessel diameter (D) we calculated the volume flow (Vapp). The dynamic hematocrit was directly derived as the ratio of Qrbc to Vapp. During antidiuresis Vrbc was 1.35 +/- 0.15 mm X s-1 (mean +/- SE) in DVR and 0.47 +/- 0.07 mm X s-1 in AVR. Qrbc in the same vessels averaged 3.26 +/- 0.9 and 1.72 +/- 0.35 nl X min-1, respectively. The diameter in DVR was 14.3 +/- 0.9 and in AVR 17.9 +/- 0.9 micron. From these values we calculated a dynamic hematocrit of 26 +/- 4 in DVR and 25 +/- 4% in AVR. The systemic hematocrit was 44 +/- 1%. The dynamic hematocrit in vasa recta represented 59 +/- 9 and 57 +/- 8% of the value in the systemic circulation, respectively.