Effects of diuretic states on collecting duct fluid flow resistance in the hamster kidney.

Hydrostatic pressures were measured in cortical tubules, long loops of Henle, terminal collecting ducts, and in vasa recta in hamsters. In hydropenia, the loops of Henle and terminal collecting ducts provided the major fluid flow resistances, as judged by the location of hydrostatic pressure drops. In mannitol or saline diuresis, hydrostatic pressures in all tubular segments increased, but pressure drops in loops of Henle disappeared, indicating dilatation of loops. The major pressure drop was in terminal collecting ducts, especially in ducts of Bellini, even though these tubular segments also dilated. At highest urine flows, cortical tubule pressures were higher with the ureter and renal pelvis intact than when they were excised, suggesting laminar flow in the ureter adds a flow resistance at high flows. The pressure drop across the medullary capillary bed was 1-2 mmHg. The summation of medullary capillary hydrostatic and colloid osmotic pressures favored fluid uptake from the interstitium, a relationship enhanced by vasa recta geometry which ensures that descending vasa recta offer 4 times the flow resistance of ascending vessels.