Flow rate of urine as a determinant of renal countercurrent multiplier system

¹ Department of Physiology, University of Michigan, Ann Arbor, Michigan

The mathematical treatment of the renal countercurrent multiplier system predicts that the rates of flow of urine and blood through the medullary loops are determinants of the maximal osmolarity which may be established in the medullary interstitium. It was found that acute increases in the urine flow rate resulted in a concomitant increase in the sodium concentration of the urine. The relation between urine flow rates and urine osmolarity was studied more closely. During osmotic diuresis the GFR of one kidney was reduced independently of the renal plasma flow by partially occluding one ureteral catheter. This resulted in an increase in urine osmolarity from that kidney. The increase in urine osmolarity correlated with an increase in the medullary sodium gradient. A smooth curve was obtained relating urine osmolarity to percent reduction of GFR. Urine osmolarity increased as the GFR was reduced, reaching a maximum at 70% reduction in GFR. Greater reductions of GFR resulted in decreasing osmolarities.