Thin Limbs of Henle and Inner Medullary Collecting Ducts are the Major Points of Flow Resistance Along the Renal Tubule

Abstract

The driving force for flow along the nephron and collecting ducts is generally thought to derive from the pressure in the glomerular capsule (Bowman's space). In superficial nephrons of rats, the glomerular capsular pressure has been reported to be 12–13 mmHg under euvolemic conditions. To test the adequacy of this driving force, we calculated the theoretical pressure drop along each segment of the rat superficial renal tubule using the Hagen‐Poiseuille laminar flow equation with measured flow rates, tubule diameters, and tubule lengths. We used both a rigid tube model and an elastic tube model based on compliances from the literature. The rigid tube model predicted a net pressure drop from glomerular capsule to renal pelvis of 8.8 mmHg during antidiuresis and 15.5 mmHg during water diuresis. The latter value exceeds the measured value in the glomerular capsule. In contrast, the elastic tube model predicted a 9.5 mmHg pressure drop along the length of the renal tubule during water diuresis. In both models, the greatest flow resistance (pressure drop per unit length) was in the thin descending limb of Henle's loop and the final millimeter of the inner medullary collecting duct. We extended the elastic tube model during water diuresis to long‐looped nephrons (those with loops of Henle extending into the inner medulla). The predicted net tubular pressure drop for these nephrons was 18.3–20.3 mmHg depending on the location of the loop bend. The calculated overall flow resistance was higher in deep nephrons chiefly due to the thin descending and thin ascending limbs of Henle's loop. We do not have reliable glomerular capsular pressure values for deep nephrons. However, the glomerular capsular pressures in deep nephrons are unlikely to be substantially higher than in superficial nephrons because the single‐nephron glomerular filtration rate in deep nephrons is equal to or greater than in superficial nephrons. Thus, the calculated glomerular capsular pressure is 6–8 mmHg higher than the likely actual value with the elastic tube model. This leaves an apparent discrepancy that could be explained either by 1) an underestimate of the compliance of the thin limb segments of the long‐looped nephrons; or 2) an additional driving force for flow such as the peristaltic action of the renal pelvic contractions, which could boost the flow across the restriction points in the thin descending limbs and inner medullary collecting duct.Support or Funding InformationG.G. was a member of the 2017 NIH Summer Internship Program. V.D. was a member of the 2017 Biomedical Engineering Student Internship Program at the NIH.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.