Water-pumping turbulent flow in thin segments of the loop of Henle concentrates urine

Abstract

In the mammalian kidney, urine is concentrated by passing through the thin descending limb (TDL) of Henle's loop, correlated with an osmotic gradient in the interstitial fluid that is increased from the outer medulla (OM) to the tip of the in- ner medulla (IM). The water-pumping turbulence hypothe- sis states that both the urinary concentrat- ing and the osmotic gradient of the renal medulla result from turbulent urine flow in the TDL. In turbulent flow, molecules move crosswise as well as along the tubule so H 2 O molecules are driven out through aquaporins; this concentrates urine. Since the TDL has fewer aquaporins in the IM than in the OM, the IM interstitium re- ceives less water; this can establish an osmotic gradient. Turbulence in thin seg- ments results from a greater Reynolds' number (RN), the measure of tendency for turbulence, calculated based on velocity, density, and viscosity of urinary flow and tubule diameter. RN (and the tendency for turbulence) of urinary flow in the TDL is approximately 2 times the RN in a thick segment. It makes the flow a water-pump and results in urinary concentration and the osmotic gradient of the renal medulla. INTRODUCTION The mammalian kidney continuously changes water excretion and urinary concentration depending on different conditions in order to maintain a nearly constant blood plasma osmolality. For most mammals that do not have free access to water, the ability to change wa- ter excretion is essential for survival 1 . In order to change urinary concentration, nephrons have several mechanisms, the best known of which is urinary concen- tration through the generation of a pro- gressive osmotic gradient extending from the cortico-medullary boundary to the inner medullary tip 2 and a hyperos-