Thick Ascending Loop of Henle Transport

Overview
  • The thick ascending loop of Henle is a major resorptive segment of the nephron and accounts for resorption of nearly a quarter of the filtered load of sodium, chloride, and potassium ions. In addition, Henle's thick segment is a major location of magnesium and calcium ion resorption. Importantly, the tight junctions of this segment are virtually impermeable to water. Given the large amount of solute resorption that occurs in the absence of water resorption, the tubular fluid becomes progressively dilute as it travels through the thick ascending loop. This feature is why this segment is frequently referred to as the "Diluting Segment" of the nephron.

Thick Ascending Loop of Henle Transport
Thick Ascending Loop of Henle Transport is characterized by a Na-2Cl-K symporter on the luminal surface that allows for resorption of large amounts of these ions. Resorption is powered by a basolateral Na-K ATPase. Importantly, the ascending Henle is highly impermeable to water and the resorption of large amounts of sodium in the absence of water results in significant dilution of the tubular fluid. A small amount of potassium back-leak into the lumen via a potassium channel yields a positive luminal charge that powers paracellular resorption of positive ions, including magnesium and calcium.

Transport Mechanisms
  • The resorptive capacity of the thick ascending henle is largely powered by a basolateral NaK ATPase that generates low Na+ concentrations within the tubular epithelial cells. This basolateral primary active transport of sodium is then used to actuate luminal resorption of sodium as well as other solutes. Luminal resorption of sodium is molecularly linked to that of chloride and potassium via a single entity known as the Na2ClK Symporter which allows the electrochemical gradient for sodium to power luminal secondary active transport of chloride and potassium. Importantly, the luminal membrane of tubular epithelial cells in the ascending henle display a mild intrinsic permeability to potassium, allowing backleak of some potassium ions into the lumen. This backleak of positive potassium ions generates a slight positive potential in the luminal fluid which is thought to actuate paracellular resorption of a host of positively charged ions, including sodium and potassium as well as magnesium and calcium.