Loop of Henle

Bowman’s capsules of cortical nephrons are in the cortex. These are the most abundant nephrons, accounting for some 80% of all nephrons. They have short loops of Henle. Bowman’s capsules of juxtamedullary nephrons are next to medulla. Their loops of Henle penetrate deep into the medulla along with the vasa recta capillaries.

Water reabsorption in the thin descending limb of the loop of Henle is driven by ion reabsorption in thick ascending limb. The direction of blood flow in the vasa recta is opposite to the flow in loop of Henle. This serves as a countercurrent exchanger that efficiently removes water from the tubules. In concurrent exchange, the two streams never come to equilibrium. Countercurrent exchange causes the osmolarity within the tubules to always be matched to the osmolarity of the surrounding vasa recta and interstitial fluids. The vasa recta remove water from the loop of Henle. Because active transport is used to build a high osmotic gradient, the loop of Henle is called a countercurrent multiplier.

The descending limb of the loop of Henle is permeable to water, which passes out by osmosis. Tubule fluid becomes more concentrated (matching the interstitial fluid) as it descends into the medulla. The thick ascending limb is impermeable to water. The NKCC2 transporter on the apical membrane transports Na-K-2Cl from the tubule lumen into the cells of the ascending limb. A Na-K ATPase on the basolateral membrane pumps Na into the peritubular space, increasing the osmolarity of the medulla. This sets up a gradient of increasing osmolarity that reaches a peak at the tip of the loop of Henle. The overall balance causes more ion reabsorption than water reabsorption, so that the fluid entering the distal convoluted tubule is hyposmotic.