the benefit of the counter-current system of the loop of Henle and Vasa recta?

The Loop of Henle is the part of the tubule which dips or "loops" from the cortex into the medulla, (descending limb), and then returns to the cortex, (ascending limb). It is this part of the tubule where urine is concentrated if necessary. This is possible because of the high concentration of solute in the substance or interstitium of the medulla. This high concentration of solutes is maintained by the counter current multiplier. A counter current multiplier system is an arrangement by which the high medullary interstitial concentration of solute is maintained, giving the kidney the ability to concentrate urine. The loop of Henle is the counter current multiplier and the vasa recta is the counter current exchanger, the mechanism being described below.

Actions of different parts of the loop of Henle (see fig 2 - IN LINK):


A: The descending loop of Henle is relatively impermeable to solute but permeable to water so that water moves out by osmosis, and the fluid in the tubule becomes hypertonic.

B: The thin section of the ascending loop of Henle is virtually impermeable to water, but permeable to solute especially sodium and chloride ions. Thus sodium and chloride ions move out down the concentration gradient, the fluid within the tubule becomes firstly isotonic then hypotonic as more ions leave. Urea which was absorbed into the medullary interstium from the collecting duct, diffuses into the ascending limb. This keeps the urea within the interstitium of the medulla where it also has a role in concentrating urine.

C: The thick section of the ascending loop of Henle and early distal tubule are virtually impermeable to water. However sodium and chloride ions are actively transported out of the tubule, making the tubular fluid very hypotonic.


The Vasa Recta (figure 3 - IN LINK) is a portion of the peritubular capillary system which enters the medulla where the solute concentration in the interstitium is high. It acts with the loop of Henle to concentrate the urine by a complex mechanism of counter current exchange. If the vasa recta did not exist, the high concentration of solutes in the medullary interstitium would be washed out.