Countercurrent flow (flow in opposite directions) in the ascending and descending limbs and the close proximity of the two limbs allow for interaction between them. Since the concentration of the tubular fluid in the descending limb reflects the concentration of surrounding interstitial fluid, and since the concentration of this fluid is raised by the active extrusion of salt from the ascending limb, a positive feedback mechanism is created. The more salt the ascending limb extrudes, the more concentrated will be the fluid that is delivered to it from the descending limb. This positive feedback mechanism multiplies the concentration of interstitial fluid and descending limb fluid, and is thus called the countercurrent multiplier system.
Loop of Henle
Passively permeable to water
Active transport of Na+, Cl- follows passively; impermeable to water
■ Figure 17.16 The countercurrent multiplier system. The extrusion of sodium chloride from the ascending limb makes the surrounding interstitial fluid more concentrated. Multiplication of this concentration is due to the fact that the descending limb is passively permeable to water, which causes its fluid to increase in concentration as the surrounding interstitial fluid becomes more concentrated. The values of these changes in osmolality, together with the effect on surrounding interstitial fluid concentration, are shown in milliosmolal units.
The countercurrent multiplier system recirculates salt and thus traps some of the salt that enters the loop of Henle in the interstitial fluid of the renal medulla. This system results in a gradually increasing concentration of renal interstitial fluid from the cortex to the inner medulla; the osmolality of interstitial fluid increases from 300 mOsm (isotonic) in the cortex to between 1,200 and 1,400 mOsm in the deepest part of the medulla. This hypertonicity is required for water reabsorption, as will be explained shortly.
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