Anatomy and Physiology

The nephron loop of juxtamedullary nephrons is the apparatus that allows the nephron to concentrate urine. The loop is a countercurrent multiplier system in which fluids move in opposite directions through side‐by‐side, semi‐permeable tubes. Substances are transported horizontally, by passive or active mechanisms, from one tube to the other. The movement of the transported substances up and down the tubes results in a higher concentration of substances at the bottom of the tubes than at the top of the tubes. Details of the process follow and are also shown in Figure 1:

1. The descending limb of the nephron loop is permeable to H ₂O, so H ₂O diffuses out into the surrounding fluids. Because the loop is impermeable to Na ⁺ and Cl ^– and because these ions are not pumped out by active transport, Na ⁺ and Cl ^– remain inside the loop.

2. As the fluid continues to travel down the descending limb of the loop, it becomes more and more concentrated, as water continues to diffuse out. Maximum concentration occurs at the bottom of the loop.

3. The ascending limb of the nephron loop is impermeable to water, but Na ⁺ and Cl ^– are pumped out into the surrounding fluids by active transport.

4. As fluid travels up the ascending limb, it becomes less and less concentrated because Na ⁺ and Cl ^– are pumped out. At the top of the ascending limb, the fluid is only slightly less concentrated than at the top of the descending limb. In other words, there is little change in the concentration of the fluid in the tubule as a result of traversing the nephron loop.

5. In the fluid surrounding the nephron loop, however, a gradient of salt (Na ⁺, Cl ^–) is established, increasing in concentration from the top to the bottom of the loop.

   • Fluid at the top of the collecting duct has a concentration of salts about equal to that at the beginning of the nephron loop (some water is reabsorbed in the DCT). As the fluid descends the collecting duct, the fluid is exposed to the surrounding salt gradient established by the nephron loop. Without ADH, the collecting duct is impermeable to H ₂O. Two outcomes are possible:>

     • If water conservation is necessary, ADH stimulates the opening of water channels in the collecting duct, allowing H ₂O to diffuse out of the duct and into the surrounding fluids. The result is concentrated urine (refer to Figure 1).

     • If water conservation is not necessary, ADH is not secreted and the duct remains impermeable to H ₂O. The result is dilute urine.

   • The vasa recta delivers O ₂ and nutrients to cells of the nephron loop. The vasa recta, like other capillaries, is permeable to both H ₂O and salts and could disrupt the salt gradient established by the nephron loop. To avoid this, the vasa recta acts as a countercurrent multiplier system as well. As the vasa recta descends into the renal medulla, water diffuses out into the surrounding fluids, and salts diffuse in. When the vasa recta ascends, the reverse occurs. As a result, the concentration of salts in the vasa recta is always about the same as that in the surrounding fluids, and the salt gradient established by the nephron loop remains in place.

Figure 1. The loop is a countercurrent multiplier system in which fluids move in opposite directions through side-by-side, semi-permeable tubes. This process regulates the concentration of urine.