BSC 215 Lecture Notes - Lecture 56: Descending Limb Of Loop Of Henle, Collecting Duct System, Renal Medulla
11 Jun 2018
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Regulation of Urine Concentration
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.
o 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.
o 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
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