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CHAPTER TWENTY
Urinary System
rounded by hypertonic fl
uid created by the ascending limb,
so water tends to leave the descending limb by osmosis. The
contents of the descending limb become more concentrated,
or hypertonic
(f
g. 20.25)
.
The very concentrated tubular fl uid now moves into the
ascending limb, and sodium chloride (NaCl) is again actively
reabsorbed into the medullary interstitial fl uid, raising the
interstitial NaCl concentration further. With the increased
interstitial fl uid solute concentration, even more water dif-
fuses out of the descending limb, further increasing the salt
concentration of the tubular fl uid. Each time this circuit is
completed, the concentration of NaCl increases, or multi-
plies. For this reason, the mechanism is called a
countercur-
rent multiplier.
In humans, this mechanism creates a tubular
uid solute concentration near the tip of the loop more than
four times the solute concentration of plasma (±
g. 20.25).
The descending limb of the loop is permeable to water,
so the interstitial fl
uid at any level of the loop is essentially
in equilibrium with the fl uid in the tubule. Thus, the con-
centration gradient in the loop is also found in the interstitial
uid. The solute concentration of the tubular fl
uid progres-
sively decreases as the fl uid moves toward the renal cortex
g. 20.25).
The vasa recta is another countercurrent mechanism
that maintains the NaCl concentration gradient in the renal
medulla. Blood fl ows slowly down the descending portion of
the vasa recta, and NaCl enters it by diffusion. Then, as the
blood moves back up toward the renal cortex, most of the NaCl
diffuses from the blood and reenters the medullary interstitial
fl uid. Consequently, the bloodstream carries little NaCl away
from the renal medulla, preserving the gradient
(f g. 20.26)
.
concentration of water in the body fl
uids or to decreasing
blood volume and blood pressure. When ADH reaches the
kidney, it stimulates cells in the distal convoluted tubules
and collecting ducts to insert proteins called aquaporins
into their cell membranes, which form water channels.
These channels greatly increase permeability to water; con-
sequently, water rapidly moves out of these structures by
osmosis, especially where the distal tubules and collecting
ducts pass through the extremely hypertonic medulla. The
urine becomes more concentrated, and water is retained in
the internal environment
(f
g. 20.24)
.
A
countercurrent mechanism
in the nephron loops,
particularly of the juxtamedullary nephrons, ensures that the
medullary interstitial fl
uid becomes hypertonic. This mecha-
nism is possible because the descending and ascending limbs
of the nephron loops lie parallel and close to one another.
The mechanism is named partly for the fact that fl
uid mov-
ing down the descending limb creates a current that is coun-
ter to that of the fl
uid moving up in the ascending limb.
The different parts of the nephron loop have important
functional distinctions. For example, the epithelial lining in
the thick upper portion of the ascending limb (thick segment)
is relatively impermeable to water. However, the epithe-
lium does actively reabsorb sodium and chloride ions (some
potassium is actively reabsorbed as well). As these solutes
accumulate in the interstitial fluid outside the ascending
limb, it becomes hypertonic, while the tubular fl
uid inside
becomes hypotonic because it is losing its solute.
In contrast to the ascending limb, the epithelium of
the descending limb (thin segment) is permeable to water,
but relatively impermeable to solutes. This segment is sur-
FIGURE 20.24
Urine concentrating mechanism. (
a
) The distal convoluted tubule and collecting duct are impermeable to water, so water may be
excreted as dilute urine. (
b
) If ADH is present, however, these segments become permeable, and water is reabsorbed by osmosis into the hypertonic
medullary interstitial F
uid.
Distal convoluted
tubule
Medullary
interstitial fluid
Medullary
interstitial fluid
Collecting duct
low ADH levels
high ADH levels
Dilute urine
Hypertonic
interstitial
fluid
Hypertonic
interstitial
fluid
Concentrated urine
Collecting duct
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
(a)
(b)
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