504
Chapter 14
Now back to the descending limb. This segment, in con-
trast to the ascending limb, does not reabsorb sodium chloride
and is highly permeable to water (
Figure 14–16b
). Therefore,
a net diffusion of water occurs out of the descending limb into
the more concentrated interstitial fl uid until the osmolarities
inside this limb and in the interstitial fl uid are again equal.
The interstitial hyperosmolarity is maintained during this
equilibration because the ascending limb continues to pump
sodium chloride to maintain the concentration difference
between it and the interstitial fl
uid.
Therefore, because of the diffusion of water, the osmolari-
ties of the descending limb and interstitial fl uid become equal,
and both are higher—by 200 mOsmol/L in our example—
than that of the ascending limb. This is the essence of the sys-
tem: the loop countercurrent multiplier causes the interstitial
uid of the medulla to become concentrated. It is this hyper-
osmolarity that will draw water out of the collecting ducts and
concentrate the urine. However, one more crucial feature—
the “multiplication”—must be considered.
So far we have been analyzing this system as though the
fl ow through the loop of Henle stops while the ion pumping
and water diffusion are occurring. Now, let us see what hap-
pens when we allow fl ow through the entire length of the
descending and ascending limbs of the loop of Henle (
Figure
14–16c
). The osmolarity difference—200 mOsmol/L—that
exists at each horizontal level is “multiplied” as the fl
uid
goes deeper into the medulla. By the time the fl
uid reaches
the bend in the loop, the osmolarity of the tubular fl
uid and
interstitium has been multiplied to a very high osmolar-
ity that can be as high as 1400 mOsmol/L
.
Keep in mind
that the active sodium chloride transport mechanism in the
ascending limb (coupled with low water permeability in this
segment) is the essential component of the system. Without
it, the countercurrent fl ow would have no effect on loop and
medullary interstitial osmolarity, which would simply remain
300 mOsmol/L throughout.
Now we have a concentrated medullary interstitial fl
uid,
but we must still follow the fl uid within the tubules from the
loop of Henle through the distal convoluted tubule and into
the collecting duct system, using
Figure 14–17
as our guide.
Furthermore, urea reabsorption and trapping (described in
detail later) contributes to the maximal medullary interstitial
osmolarity. The countercurrent multiplier system concen-
trates the descending-loop fl
uid, but then lowers the osmolar-
ity in the ascending loop so that the fl uid entering the distal
convoluted tubule is actually more dilute (hypoosmotic)—
100 mOsmol/L in Figure 14–17—than the plasma. The fl
uid
becomes even more dilute during its passage through the
distal convoluted tubule because this tubular segment, like
the ascending loop, actively transports sodium and chloride
out of the tubule but is relatively impermeable to water. This
hypoosmotic fl uid then enters the cortical collecting duct.
Because of the signifi cant volume reabsorption, the fl ow of
fl uid at the end of the ascending limb is much less than the
fl ow that entered the descending limb.
As noted earlier, vasopressin increases tubular perme-
ability to water in both the cortical and medullary collect-
ing ducts. In contrast, vasopressin does not infl uence water
reabsorption in the parts of the tubule prior to the collect-
ing ducts. Thus, regardless of the plasma concentration of
this hormone, the fl uid entering the cortical collecting duct
is hypoosmotic. From there on, however, vasopressin is
crucial. In the presence of high levels of vasopressin, water
reabsorption occurs by diffusion from the hypoosmotic
fl uid in the cortical collecting duct until the fl
uid in this
Descendi
0
0
0
3
0
0
0
1
Ascending
Descending
Ascending
Descending
Ascending
H
2
O
NaCl
H
2
O
NaCl
H
2
O
NaCl
H
2
O
NaCl
300
0
0
0
6
0
0
0
4
600
0
0
0
9
0
0
0
7
900
20
0
00
00
00
00
00
00
00
00
2
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
100
0
00
00
00
00
00
00
00
00
00
00
00
00
00
00
10
10
10
10
10
10
10
10
10
1
1200
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
Interstitial
osmolarity
= Active transport
= Diffusion
400
400
NaCl
NaCl
H
2
O
H
2
O
400
400
NaCl
NaCl
(a)
(b)
(c)
Figure 14–16
Generating a hyperosmolar medullary renal interstitium.(a) NaCl
active transport in ascending limbs (impermeable to H
2
O); (b) passive
reabsorption of H
2
O in descending limb; (c) add fl ow of fl
uid in
lumen resulting in multiplication of osmolarity.
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