The Kidneys and Regulation of Water and Inorganic Ions
505
segment becomes isoosmotic to the interstitial fl
uid and
peritubular plasma of the cortex—that is, until it is once
again at 300 mOsmol/L.
The isoosmotic tubular fl uid then enters and fl ows
through the
medullary
collecting ducts. In the presence of
high plasma concentrations of vasopressin, water diffuses out
of the ducts into the medullary interstitial fl uid as a result of
the high osmolarity that the loop countercurrent multiplier
system and urea trapping establish there. This water then
enters the medullary capillaries and is carried out of the kid-
neys by the venous blood. Water reabsorption occurs all along
the lengths of the medullary collecting ducts so that, in the
presence of vasopressin, the fl uid at the end of these ducts has
essentially the same osmolarity as the interstitial fl
uid sur-
rounding the bend in the loops—that is, at the bottom of the
medulla. By this means, the fi nal urine is hyperosmotic. By
retaining as much water as possible, the kidneys minimize the
rate at which dehydration occurs during water deprivation.
In contrast, when plasma vasopressin concentration is
low, both the cortical and medullary collecting ducts are rel-
atively impermeable to water. As a result, a large volume of
hypoosmotic urine is excreted, thereby eliminating an excess
of water in the body.
The Medullary Circulation
A major question arises with the countercurrent system as
described previously: Why doesn’t the blood fl owing through
medullary capillaries eliminate the countercurrent gradient
set up by the loops of Henle? One would think that as plasma
with the usual osmolarity of 300 mOsm/L enters the highly
concentrated environment of the medulla, there would be
massive net diffusion of sodium and chloride into the capil-
laries and water out of them, and thus the interstitial gradient
would be “washed away.” However, the blood vessels in the
medulla (vasa recta) form hairpin loops that run parallel to
the loops of Henle and medullary collecting ducts. As shown
in
Figure 14–18
, blood enters the top of the vessel loop at
an osmolarity of 300 mOsm/L, and as the blood fl ows down
the loop deeper and deeper into the medulla, sodium and
chloride do indeed diffuse into, and water out of, the vessel.
However, after the bend in the loop is reached, the blood
then fl ows up the ascending vessel loop, where the process
= Active transport
= Facilitated diffusion
= Diffusion
Descending
limb
Ascending
limb
Urea
Urea
Medullary
collecting
duct
Cortical
collecting
duct
Distal convoluted
tubule
300
300
300
600
900
100
100
80
400
700
1200
600
900
1200
1400
300
900
1200
1400
1000
1400
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
NaCl
Figure 14–17
Simplifi ed depiction of the generation of an interstitial fl
uid
osmolarity gradient by the renal countercurrent multiplier system
and its role in the formation of hyperosmotic urine in the presence
of vasopressin. Notice that the hyperosmotic medulla depends on
NaCl reabsorption and urea trapping (described in Figure 14–19).
Figure 14–17
physiological
inquiry
Certain types of lung tumors secrete one or more hormones.
What would happen to plasma and urine osmolarity and urine
volume in a patient with a lung tumor that secretes vasopressin?
Answer can be found at end of chapter.
Figure 14–18
Function of the vasa recta to maintain the hypertonic interstitial
renal medulla. All movements of water and solutes are by diffusion.
Not shown is the simultaneously occurring uptake of interstitial
fl uid by bulk fl ow.
325
300
Interstitial fluid
475
350
625
425
775
575
925
725
1075
875
1200
450
375
600
750
900
1050
1200
1200
1025
H
Solutes
(mainly Na
+
and Cl
)
Ascending
limb of
vasa recta
Descending
limb of
vasa recta
2
O
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