798
P A R T 9
Drugs acting on the renal system
In contrast, the ascending loop of Henle is imper-
meable to water, and so water that remains in the tubule
is trapped there. Chloride is actively transported out of
the tubule using energy in a process that is referred to as
the chloride pump; sodium leaves with the chloride to
maintain electrical neutrality. As a result, the fluid in the
ascending loop of Henle becomes hypotonic in compari-
son to the hypertonic situation in the peritubular tissue.
Antidiuretic hormone (ADH)
, which is produced
by the hypothalamus and stored in the posterior pitu-
itary gland, is important in maintaining fluid balance.
ADH is released in response to falling blood volume,
sympathetic stimulation, or rising sodium levels (a con-
centration that is sensed by the osmotic cells of the
hypothalamus).
If ADH is present at the distal convoluted tubule and
the collecting duct, the permeability of the membrane to
water is increased. Consequently, the water remaining in
the tubule rapidly flows into the hypertonic tissue sur-
rounding the loop of Henle, where it either is absorbed
by the peritubular capillaries or re-enters the descending
loop of Henle in a countercurrent style. The resulting
urine is hypertonic and of small volume. If ADH is
not present, the tubule remains impermeable to water.
The water that has been trapped in the ascending loop
of Henle passes into the collection duct, resulting in
hypotonic urine of greater volume. This countercur-
rent mechanism allows the body to finely regulate fluid
volume by regulating the control of sodium and water
(Figure 50.3).
Chloride regulation
Chloride is an important negatively charged ion that
helps to maintain electrical neutrality with the movement
of cations across the cell membrane. Chloride is prim
arily reabsorbed in the loop of Henle, where it promotes
the movement of sodium out of the cell.
Potassium regulation
Potassium is another cation that is vital to proper function-
ing of the nervous system, muscles and cell membranes.
About 65% of the potassium that is filtered at the glomer-
ulus is reabsorbed at Bowman’s capsule and the proximal
convoluted tubule. Another 25% to 30% is reabsorbed
in the ascending loop of Henle. The fine-tuning of potas-
sium levels occurs in the distal convoluted tubule, where
aldosterone activates the sodium–potassium exchange,
leading to a loss of potassium. If potassium levels are
very high, the retention of sodium in exchange for potas-
sium also leads to a retention of water and a dilution of
blood volume, which further decreases the potassium
concentration (see Figure 50.3).
Bowman’s
capsule
65%
Impermeable to Na
+
/Cl
-
Impermeable to
H
2
O
H
2
O
,
Na
+
,
Cl
-
Ca
++
,
K
+
Proximal
convoluted
tubule
Distal
convoluted
tubule
Collecting
duct
Loop of Henle
ADH
ADH
Efferent arteriole
Glomerulus
Afferent arteriole
PTH
Aldosterone
H
2
O
H
2
O
Ca
++
,
H
2
O
K
+
Na
+
H
2
O
,
Na
+
,
Cl
-
Ca
++
,
K
+
FIGURE 50.3
Nephron and points of regulation of sodium, chloride, potassium, calcium and water. ADH, antidiuretic hormone;
PTH, parathyroid hormone.
