794
UNIT FIVE
Regulation of Urine Concentration
and Volume
Hormones such as aldosterone and ANP affect the solute
concentration of urine, particularly sodium. However, the
ability of the kidneys to maintain the internal environment
rests in large part on their ability to concentrate urine by
reabsorbing large volumes of water.
In contrast to conditions in the proximal convoluted
tubule, the tubular fl
uid reaching the distal convoluted tubule
is hypotonic because of changes that occur through the loop
segment of each nephron. The cells lining the distal convo-
luted tubule and the collecting duct that follows continue
to reabsorb sodium ions (chloride ions follow passively)
under the infl
uence of aldosterone, which the adrenal cortex
secretes (see chapter 13, p. 506). In addition, the interstitial
fl uid surrounding the collecting ducts is hypertonic, particu-
larly in the medulla. These might seem to be ideal conditions
for water reabsorption as well. However, the cells lining the
later portion of the distal convoluted tubule and the collect-
ing duct are impermeable to water unless antidiuretic hor-
mone (ADH) is present. Thus, water inside the tubule may
be excreted, forming dilute urine.
As discussed in chapter 13 (p. 498), neurosecretory
cells in the hypothalamus produce ADH. The posterior lobe
of the pituitary gland releases ADH in response to decreasing
Most of the potassium ions in the glomerular F ltrate are
actively reabsorbed in the proximal convoluted tubule, but
some may be secreted in the distal convoluted tubule and
collecting duct. During this process, the active reabsorption
of sodium ions out of the tubular fl
uid under the infl
uence of
aldosterone produces a negative electrical charge in the tube.
Positively charged potassium ions (K
+
) are attracted to nega-
tively charged regions, so these ions move passively through
the tubular epithelium and enter the tubular fl
uid. Potassium
ions are also actively secreted
(f
g. 20.23)
.
To summarize, urine forms as a result of the following:
Glomerular F
ltration of materials from blood plasma.
Tubular reabsorption of substances, including glucose;
water; urea; proteins; creatine; amino, lactic, citric, and
uric acids; and phosphate, sulfate, calcium, potassium,
and sodium ions.
Tubular secretion of substances, including penicillin,
histamine, phenobarbital, hydrogen ions, ammonia, and
potassium ions.
PRACTICE
18
Def
ne
tubular secretion.
19
Which substances are actively secreted? Passively secreted?
20
How does the reabsorption oF sodium a±
ect the secretion oF
potassium?
TABLE
20.2
|
Average Values for Sodium and Water Filtration, Reabsorption, and Excretion
Amount Filtered per Day
Amount Reabsorbed per Day (%)
Amount Excreted per Day
Water (L)
180
178.2 (99%)
1.8 (1%)
Na
+
(g)
630
626.8 (99.5%)
3.2 (0.5%)
FIGURE 20.23
In the distal convoluted tubule, potassium ions (or hydrogen ions) may be passively secreted in response to the active
reabsorption oF sodium ions.
Blood flow
Peritubular capillary
Distal convoluted tubule
Ascending limb
of nephron loop
Na
+
Na
+
Na
+
Na
+
Na+
Na+
Na+
Na
+
Na+
Na
+
K
+
K
+
K
+
K
+
H
+
H
+
Tubular fluid
Collecting duct
Tubular reabsorption
Tubular secretion
K
+
or H
+
Na
+
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