796
UNIT FIVE
TABLE
20.3
|
Role of ADH in Regulating Urine
Concentration and Volume
1. Concentration of water in the blood decreases.
2. Increase in the osmotic pressure of body F
uids stimulates
osmoreceptors in the hypothalamus.
3. Hypothalamus signals the posterior pituitary gland to release ADH.
4. Blood carries ADH to the kidneys.
5. ADH causes the distal convoluted tubules and collecting ducts to
increase water reabsorption by osmosis.
6. Urine becomes more concentrated, and urine volume decreases.
Urea and Uric Acid Excretion
Urea
is a by-product of amino acid catabolism in the liver.
Therefore, the amount of urea that must be eliminated in
the urine refl
ects the amount of protein in the diet. Urea
enters the renal tubule by F
ltration and is both reabsorbed
and secreted by different portions of the renal tubule. The
pattern of these processes effectively recycles up to 80%
of the F
ltered urea, which provides much of the osmotic
concentration of the medullary interstitial fl
uid. As a result,
To summarize, the countercurrent multiplier creates a
large osmotic gradient for water reabsorption in the intersti-
tial fl uid surrounding the distal convoluted tubules and the
collecting ducts of the nephron. The epithelial lining of these
structures is impermeable to water, unless ADH is present.
The higher the blood levels of ADH, the more permeable
the epithelial lining becomes, increasing water reabsorption
and concentrating the urine. In this way, soluble wastes and
other substances are excreted in minimal water, preserving
body water when dehydration is a threat. If the body fl
u-
ids contain excess water, ADH secretion decreases and the
epithelial linings of the distal convoluted tubule and the col-
lecting duct become less permeable to water. Less water is
reabsorbed, and the urine becomes more dilute.
Table 20.3
summarizes the role of ADH in urine production.
Table 20.4
summarizes the functions of different parts of the nephron.
A substance that causes diuresis is called a
diuretic.
Medically, a “water
pill” may help patients abnormally retaining water. However, other
diuretics are encountered more commonly. Ca±
eine inhibits proximal
tubular sodium reabsorption, leading to an osmotic diuresis. Alcohol
inhibits secretion of ADH from the posterior pituitary gland, directly
decreasing water reabsorption.
Isotonic fluid
Increasing
NaCl
concentration
Descending
limb
(permeable
to water)
Hypotonic fluid
Medullary
interstitial
fluid
Thick ascending
limb (impermeable
to water)
Na
+
Cl
Na
+
Cl
Na
+
Cl
(a)
(b)
Na
+
Cl
Salty
H
2
O
Na
+
Cl
More
salty
H
2
O
Na
+
Cl
Even
more
salty
H
2
O
1
2
3
Hypertonic
fluid
H
2
O
H
2
O
H
2
O
FIGURE 20.25
The countercurrent multiplier. (
a
) The solute concentration of interstitial F
uid in the medulla equilibrates with tubular F
uid, which
loses water in the descending limb, and thus becomes hypertonic by the tip of the nephron loop. The ascending limb of the loop actively reabsorbs
solute. (
b
) Active solute reabsorption from the ascending limb of the loop causes even more water loss from the descending limb as tubular F
uid
continues to F
ow. The countercurrent multiplier progressively increases the solute concentration of the interstitial F
uid, up to a maximum near the
tip of the loop more than four times that of plasma.
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