793
CHAPTER TWENTY
Urinary System
proteinemia), the plasma colloid osmotic pres-
sure falls, increasing net filtration pressure in
capillaries throughout the body. This may lead
to widespread, severe edema as a large volume
of fluid accumulates in interstitial spaces in tis-
sues and in body spaces such as the abdominal
cavity, pleural cavity, pericardial cavity, and joint
cavities.
Also, as edema develops, blood volume
decreases and blood pressure drops. These
changes may activate the renin-angiotensin
T
he
nephrotic syndrome
is a set of symp-
toms that often appears in patients
with renal diseases. Considerable loss
of plasma proteins into the urine (proteinuria),
results in widespread edema, and increased sus-
ceptibility to infection.
Plasma proteins enter the urine because of
increased permeability of the glomerular mem-
branes, which accompanies renal disorders such
as glomerulonephritis. As a consequence of a
decreasing plasma protein concentration (hypo-
system, leading to the release of aldosterone
from the adrenal cortex (see chapter 13, p. 506),
which, in turn, stimulates the kidneys to conserve
sodium ions and water. This action reduces the
urine output and may aggravate the edema.
The nephrotic syndrome sometimes appears
in young children who have
lipoid nephrosis.
The
cause of this condition is unknown, but it alters
the epithelial cells of the glomeruli so that the
glomerular membranes enlarge and distort,
allowing proteins through.
20.3
CLINICAL APPLICATION
The Nephrotic Syndrome
Tubular Secretion
In
tubular secretion
, certain substances move from the
plasma of the peritubular capillary into the fl uid of the renal
tubule. The amount of a particular chemical excreted in the
urine may exceed the amount F ltered from the plasma in the
glomerulus (see F
g. 20.21). As in the case of tubular reab-
sorption, the term
tubular
is used because the epithelial cells
of the renal tubules control the process.
Active transport mechanisms similar to those that func-
tion in reabsorption secrete some substances. However, the
secretory mechanisms transport substances in the opposite
direction. ±or example, the epithelium of the proximal con-
voluted tubules actively secretes certain organic compounds,
including penicillin and histamine, into the tubular fl
uid.
Hydrogen ions are actively secreted throughout the
entire renal tubule. Urine is usually acidic by the time it is
excreted, although the urinary pH can vary considerably.
The secretion of hydrogen ions is important in regulating the
pH of body fl
uids, as chapter 21 (p. 823) explains.
The kidneys respond to certain chemicals, including antibiotics, by
eliminating them from the body as rapidly as possible. In some cases,
such as eliminaiton of toxins, this is beneF
cial. In the case of helpful
drugs, however, it is undesirable. The drug probenecid is given with
certain antibiotics to block their tubular secretion, increasing their
levels in the plasma. ±or example, probenecid increases plasma con-
centration of penicillin two to four fold. It also acts on ampicillin, methi-
cillin, and certain other drugs in the penicillin family, as well as certain
cephalosporin antibiotics.
Blood
flow
Peritubular capillary
Blood
flow
Glomerular
capsule
Glomerulus
Glomerular
filtrate
Isotonic
tubular fluid
Proximal
convoluted
tubule
+
Na
+
Na
+
+
Cl
Cl
PO
4
3
HCO
3
+
Na
+
+
+
+
+
+
+
Na
+
H
2
O
+
+
+
+
H
2
O
+
+
H
2
O
Sodium ions
are reabsorbed by
active transport
Negatively charged ions
are attracted to positively
charged ions
As concentration of ions
(solute) increases in plasma,
osmotic pressure increases
Water moves from proximal
tubule to capillary by
osmosis
1
2
3
4
Blood
flow
FIGURE 20.22
In the proximal part of the renal tubule, osmosis
reabsorbs water in response to active transport reabsorbing sodium
and other solutes.
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