792
UNIT FIVE
convoluted tubule, if sodium reabsorption increases, water
reabsorption increases; if sodium reabsorption decreases,
water reabsorption decreases also.
Much of the sodium ion reabsorption occurs in the prox-
imal segment of the renal tubule by active transport (sodium
pump mechanism). When the positively charged sodium
ions (Na
+
) are moved through the tubular wall, negatively
charged ions, including chloride ions (Cl
), phosphate ions
(PO
4
–3
), and bicarbonate ions (HCO
3
), accompany them.
This movement of negatively charged ions is due to the elec-
trochemical attraction between particles of opposite electri-
cal charge. Although this movement of negatively charged
ions depends on active transport of sodium, it is considered a
passive process because it does not require a direct expendi-
ture of cellular energy. Active transport also reabsorbs some
of these ions directly, such as HCO
3
and PO
4
–3
.
As more sodium ions are reabsorbed into the peritubular
capillary along with negatively charged ions, the concentra-
tion of solutes in the peritubular blood might be expected
to increase. However, because water moves by osmosis
through cell membranes from regions of lesser solute con-
centration (hypotonic) toward regions of greater solute con-
centration (hypertonic), water is also reabsorbed, following
the ions from the renal tubule into the peritubular capillary.
The proximal convoluted tubule reabsorbs about 70% of
the F ltered sodium, other ions, and water. By the end of the
proximal convoluted tubule, osmotic equilibrium is reached,
and the remaining tubular fl
uid is isotonic
(f g. 20.22)
.
Active transport continues to reabsorb sodium ions as
the tubular fl
uid moves through the nephron loop, the dis-
tal convoluted tubule, and the collecting duct. Consequently,
almost all of the sodium and water (97% to 99%) that enters
the renal tubules as part of the glomerular filtrate may be
reabsorbed before the urine is excreted. However, aldoster-
one controls sodium reabsorption, and antidiuretic hormone
controls water reabsorption. Under the infl uence of these hor-
mones, reabsorption of sodium and water can change to keep
conditions in the body fl uids constant. Chapter 21 (pp. 818
and 815) discusses the speciF c effects of these hormones.
Recall that the kidneys F lter an extremely large volume
of fl
uid (180 liters) each day. If 99% of the glomerular F l-
trate is reabsorbed, the remaining 1% excreted includes a
relatively large amount of sodium and water
(table 20.2)
. On
the other hand, if sodium and water reabsorption decrease
to 97% of the amount F
ltered, the amount excreted triples!
Therefore, small changes in the tubular reabsorption of
sodium and water result in large changes in urinary excre-
tion of these substances.
PRACTICE
13
How is the peritubular capillary adapted for reabsorption?
14
Which substances in glomerular F
ltrate are not normally present
in urine?
15
Which mechanisms reabsorb solutes from glomerular F
ltrate?
16
DeF
ne
renal plasma threshold.
17
Describe the role of passive transport in urine formation.
Usually all of the glucose in the glomerular F ltrate is reab-
sorbed because there are enough carrier molecules to trans-
port it. When the plasma glucose concentration increases to
a critical level, called the
renal plasma threshold,
more glu-
cose molecules are in the F ltrate than the active transport
mechanism can handle. As a result, some glucose remains
in the F ltrate and is excreted in the urine. This explains why
the elevated blood glucose of diabetes mellitus results in glu-
cose in the urine.
Any increase in urine volume is called
diuresis.
Nonreabsorbed glucose in the tubular fluid increases the
osmotic concentration of the tubular fl
uid, which reduces the
volume of water reabsorbed by osmosis from the proximal
tubule and collecting duct. The resultant increase in urine
volume is called an
osmotic diuresis.
Amino acids
enter the glomerular F ltrate and are reab-
sorbed in the proximal convoluted tubule. Three differ-
ent active transport mechanisms reabsorb different groups
of amino acids, whose members have similar structures.
Normally only a trace of amino acids remains in the urine.
The glomerular filtrate is nearly free of protein, but a
number of smaller protein molecules, such as albumin, may
squeeze through the glomerular capillaries. These proteins are
transported by
endocytosis
through the brush border of epithe-
lial cells lining the proximal convoluted tubule. Once inside
an epithelial cell, the proteins are degraded to amino acids,
which are moved into the blood of the peritubular capillary.
The epithelium of the proximal convoluted tubule also
reabsorbs creatine; lactic, citric, uric, and ascorbic (vita-
min C) acids; and phosphate, sulfate, calcium, potassium,
and sodium ions. Active transport mechanisms with limited
transport capacities reabsorb all of these chemicals. Such
substances begin to appear in the urine when their concen-
trations in the glomerular filtrate exceed their respective
renal plasma thresholds. Clinical Application 20.3 discusses
how the nephrotic syndrome causes plasma proteins to
appear in the urine.
Glucose in the urine is called
glucosuria.
It may follow intravenous
administration of glucose, or eating candy, or it may occur in a person
with diabetes mellitus. In type 1 diabetes, blood glucose concentra-
tion rises because of insu±
cient insulin secretion from the pancreas
(see Clinical Application 13.4, page 512).
One in three people who have diabetes mellitus sustains kidney
damage (nephropathy). In the past, large amounts of the protein
albumin in the urine indicated high risk of developing kidney dam-
age. Recent studies show that small amounts of albumin (microal-
buminuria) predict kidney damage in people with type 1 diabetes.
²ollowing a low-protein diet can slow the loss of kidney function.
Sodium and Water Reabsorption
Water reabsorption occurs passively by osmosis, primarily
in the proximal convoluted tubule, and is closely associated
with the active reabsorption of sodium ions. In the proximal
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