496
Chapter 14
out of the cell across the basolateral membrane via Na
+
/K
+
-
ATPases in this membrane.
The reabsorption of many substances is coupled to the
reabsorption of sodium. The cotransported substance moves
uphill into the cell via a secondary active cotransporter as
sodium moves downhill into the cell via this same cotrans-
porter. This is precisely how glucose, many amino acids, and
other organic substances undergo tubular reabsorption. The
reabsorption of several inorganic ions is also coupled in a vari-
ety of ways to the reabsorption of sodium.
Many of the mediated transport reabsorptive systems in
the renal tubule have a limit to the amounts of material they
can transport per unit time (
transport maximum,
T
m
). This
is because the binding sites on the membrane transport proteins
become saturated when the concentration of the transported
substance increases to a certain level. An important example is
the secondary active-transport proteins for glucose, located in
the proximal tubule. As noted earlier, glucose does not usually
appear in the urine because all of the fi ltered glucose is reab-
sorbed. This is illustrated in
Figure 14–11
, which shows the
relationship between plasma glucose concentrations and the
fi ltered load, reabsorption, and excretion of glucose. Plasma
glucose concentration in a healthy person normally does not
exceed 150 mg/100mL even after the person eats a sugary
meal. Notice that this level of plasma glucose is below the
threshold at which glucose starts to appear in urine (
glucos-
uria
). Also notice that the
T
m
for the entire kidney is higher
than the threshold for glucosuria. This is because the nephrons
have a range of
T
m
values that, when averaged, give a
T
m
for the
entire kidney, as shown in Figure 14–11. When plasma glucose
concentration exceeds the transport maximum for a signifi -
cant number of nephrons, glucose starts to appear in urine. In
people with signifi cant hyperglycemia (for example, in poorly
controlled
diabetes mellitus
), the plasma glucose concentra-
tion often exceeds the threshold value of 200 mg/100 mL,
so that the fi ltered load exceeds the ability of the nephrons to
reabsorb glucose. In other words, although the capacity of the
kidney to reabsorb glucose can be normal in diabetes mellitus,
the tubules cannot reabsorb the large increase in the fi ltered
load of glucose. As you will learn in Chapter 16, the high fi l-
tered load of glucose can also lead to signifi cant disruption of
normal renal function (
diabetic nephropathy
).
The pattern described for glucose is also true for a large
number of other organic nutrients. For example, most amino
acids and water-soluble vitamins are fi ltered in large amounts
each day, but almost all of these fi ltered molecules are reab-
sorbed by the proximal tubule. If the plasma concentration
becomes high enough, however, reabsorption of the fi ltered
load will not be as complete, and the substance will appear
in larger amounts in the urine. Thus, people who ingest very
large quantities of vitamin C have increased plasma concentra-
tions of vitamin C. Eventually, the fi ltered load may exceed
the tubular reabsorptive
T
m
for this substance, and any addi-
tional ingested vitamin C is excreted in the urine.
Tubular Secretion
Tubular secretion moves substances from peritubular capillaries
into the tubular lumen. Like glomerular fi ltration, it constitutes
a pathway from the blood into the tubule. Like reabsorption,
secretion can occur by diffusion or by transcellular mediated
transport.
The
most
important
substances
secreted
by
the
tubules are hydrogen ions and potassium. However, a large
number of normally occurring organic anions, such as choline
and creatinine, are also secreted; so are many foreign chemicals
such as penicillin. Active secretion of a substance requires active
transport either from the blood side (the interstitial fl
uid) into
the cell (across the basolateral membrane) or out of the cell
into the lumen (across the luminal membrane). As in reabsorp-
tion, tubular secretion is usually coupled to the reabsorption of
sodium. Secretion from the interstitial space into the tubular
fl uid, which draws substances from the peritubular capillaries,
is a mechanism to increase the ability of the kidney to dispose
of substances at a higher rate rather than depending only on
the fi ltered load.
Metabolism by the Tubules
We noted earlier that, during fasting, the cells of the renal
tubules synthesize glucose and add it to the blood. They can
also catabolize certain organic substances, such as peptides,
taken up from either the tubular lumen or peritubular capil-
laries. Catabolism eliminates these substances from the body
just as if they had been excreted into the urine.
100
200
300
400
500
600
700
800
100
200
300
400
500
600
700
800
900
0
Filtered
load
Excretion
Transport
maximum
Reabsorption
Plasma glucose concentration
(mg/100 ml)
Glucose filtered load, reabsorption
or excretion (mg/min)
Threshold
Normal
Figure 14–11
The relationship between plasma glucose concentration and the
rate of glucose fi ltered (fi ltered load), reabsorbed, or excreted. The
dotted line shows the transport maximum, which is the maximum
rate at which glucose can be reabsorbed. Notice that as plasma
glucose exceeds its threshold, glucose begins to appear in the urine.
Figure 14–11
physiological
inquiry
How would you calculate the fi ltered load and excretion rate of
glucose?
Answer can be found at end of chapter.
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