970
UNIT 4
Maintenance of the Body
25
When the transporters are saturated—that is, all bound to
the substance they transport—the excess is excreted in urine.
Tis is what happens in individuals who become hyperglycemic
because of uncontrolled diabetes mellitus. As plasma levels of
glucose approach and exceed 180 mg/dl, the glucose ±
m
is ex-
ceeded and large amounts of glucose may be lost in the urine
even though the renal tubules are still functioning normally.
Reabsorptive Capabilities of the Renal Tubules
and Collecting Ducts
Table 25.1
compares the reabsorptive abilities of various re-
gions of the renal tubules and collecting ducts.
Proximal Convoluted Tubule
Te entire renal tubule is in-
volved in reabsorption to some degree, but the PC± cells are by
far the most active “reabsorbers” and the events just described
occur mainly in this tubular segment. Normally, the PC± reab-
sorbs
all
of the glucose and amino acids in the filtrate and 65%
of the Na
1
and water. Te bulk of the electrolytes are reabsorbed
part why lipid-soluble drugs and environmental pollutants are
difficult to excrete: Since lipid-soluble compounds can generally
pass through membranes, they will follow their concentration
gradients and be reabsorbed, even if this is not “desirable.”
As Na
1
ions move through the tubule cells into the peritubu-
lar capillary blood, they also establish an electrical gradient that
favors passive reabsorption of anions (primarily Cl
2
) to restore
electrical neutrality in the filtrate and plasma.
Transport Maximum
Te transcellular transport systems for the various solutes are
quite specific and
limited
. Tere is a
transport maximum (T
m
)
for nearly every substance that is reabsorbed using a transport
protein in the membrane. Te ±
m
(reported in mg/min) reflects
the number of transport proteins in the renal tubules available
to ferry a particular substance. In general, there are plenty of
transporters (and therefore high ±
m
values) for substances such
as glucose that need to be retained, and few or no transporters
for substances of no use to the body.
Filtrate
in tubule
lumen
Glucose
Amino acids
Some ions
Vitamins
Lipid-soluble
substances
Various ions
and urea
3Na
+
2K
+
3Na
+
2K
+
K
+
H
2
O
Na
+
Nucleus
Tubule cell
Paracellular
route
Tight junction
Interstitial
fluid
Peri-
tubular
capillary
1
At the basolateral membrane, Na
+
is pumped into the interstitial space by
the Na
+
-K
+
ATPase. Active Na
+
transport creates concentration
gradients that drive:
2
“Downhill” Na
+
entry at the
apical membrane.
4
Reabsorption of water by
osmosis through aquaporins. Water
reabsorption increases the
concentration of the solutes that are
left behind. These solutes can then
be reabsorbed as they move down
their gradients:
3
Reabsorption of organic nutrients
and certain ions by cotransport at the
apical membrane.
5
Lipid-soluble substances
diffuse by the transcellular route.
6
Various ions (e.g., Cl
-
, Ca
2
+
, K
+
)
and urea diffuse by the paracellular
route.
6
5
4
3
2
1
Primary active transport
Passive transport (diffusion)
Secondary active transport
Transport protein
Ion channel
Aquaporin
Figure 25.14
Reabsorption by PCT cells.
Though not illustrated here, most organic
nutrients reabsorbed in the PCT move through the basolateral membrane by facilitated
diffusion. Microvilli have been omitted for simplicity.
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