Chapter 26
Fluid, Electrolyte, and Acid-Base Balance
1007
26
the kidneys must do more than just eliminate enough hydro-
gen ions to counter rising blood H
1
levels. Depleted stores of
HCO
3
2
have to be replenished. Tis task is more complex than
it seems because the tubule cells are almost completely imper-
meable to the HCO
3
2
in the filtrate—they cannot reabsorb it.
However, the kidneys can conserve filtered HCO
3
2
in a
rather roundabout way. As you can see, dissociation of carbonic
acid liberates HCO
3
2
as well as H
1
(Figure 26.12
2
). Although
the tubule cells cannot reclaim HCO
3
2
directly from the fil-
trate, they can and do shunt HCO
3
2
generated within them (as
a result of splitting H
2
CO
3
) into the peritubular capillary blood.
HCO
3
2
leaves the tubule cell either accompanied by Na
1
or
in exchange for Cl
2
(Figure 26.12
3b
). H
1
is actively secreted,
mostly by a Na
1
-H
1
antiporter, but also by a H
1
A±Pase (Fig-
ure 26.12
3a
). In the filtrate, H
1
combines with filtered HCO
3
2
,
as we saw earlier (Figure 26.12
4
,
5
). For this reason, reab-
sorption of HCO
3
2
depends on the active secretion of H
1
.
In short, for each filtered HCO
3
2
that “disappears” from the
filtrate, a HCO
3
2
generated within the tubule cells enters the
blood—a one-for-one exchange. When large amounts of H
1
are
secreted, correspondingly large amounts of HCO
3
2
enter the
peritubular blood. Te net effect is to remove HCO
3
2
almost
completely from the filtrate.
Generating New Bicarbonate Ions
±wo renal mechanisms commonly carried out by cells of the
PC± and collecting ducts generate
new
HCO
3
2
that can be
added to plasma. Both mechanisms involve renal excretion of
to reabsorb bicarbonate, the kidney has to secrete H
1
, and when
it excretes excess HCO
3
2
, H
1
is retained (not secreted).
Because the mechanisms for regulating acid-base bal-
ance depend on H
1
being secreted into the filtrate, we con-
sider that process first. Secretion of H
1
occurs mainly in the
PC± and in type A intercalated cells of the collecting duct.
Te H
1
secreted comes from the dissociation of carbonic
acid, created from the combination of CO
2
and water within
the tubule cells, a reaction catalyzed by
carbonic anhydrase
(
Figure 26.12
1
,
2
). As H
1
is secreted into the lumen of
the PC±, Na
1
is reabsorbed from the filtrate, maintaining the
electrical balance (Figure 26.12
3a
).
Te rate of H
1
secretion rises and falls with CO
2
levels in
the ECF. Te more CO
2
in the peritubular capillary blood, the
faster the rate of H
1
secretion. Because blood CO
2
levels di-
rectly relate to blood pH, this system can respond to both ris-
ing and falling H
1
concentrations. Notice that secreted H
1
can
combine with HCO
3
2
in the filtrate, generating CO
2
and water
(Figure 26.12
4
,
5
). In this case, H
1
is bound in water. Te
rising concentration of CO
2
in the filtrate creates a steep diffu-
sion gradient for its entry into the tubule cell, where it promotes
still more H
1
secretion (Figure 26.12
6
).
Conserving Filtered Bicarbonate Ions:
Bicarbonate Reabsorption
Bicarbonate ions (HCO
3
2
) are an important part of the bicarbo-
nate buffer system, the most important inorganic blood buffer.
If this reservoir of base, the
alkaline reserve
, is to be maintained,
CO
2
CO
2
+
+
H
2
O
2K
+
2K
+
Na
+
Na
+
3Na
+
3Na
+
Tight junction
H
2
CO
3
H
2
CO
3
PCT cell
Nucleus
Filtrate in
tubule lumen
Cl
-
Cl
-
HCO
3
-
HCO
3
-
H
2
O
CO
2
CA
CA
H
+
H
+
HCO
3
-
HCO
3
-
Na
+
HCO
3
-
ATPase
ATPase
Peri-
tubular
capillary
1
2
4
5
6
3a
3b
1
CO
2
combines with water
within the tubule cell, forming
H
2
CO
3
.
2
H
2
CO
3
is quickly split, forming
H
+
and bicarbonate ion (HCO
3
-
).
3a
H
+
is secreted into the filtrate.
3b
For each H
+
secreted, a HCO
3
-
enters the peritubular capillary
blood either via symport with Na
+
or via antiport with CI
-
.
4
Secreted H
+
combines with
HCO
3
-
in the filtrate, forming
carbonic acid (H
2
CO
3
).
HCO
3
-
disappears from the filtrate at the
same rate that HCO
3
-
(formed
within the tubule cell) enters the
peritubular capillary blood.
5
The H
2
CO
3
formed in the
filtrate dissociates to release CO
2
and H
2
O.
6
CO
2
diffuses into the tubule
cell, where it triggers further H
+
secretion.
Primary active transport
Simple diffusion
Secondary active transport
Carbonic anhydrase
Transport protein
CA
CA
*
Figure 26.12
Reabsorption of filtered HCO
3
2
is coupled to H
1
secretion.
*The breakdown of H
2
CO
3
to CO
2
and H
2
O in the tubule lumen is catalyzed by carbonic anhydrase only in
the PCT.
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