1008
UNIT 4
Maintenance of the Body
26
reabsorbed. However, their reabsorption is inhibited during
acidosis. As a result, the buffer pair becomes more and more
concentrated as the filtrate moves through the renal tubules.
As shown in
Figure 26.13
3a
, the type A intercalated cells
secrete H
1
actively via a H
1
ATPase pump and via a K
1
-H
1
antiporter (not illustrated). ±e secreted H
1
combines with
HPO
4
2
2
, forming H
2
PO
4
2
which then flows out in urine (Fig-
ure 26.13
4
and
5
).
Bicarbonate ions generated in the cells during the same reac-
tion move into the interstitial space via a HCO
3
2
2
Cl
2
antiport
process and then move passively into the peritubular capillary
blood (Figure 26.13
3b
). Notice again that when H
1
is being ex-
creted, “brand new” bicarbonate ions are added to the blood—
over and above those reclaimed from the filtrate. As you can
see, in response to acidosis, the kidneys generate new HCO
3
2
and add it to the blood (alkalinizing the blood) while adding an
equal amount of H
1
to the filtrate (acidifying the urine).
Via NH
4
1
Excretion
±e second and more important mecha-
nism for excreting acid uses the ammonium ion (NH
4
1
) pro-
duced by glutamine metabolism in PCT cells. Ammonium ions
are weak acids that donate few H
1
at physiological pH.
As
Figure 26.14
step
1
shows, for each glutamine metabo-
lized (deaminated, oxidized, and acidified by combining with H
1
),
two NH
4
1
and two HCO
3
2
result. ±e HCO
3
2
moves through the
basolateral membrane into the blood (Figure 26.14
2b
). ±e NH
4
1
,
in turn, is excreted and lost in urine (Figure 26.14
2a
,
3
). As with
the phosphate buffer system, this buffering mechanism replenishes
the alkaline reserve of the blood, because the newly made HCO
3
2
enters the blood as NH
4
1
is secreted.
acid,
via secretion and excretion
of either H
1
or ammonium ions
in urine. Let’s examine how these mechanisms differ.
As long as
filtered bicarbonate
is reclaimed, as we saw in
Figure 26.12, the secreted H
1
is
not excreted or lost
from the
body in urine. Instead, the H
1
is buffered by HCO
3
2
in the
filtrate and ultimately becomes part of water molecules (most
of which are reabsorbed).
However, once the filtered HCO
3
2
is “used up” (usually by
the time the filtrate reaches the collecting ducts), any addi-
tional H
1
secreted is excreted in urine. More o²en than not,
this is the case.
Reclaiming filtered HCO
3
2
simply restores the bicarbonate
concentration of plasma that exists at the time. However, me-
tabolism of food normally releases new H
1
into the body. ±is
additional H
1
uses up HCO
3
2
and so must be balanced by gen-
erating
new
HCO
3
2
(as opposed to filtered HCO
3
2
) that moves
into the blood to counteract acidosis. ±is process of alkaliniz-
ing the blood is the way the kidneys compensate for acidosis.
Via Excretion of Buffered H
1
Binding H
1
to buffers in the
filtrate minimizes the H
1
concentration gradient, allowing the
proton pumps of the type A intercalated cells to secrete the large
numbers of H
1
that the body must get rid of to prevent acidosis.
(H
1
secretion ceases when urine pH falls to 4.5 because the pro-
ton pumps cannot pump against this large gradient.) ±e most
important urine buffer is the
phosphate buffer system
, specifi-
cally its weak base
monohydrogen phosphate
(HPO
4
2
2
).
±e components of the phosphate buffer system filter freely
into the tubules, and about 75% of the filtered phosphate is
Nucleus
Type A intercalated
cell of collecting duct
Filtrate in
tubule lumen
Peri-
tubular
capillary
H
+
+
Cl
-
Cl
-
out in urine
H
2
O
+
CO
2
H
2
CO
3
H
+
Primary active transport
Simple diffusion
Secondary active transport
Facilitated diffusion
Carbonic anhydrase
Transport protein
Ion channel
Cl
-
HCO
3
-
(new)
HCO
3
-
CA
CA
ATPase
Tight junction
1
2
4
5
3a
3b
1
CO
2
combines with water
within the type A intercalated
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
by a H
+
ATPase pump.
3b
For each H
+
secreted, a HCO
3
-
enters the peritubular capillary
blood via an antiport carrier in a
HCO
3
-
-CI
-
exchange process.
4
Secreted H
+
combines with
HPO
4
2
-
in the tubular filtrate,
forming H
2
PO
4
-
.
5
The H
2
PO
4
-
is excreted in the
urine.
HPO
4
H
2
PO
4
-
2
-
Figure 26.13
New HCO
3
2
is generated via buffering of secreted H
1
by HPO
4
2
2
(monohydrogen phosphate).
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