520
Chapter 14
with a buffer other than bicarbonate, the overall effect is not
merely one of bicarbonate conservation, as in Figure 14–31, but
rather of addition to the plasma of a
new
bicarbonate. This raises
the bicarbonate concentration of the plasma and alkalinizes it.
To repeat, signifi
cant numbers of hydrogen ions com-
bine with fi ltered nonbicarbonate buffers like HPO
4
2–
only
after the fi ltered bicarbonate has virtually all been reabsorbed.
The main reason is that there is such a large load of fi ltered
bicarbonate—25 times more than the load of fi
ltered nonbi-
carbonate buffers—competing for the secreted hydrogen ions.
There is a second mechanism by which the tubules contrib-
ute new bicarbonate to the plasma that involves not hydrogen
ion secretion, but rather the renal production and secretion
of ammonium ions (NH
4
+
) (
Figure 14–33
). Tubular cells,
mainly those of the proximal tubule, take up glutamine
from both the glomerular fi ltrate and peritubular plasma and
metabolize it. In the process, both NH
4
+
and bicarbonate are
formed inside the cells. The NH
4
+
is actively secreted via Na
+
/
NH
4
+
countertransport into the lumen and excreted, while
the bicarbonate moves into the peritubular capillaries and
constitutes new plasma bicarbonate.
A comparison of Figures 14–32 and 14–33 demonstrates
that the overall result—renal contribution of new bicarbonate
to the plasma—is the same regardless of whether it is achieved
by: (1) H
+
secretion and excretion on nonbicarbonate buffers
such as phosphate (see Figure 14–32); or (2) by glutamine
metabolism with NH
4
+
excretion (see Figure 14–33). It is con-
venient, therefore, to view the latter as representing H
+
excre-
tion “bound” to NH
3
, just as the former case constitutes H
+
excretion bound to nonbicarbonate buffers. Thus, the amount
of H
+
excreted in the urine in these two forms is a measure of
the amount of new bicarbonate added to the plasma by the
kidneys. Indeed, “urinary H
+
excretion” and “renal contribu-
tion of new bicarbonate to the plasma” are really two sides of
the same coin.
The kidneys normally contribute enough new bicarbonate
to the blood by excreting hydrogen ions to compensate for the
hydrogen ions from nonvolatile acids generated in the body.
Renal Responses to Acidosis and Alkalosis
We can now apply this material to the renal responses to the
presence of an acidosis or alkalosis. These are summarized in
Table 14–7
.
Clearly, these homeostatic responses require that the
rates of hydrogen ion secretion, glutamine metabolism, and
ammonium excretion be subject to physiological control by
changes in blood hydrogen ion concentration. The specifi c
pathways and mechanisms that bring about these rate changes
are very complex, however, and are not presented here.
HPO
4
2–
(filtered)
HPO
4
2–
+ H
+
H
2
PO
4
HCO
3
HCO
3
H
2
O + CO
2
H
2
CO
3
H
+
Carbonic
anhydrase
Begin
Tubular
lumen
Tubular epithelial
cells
Interstitial fluid
Excreted
Figure 14–32
Renal contribution of new HCO
3
to the plasma as achieved by
tubular secretion of H
+
. The process of intracellular H
+
and HCO
3
generation, with H
+
moving into the lumen and HCO
3
into the
plasma, is identical to that shown in Figure 14–31. Once in the
lumen, however, the H
+
combines with fi ltered phosphate (HPO
4
2–
)
rather than fi
ltered HCO
3
and is excreted as H
2
PO
4
. As described
in the legend for Figure 14–31, the transport of hydrogen ions into
the lumen is accomplished not only by H
+
-ATPase pumps but, in
several tubular segments, by Na
+
/H
+
countertransporters and/or
H
+
/K
+
-ATPase pumps as well.
Tubular
lumen
Tubular epithelial
cells
Interstitial fluid
(filtered)
NH
4
+
NH
4
+
Na
+
Na
+
Na
+
Begin
HCO
3
HCO
3
Glutamine
Excreted
Glutamine
Glutamine
Begin
Begin
Figure 14–33
Renal contribution of new HCO
3
to the plasma as achieved by
renal metabolism of glutamine and excretion of ammonium (NH
4
+
).
Compare this fi gure to Figure 14–32. This process occurs mainly in
the proximal tubule.
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