Chapter 26
Fluid, Electrolyte, and Acid-Base Balance
1009
26
Bicarbonate Ion Secretion
When the body is in alkalosis, another population of interca-
lated cells (type B) in the collecting ducts exhibit net HCO
3
2
secretion (rather than net HCO
3
2
reabsorption) while reclaim-
ing H
1
to acidify the blood. Overall we can think of the type B
cells as “flipped” type A cells, and we can visualize the HCO
3
2
secretion process as the exact opposite of the HCO
3
2
reabsorp-
tion process illustrated in Figure 26.12. However, the predomi-
nant process in the nephrons and collecting ducts is HCO
3
2
reabsorption, and even during alkalosis, much more HCO
3
2
is
conserved than excreted.
Check Your Understanding
14.
Reabsorption of HCO
3
2
is always tied to the secretion of
which ion?
15.
What is the most important urinary buffer of H
1
?
16.
List the two mechanisms by which tubule and collecting duct
cells generate new HCO
3
2
.
For answers, see Appendix H.
Abnormalities of Acid-Base Balance
Distinguish between acidosis and alkalosis resulting from
respiratory and metabolic factors. Describe the importance of
respiratory and renal compensations to acid-base balance.
All cases of acidosis and alkalosis can be classed according to
cause as
respiratory
or
metabolic
(Table 26.3)
.
A Closer Look
on
out in urine
Nucleus
PCT tubule cells
Filtrate in
tubule lumen
Peri-
tubular
capillary
2NH
4
+
Na
+
Na
+
Na
+
Na
+
Na
+
3Na
+
3Na
+
Glutamine
Glutamine
Glutamine
Tight junction
Deamination,
oxidation, and
acidification
(+H
+
)
2K
+
2K
+
NH
4
+
NH
4
+
HCO
3
-
2HCO
3
-
HCO
3
-
(new)
ATPase
1
PCT cells metabolize
glutamine to NH
4
+
and HCO
3
-
.
2a
This weak acid NH
4
+
(ammonium) is secreted into the
filtrate, taking the place of H
+
on
a Na
+
- H
+
antiport carrier.
2b
For each NH
4
+
secreted, a
bicarbonate ion (HCO
3
-
) enters
the peritubular capillary blood via
a symport carrier.
3
The NH
4
+
is excreted in the
urine.
Primary active transport
Simple diffusion
Secondary active transport
Transport protein
1
2a
2b
3
Figure 26.14
New HCO
3
2
is generated via glutamine metabolism and NH
4
1
secretion.
p. 1011 discusses methods for determining the cause of an acid-
base disturbance and whether it is being compensated (whether
the lungs or kidneys are taking steps to correct the imbalance).
Respiratory Acidosis and Alkalosis
Respiratory pH imbalances result from some failure of the respira-
tory system to perform its normal pH-balancing role. Te partial
pressure of carbon dioxide (P
CO
2
) is the single most important in-
dicator of the adequacy of respiratory function. When respiratory
function is normal, the P
CO
2
fluctuates between 35 and 45 mm Hg.
Generally speaking, values above 45 mm Hg indicate respiratory
acidosis, and values below 35 mm Hg signal respiratory alkalosis.
Respiratory acidosis
is a common cause of acid-base imbal-
ance. It most o±en occurs when a person breathes shallowly or
when gas exchange is hampered by diseases such as pneumonia,
cystic fibrosis, or emphysema. Under such conditions, CO
2
ac-
cumulates in the blood. Tus, respiratory acidosis is character-
ized by falling blood pH and rising P
CO
2
.
Respiratory alkalosis
results when carbon dioxide is elimi-
nated from the body faster than it is produced. Tis is called
hyperventilation
(see p. 836), and results in the blood becom-
ing more alkaline. While respiratory acidosis is frequently asso-
ciated with respiratory system pathology, respiratory alkalosis is
o±en due to stress or pain.
Metabolic Acidosis and Alkalosis
Metabolic pH imbalances include all abnormalities of acid-base
imbalance
except
those caused by too much or too little carbon
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