522
Chapter 14
be unchanged, but this is not the case. As emphasized earlier
in this chapter, the elevated hydrogen ion concentration asso-
ciated with metabolic acidosis
refl
exly
stimulates ventilation
and lowers arterial
P
CO
2
. By mass action, this helps restore
the hydrogen ion concentration toward normal. Conversely,
a person with metabolic alkalosis will refl exly have ventilation
inhibited. The result is an increase in arterial
P
CO
2
and, by
mass action, an associated restoration of hydrogen ion concen-
tration toward normal.
To reiterate, the plasma
P
CO
2
changes in metabolic aci-
dosis and alkalosis are not the
cause
of the acidosis or alkalosis,
but the
result
of compensatory refl ex responses to nonrespira-
tory abnormalities. Thus, in metabolic as opposed to respira-
tory conditions, the arterial plasma
P
CO
2
and hydrogen ion
concentration move in opposite directions, as summarized in
Table 14–8.
SECTION C SUMMARY
Sources of Hydrogen Ion Gain or Loss
I. Total-body balance of hydrogen ions is the result of both
metabolic production of these ions and of net gains or losses
via the respiratory system, gastrointestinal tract, and urine
(Table 14–6).
II. A stable balance is achieved by regulation of urinary losses.
Buffering of Hydrogen Ions in the Body
I. Buffering is a means of minimizing changes in hydrogen ion
concentration by combining these ions reversibly with anions
such as bicarbonate and intracellular proteins.
II. The major extracellular buffering system is the CO
2
/HCO
3
system, and the major intracellular buffers are proteins and
phosphates.
Integration of Homeostatic Controls
I. The kidneys and the respiratory system are the homeostatic
regulators of plasma hydrogen ion concentration.
II. The kidneys are the organs that achieve body hydrogen ion
balance.
III. A decrease in arterial plasma hydrogen ion concentration
causes refl ex hypoventilation, which raises arterial
P
CO
2
and,
hence, raises plasma hydrogen ion concentration toward
normal. An increase in plasma hydrogen ion concentration
causes refl ex hyperventilation, which lowers arterial
P
CO
2
and,
hence, lowers hydrogen ion concentration toward normal.
Renal Mechanisms
I. The kidneys maintain a stable plasma hydrogen ion
concentration by regulating plasma bicarbonate concentration.
They can either excrete bicarbonate or contribute new
bicarbonate to the blood.
II. Bicarbonate is reabsorbed when hydrogen ions, generated in
the tubular cells by a process catalyzed by carbonic anhydrase,
are secreted into the lumen and combine with fi ltered
bicarbonate. The secreted hydrogen ions are not excreted in
this situation.
III. In contrast, when the secreted hydrogen ions combine in the
lumen with fi ltered phosphate or other nonbicarbonate buffer,
they are excreted, and the kidneys have contributed new
bicarbonate to the blood.
IV. The kidneys also contribute new bicarbonate to the blood
when they produce and excrete ammonium.
Classifi
cation of Acidosis and Alkalosis
I. Acid-base disorders are categorized as respiratory or metabolic.
a. Respiratory acidosis is due to retention of carbon dioxide,
and respiratory alkalosis to excessive elimination of carbon
dioxide.
b. All other causes of acidosis or alkalosis are termed
metabolic
and refl ect gain or loss, respectively, of hydrogen ions from
a source other than carbon dioxide.
SECTION C KEY TERMS
buffer
518
nonvolatile acid
517
SECTION C CLINICAL TERMS
acidosis
517
alkalosis
517
metabolic acidosis
521
metabolic alkalosis
521
respiratory acidosis
521
respiratory alkalosis
521
SECTION C REVIEW QUESTIONS
1. What are the sources of gain and loss of hydrogen ions in the
body?
2. List the body’s major buffer systems.
3. Describe the role of the respiratory system in the regulation of
hydrogen ion concentration.
4. How does the tubular secretion of hydrogen ions occur, and
how does it achieve bicarbonate reabsorption?
5. How does hydrogen ion secretion contribute to the renal
addition of new bicarbonate to the blood? What determines
whether a secreted hydrogen ion will achieve these results or
will instead cause bicarbonate reabsorption?
6. How does the metabolism of glutamine by the tubular cells
contribute new bicarbonate to the blood and ammonium to
the urine?
7. What two quantities make up “hydrogen ion excretion?”
Why can this term be equated with “contribution of new
bicarbonate to the plasma?”
8. How do the kidneys respond to the presence of an acidosis or
alkalosis?
9. Classify the four types of acid-base disorders according to
plasma hydrogen ion concentration, bicarbonate concentration,
and
P
CO
2
.
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