be unchanged, but this is not the case. As emphasized earlier
in this chapter, the elevated hydrogen ion concentration asso-
ciated with metabolic acidosis
and lowers arterial
. 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
mass action, an associated restoration of hydrogen ion concen-
tration toward normal.
To reiterate, the plasma
changes in metabolic aci-
dosis and alkalosis are not the
of the acidosis or alkalosis,
of compensatory reﬂ ex responses to nonrespira-
tory abnormalities. Thus, in metabolic as opposed to respira-
tory conditions, the arterial plasma
and hydrogen ion
concentration move in opposite directions, as summarized in
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
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
system, and the major intracellular buffers are proteins and
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
III. A decrease in arterial plasma hydrogen ion concentration
causes reﬂ ex hypoventilation, which raises arterial
hence, raises plasma hydrogen ion concentration toward
normal. An increase in plasma hydrogen ion concentration
causes reﬂ ex hyperventilation, which lowers arterial
hence, lowers hydrogen ion concentration toward normal.
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
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.
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
b. All other causes of acidosis or alkalosis are termed
and reﬂ ect gain or loss, respectively, of hydrogen ions from
a source other than carbon dioxide.
SECTION C KEY TERMS
SECTION C CLINICAL TERMS
SECTION C REVIEW QUESTIONS
1. What are the sources of gain and loss of hydrogen ions in the
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
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
9. Classify the four types of acid-base disorders according to
plasma hydrogen ion concentration, bicarbonate concentration,