The Kidneys and Regulation of Water and Inorganic Ions
Classiﬁ cation of Acidosis
To repeat, acidosis refers to any situation in which the hydro-
gen ion concentration of arterial plasma is elevated whereas
alkalosis denotes a reduction. All such situations ﬁ t into two
distinct categories (
As its name implies, respiratory acidosis results from
altered alveolar ventilation. Respiratory acidosis occurs when
the respiratory system fails to eliminate carbon dioxide as fast
as it is produced. Respiratory alkalosis occurs when the respi-
ratory system eliminates carbon dioxide faster than it is pro-
duced. As described earlier, the imbalance of arterial hydrogen
ion concentrations in such cases is completely explainable in
terms of mass action. Thus, the hallmark of a respiratory aci-
dosis is an elevation in both arterial
and hydrogen ion
concentration, whereas that of respiratory alkalosis is a reduc-
tion in both.
Metabolic acidosis or alkalosis includes all situations
other than those in which the primary problem is respiratory.
Some common causes of metabolic acidosis are excessive pro-
duction of lactic acid (during severe exercise or hypoxia) or of
ketone bodies (in uncontrolled diabetes mellitus or fasting, as
described in Chapter 16). Metabolic acidosis can also result
from excessive loss of bicarbonate, as in diarrhea. Another
cause of metabolic alkalosis is persistent vomiting, with its
associated loss of hydrogen ions as HCl from the stomach.
What is the arterial
in metabolic acidosis or alka-
losis? By deﬁ nition, metabolic acidosis and alkalosis must be
due to something other than excess retention or loss of carbon
dioxide, so you might have predicted that arterial
Renal Responses to Acidosis
Responses to Acidosis
1. Sufﬁ cient hydrogen ions are secreted to reabsorb all the
ﬁ ltered bicarbonate.
2. Still more hydrogen ions are secreted, and this contributes
new bicarbonate to the plasma as these hydrogen ions are
excreted bound to nonbicarbonate urinary buffers such as
3. Tubular glutamine metabolism and ammonium excretion
are enhanced, which also contributes new bicarbonate to the
More new bicarbonate ions than usual are added
to the blood, and plasma bicarbonate is increased,
thereby compensating for the acidosis. The urine is
highly acidic (lowest attainable pH = 4.4).
Responses to Alkalosis
1. Rate of hydrogen ion secretion is inadequate to reabsorb
all the ﬁ ltered bicarbonate, so signiﬁ cant amounts of
bicarbonate are excreted in the urine, and there is little or
no excretion of hydrogen ions on nonbicarbonate urinary
2. Tubular glutamine metabolism and ammonium excretion
are decreased so that little or no new bicarbonate is
contributed to the plasma from this source.
Plasma bicarbonate concentration is decreased,
thereby compensating for the alkalosis. The urine
is alkaline (pH > 7.4).
Changes in the Arterial Concentrations of Hydrogen Ions, Bicarbonate, and Carbon Dioxide
in Acid-Base Disorders
Cause of HCO
Cause of CO
Reﬂ ex ventilatory compensation
A patient has an arterial
of 50 mmHg, an arterial
of 60 mmHg, and an arterial pH of 7.36. Classify the acid-base disturbance and
hypothesize a cause.
Answer can be found at end of chapter.