1010
UNIT 4
Maintenance of the Body
26
Effects of Acidosis and Alkalosis
Te absolute blood pH limits for life are a low of 6.8 and a high
of 7.8. When blood pH falls below 6.8, the central nervous sys-
tem is so depressed that the person goes into coma and death
soon follows. When blood pH rises above 7.8, the nervous sys-
tem is overexcited, leading to such characteristic signs as mus-
cle tetany, extreme nervousness, and convulsions. Death oFen
results from respiratory arrest.
Respiratory and Renal Compensations
If one of the physiological buffer systems (lungs or kidneys)
malfunctions and disrupts acid-base balance, the other system
tries to compensate. Te respiratory system attempts to com-
pensate for metabolic acid-base imbalances, and the kidneys
(although much slower) work to correct imbalances caused by
dioxide in the blood. Bicarbonate ion levels below or above the
normal range of 22–26 mEq/L indicate a metabolic acid-base
imbalance.
Te second most common cause of acid-base imbalance,
metabolic acidosis
, is recognized by low blood pH and HCO
3
2
levels. ±ypical causes are ingesting too much alcohol (which
is metabolized to acetic acid) and excessive loss of HCO
3
2
, as
might result from persistent diarrhea. Other causes are accu-
mulation of lactic acid during exercise or shock, the ketosis that
occurs in diabetic crisis or starvation, and, infrequently, kidney
failure.
Metabolic alkalosis
, indicated by rising blood pH and
HCO
3
2
levels, is much less common than metabolic acidosis.
±ypical causes are vomiting the acidic contents of the stomach
(or loss of those secretions through gastrointestinal suctioning)
and intake of excess base (too many antacids, for example).
Table 26.3
Causes and Consequences of Acid-Base Imbalances
CONDITION AND HALLMARK
POSSIBLE CAUSES; COMMENTS
Respiratory Acidosis (Hypoventilation)
If uncompensated
(uncorrected):
P
CO
2
.
45 mm Hg;
pH
,
7.35
Impaired lung function
(e.g., chronic bronchitis, cystic fibrosis, emphysema): impaired gas exchange or alveolar
ventilation
Impaired ventilatory movement:
paralyzed respiratory muscles, chest injury, extreme obesity
Narcotic or barbiturate overdose or injury to brain stem:
depression of respiratory centers, resulting in
hypoventilation and respiratory arrest
Respiratory Alkalosis (Hyperventilation)
If uncompensated:
P
CO
2
,
35 mm Hg;
pH
.
7.45
Strong emotions:
pain, anxiety, fear, panic attack
Hypoxemia:
asthma, pneumonia, high altitude; represents effort to raise P
O
2
at the expense of excessive CO
2
excretion
Brain tumor or injury:
abnormal respiratory controls
Metabolic Acidosis
If uncompensated:
HCO
3
2
,
22 mEq/L;
pH
,
7.35
Severe diarrhea:
bicarbonate-rich intestinal (and pancreatic) secretions rushed through digestive tract before
their solutes can be reabsorbed; bicarbonate ions are replaced by renal mechanisms that generate new
bicarbonate ions
Renal disease:
failure of kidneys to rid body of acids formed by normal metabolic processes
Untreated diabetes mellitus:
lack of insulin or inability of tissue cells to respond to insulin, resulting in inability
to use glucose; fats are used as primary energy fuel, and ketoacidosis occurs
Starvation:
lack of dietary nutrients for cellular fuels; body proteins and fat reserves are used for energy—both
yield acidic metabolites as they are broken down for energy
Excess alcohol ingestion:
results in excess acids in blood
Metabolic Alkalosis
If uncompensated:
HCO
3
2
.
26 mEq/L;
pH
.
7.45
Vomiting or gastric suctioning:
loss of stomach HCl requires that H
1
be withdrawn from blood to replace
stomach acid; thus H
1
decreases and HCO
3
2
increases proportionately
Selected diuretics:
cause K
1
depletion and H
2
O loss. Low K
1
directly stimulates tubule cells to secrete H
1
.
Reduced blood volume elicits the renin-angiotensin-aldosterone mechanism, which stimulates Na
1
reabsorption
and H
1
secretion.
Ingestion of excessive sodium bicarbonate (antacid):
bicarbonate moves easily into ECF, where it enhances
natural alkaline reserve
Excess aldosterone
(e.g., adrenal tumors): promotes excessive reabsorption of Na
1
, which pulls increased
amount of H
1
into urine. Hypovolemia promotes the same relative effect because aldosterone secretion is
increased to enhance Na
1
(and H
2
O) reabsorption.
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