1006
UNIT 4
Maintenance of the Body
26
the normal range. Tese respiratory system–mediated correc-
tions of blood pH are accomplished within a minute or so.
Changes in alveolar ventilation can produce dramatic
changes in blood pH—far more than is needed. For example,
doubling alveolar ventilation can raise blood pH by about 0.2
pH unit. Likewise, cutting alveolar ventilation in half can lower
blood pH by the same amount. Because normal arterial pH is
7.4, a change of 0.2 pH unit yields a blood pH of 7.6 or 7.2—
both well beyond the normal limits. Respiratory controls of
blood pH have a tremendous reserve capacity because alveolar
ventilation can rise about 15-fold or fall (briefly) to zero.
Anything that impairs respiratory system functioning causes
acid-base imbalances. For example, net carbon dioxide reten-
tion (hypoventilation) leads to acidosis. On the other hand,
hyperventilation, which causes net elimination of CO
2
, causes
alkalosis. When respiratory system problems cause the pH im-
balance, the resulting condition is either
respiratory acidosis
or
respiratory alkalosis
(see ±able 26.3 on p. 1010).
Check Your Understanding
10.
Define acidemia and alkalemia.
11.
To minimize a shift in pH brought about by adding a
strong acid to a solution, would it be better if the solution
contained a weak base or a strong base?
12.
What are the body’s three major chemical buffer systems?
What is the most important buffer inside cells?
13.
Joanne, a diabetic patient, is at the emergency department
with acidosis due to the production of ketone bodies. Would
you expect her ventilation to be increased or decreased? Why?
For answers, see Appendix H.
Renal Mechanisms of Acid-Base Balance
Describe how the kidneys regulate hydrogen and
bicarbonate ion concentrations in the blood.
Te ultimate acid-base regulatory organs are the kidneys, which
act slowly but surely to compensate for acid-base imbalances re-
sulting from variations in diet or metabolism, or disease. Chem-
ical buffers can tie up excess acids or bases temporarily, but they
cannot eliminate them from the body. And while the lungs can
dispose of the
volatile acid
carbonic acid by eliminating CO
2
,
only the kidneys can rid the body of other acids generated by
cellular metabolism: phosphoric, uric, and lactic acids, and ke-
tone bodies. Tese acids are referred to as
nonvolatile (fixed)
acids
. Additionally, only the kidneys can regulate blood levels
of alkaline substances and renew chemical buffers that are used
up in regulating H
1
levels in the ECF.
Te most important renal mechanisms for regulating acid-
base balance of the blood involve (1) conserving (reabsorbing)
or generating new HCO
3
2
, and (2) excreting HCO
3
2
. If we look
back at the equation for the carbonic acid–bicarbonate buffer sys-
tem, we can see that losing a HCO
3
2
from the body produces the
same net effect as gaining a H
1
, because it pushes the equation to
the right, increasing the H
1
level. By the same token, generating
or reabsorbing HCO
3
2
is the same as losing H
1
because it pushes
the equation to the le², decreasing the H
1
level. For this reason,
Hemoglobin in red blood cells is an excellent example of a
protein that functions as an intracellular buffer. As we explained
in Chapter 22 (p. 829), CO
2
released from the tissues forms
H
2
CO
3
, which dissociates to liberate H
1
and HCO
3
2
in the
blood. Meanwhile, hemoglobin is unloading oxygen, becoming
reduced hemoglobin, which carries a negative charge. Because
H
1
rapidly binds to the hemoglobin anions, pH changes are
minimized. In this case, carbonic acid, a weak acid, is buffered
by an even weaker acid, hemoglobin.
Respiratory Regulation of H
1
Describe the influence of the respiratory system on acid-
base balance.
Te respiratory and renal systems together form the
physiologi-
cal buffering systems
that control pH by regulating the amount of
acid or base in the body. Although physiological buffer systems act
more slowly than chemical buffer systems, they have many times
the buffering power of all the body’s chemical buffers combined.
As we described in Chapter 22, the respiratory system elimi-
nates CO
2
, an acid, from the blood while replenishing its supply
of O
2
. Carbon dioxide generated by cellular respiration enters
erythrocytes in the circulation and is converted to bicarbonate
ions for transport in the plasma:
carbonic
anhydrase
CO
2
1
H
2
O
m
H
2
CO
3
m
H
1
1
HCO
3
2
carbonic
bicarbonate
acid
ion
Te first set of double arrows indicates a reversible equilibrium
between dissolved carbon dioxide and water on the le² and car-
bonic acid on the right. Te second set indicates a reversible
equilibrium between carbonic acid on the le² and hydrogen
and bicarbonate ions on the right. Because of these equilibria,
an increase in any of these chemical species pushes the reaction
in the opposite direction. Notice also that the right side of the
equation is equivalent to the bicarbonate buffer system.
Healthy individuals expel CO
2
from the lungs at the same
rate it is formed in the tissues. During carbon dioxide unload-
ing, the reaction shi²s to the le², and H
1
generated from car-
bonic acid is reincorporated into water. Because of the protein
buffer system, H
1
produced by CO
2
transport is not allowed to
accumulate and has little or no effect on blood pH.
If P
CO
2
rises, it activates medullary chemoreceptors (via
cerebral acidosis promoted by excessive accumulation of
CO
2
) that respond by increasing respiratory rate and depth
(see Figure 22.25, p. 836). Additionally, a rising plasma H
1
concentration resulting from any metabolic process excites
the respiratory center indirectly (via peripheral chemorecep-
tors) to stimulate deeper, more rapid respiration. As ventila-
tion increases, more CO
2
is removed from the blood, pushing
the reaction to the le² and reducing the H
1
concentration.
When blood pH rises, the respiratory center is depressed. As
respiratory rate drops and respiration becomes shallower, CO
2
accumulates, pushing the equilibrium to the right and causing
the H
1
concentration to increase. Again blood pH is restored to
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