819
CHAPTER TWENTY-ONE
Water, Electrolyte, and Acid-Base Balance
net reaction only. Remember, it is the concentration of H
+
at
equilibrium that determines the pH.)
1.
Aerobic respiration of glucose.
This process produces
carbon dioxide and water. Carbon dioxide diffuses out
of the cells and reacts with water in the extracellular
uids to form
carbonic acid:
CO
2
+ H
2
O
H
2
CO
3
The resulting carbonic acid then ionizes to release
hydrogen ions and bicarbonate ions:
H
2
CO
3
H
+
+ HCO
3
2.
Anaerobic respiration of glucose.
Glucose metabolized
anaerobically produces
lactic acid,
which adds hydrogen
ions to body fl
uids.
3.
Incomplete oxidation of fatty acids.
The incomplete
oxidation of fatty acids produces
acidic ketone bodies,
which increase hydrogen ion concentration.
4.
Oxidation of amino acids containing sulfur.
The
oxidation of sulfur-containing amino acids yields
sulfuric
acid
(H
2
SO
4
), which ionizes to release hydrogen ions.
21.5
ACID-BASE BALANCE
As discussed in chapter 2 (p. 59), electrolytes that ionize in
water and release hydrogen ions are
acids.
Substances that
combine with hydrogen ions are
bases.
Acid-base balance
entails regulation of the hydrogen ion concentration of body
fl uids. This is important because slight changes in hydrogen
ion concentrations can alter the rates of enzyme-controlled
metabolic reactions, shift the distribution of other ions, or
modify hormone actions. Recall from chapter 2 that pH num-
ber indicates the degree to which a solution is acidic or basic
(alkaline). The more acid the solution, the lower its pH, and
vice versa. The internal environment is normally maintained
between pH 7.35 and 7.45.
Sources of Hydrogen Ions
Most of the hydrogen ions in body fl uids originate as by-
products of metabolic processes, although the digestive tract
may directly absorb some hydrogen ions. The major meta-
bolic sources of hydrogen ions include the following. (These
are reversible reactions but, for clarity, are presented as the
FIGURE 21.8
If the concentration of calcium ions decreases, parathyroid hormone increases calcium ion concentration. Increased urinary
phosphate excretion oF
sets bone resorption (which increases blood phosphate levels) to maintain a normal concentration of phosphate ions.
Parathyroid glands
are stimulated
Calcium ion
concentration decreases
Activity of bone-resorbing
osteoclasts increases
Intestinal absorption
of calcium increases
Calcium ion concentration
returns toward normal
Normal phosphate
concentration is maintained
Addition of phosphate
to bloodstream
Increased phosphate
excretion in urine
Renal tubules conserve
calcium and increase
secretion of phosphate
Parathyroid hormone
is secreted
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