821
CHAPTER TWENTY-ONE
Water, Electrolyte, and Acid-Base Balance
In the presence of excess hydrogen ions,
monohydrogen phosphate ions act as a weak base,
combining with hydrogen ions to form dihydrogen
phosphate, minimizing increase in the hydrogen ion
concentration of body fl
uids.
H
+
+ HPO
4
–2
H
2
PO
4
On the other hand, if conditions are basic or alkaline,
dihydrogen phosphate, acting as a weak acid,
dissociates to release hydrogen ion:
H
2
PO
4
H
+
+
HPO
4
–2
3.
Protein buffer system.
The protein acid-base buffer
system consists of the plasma proteins, such as
albumins, and certain proteins in cells, including
hemoglobin in red blood cells.
As described in chapter 2 (p. 64), proteins are
chains of amino acids. Some of these amino acids
have freely exposed groups of atoms, called carboxyl
groups. If the H
+
concentration drops, a carboxyl group
(—COOH) can become ionized, releasing a hydrogen
ion, thus resisting the pH change:
—COOH
—COO
+ H
+
This is a reversible reaction. (However, the degree to
which it is reversible depends on the particular amino
acids.) In the presence of excess hydrogen ions, the
—COO
parts of the protein molecules accept hydrogen
ions and become —COOH groups again. This action
decreases the number of free hydrogen ions in the body
uids and again minimizes the pH change.
Some of the amino acids of a protein molecule also
have freely exposed amino groups (—NH
2
). If the H
+
concentration rises, these amino groups can accept
hydrogen ions in another reversible reaction (Once
again, the degree to which it is reversible depends on
the particular amino acids.):
—NH
2
+ H
+
—NH
3
+
In the presence of excess hydroxyl ions (OH
), the
—NH
3
+
groups of protein molecules give up hydrogen
despite the addition of an acid or a base. More speciF
cally,
the chemical components of a buffer system can combine
with strong acids to convert them into weak acids. Likewise,
these buffers can combine with strong bases to convert them
into weak bases. Such activity helps minimize pH changes in
the body fl uids. The three most important buffer systems in
body fl
uids are the bicarbonate buffer system, the phosphate
buffer system, and the protein buffer system.
In the following discussion, associated anions and cat-
ions have been omitted for clarity. ±or example, the weak
base sodium bicarbonate (NaHCO
3
) is represented by bicar-
bonate (HCO
3
). Sodium is also the cation associated with
the phosphate ions.
1.
Bicarbonate buffer system.
In the bicarbonate buffer
system, present in both intracellular and extracellular
fl uids, the bicarbonate ion (HCO
3
) acts as a weak
base, and carbonic acid (H
2
CO
3
) acts as a weak acid.
In the presence of excess hydrogen ions, bicarbonate
ions combine with hydrogen ions to form carbonic
acid, minimizing any increase in the hydrogen ion
concentration of body fl
uids:
H
+
+ HCO
3
H
2
CO
3
On the other hand, if conditions are basic or alkaline,
carbonic acid dissociates to release bicarbonate ion and
hydrogen ion:
H
2
CO
3
H
+
+ HCO
3
Although this reaction releases bicarbonate ion, it is
the increase of free hydrogen ions at equilibrium that
is important in minimizing the shift toward a more
alkaline pH.
2.
Phosphate buffer system.
The phosphate buffer
system is also present in both intracellular and
extracellular fl
uids. However, it is particularly important
in the control of hydrogen ion concentration in the
intracellular fl
uid and in renal tubular fl
uid and urine.
This buffer system consists of two phosphate ions,
dihydrogen phosphate (H
2
PO
4
) and monohydrogen
phosphate (HPO
4
2–
).
Aerobic
respiration
of glucose
Anaerobic
respiration
of glucose
Incomplete
oxidation of
fatty acids
Oxidation of
sulfur-containing
amino acids
Hydrolysis of
phosphoproteins
and nucleic acids
Carbonic
acid
H
+
Internal environment
Lactic
acid
Acidic ketone
bodies
Sulfuric
acid
Phosphoric
acid
FIGURE 21.9
Some of the metabolic processes that provide hydrogen ions.
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