C h a p t e r 8
Disorders of Fluid, Electrolyte, and Acid–Base Balance
193
CO
2
CO
2
HbCO
2
CO
2
dissolved
in plasma
20%
carried as
carbaminohemoglobin
(HbCO
2
)
Red blood
cell
Hemoglobin
(Hb)
Hemoglobin.
The remaining CO
2
in the red blood cells combines with
hemoglobin to form carbaminohe-
moglobin (HbCO
2
). The combina-
tion of CO
2
with hemoglobin is a
reversible reaction characterized by
a loose bond, so that CO
2
can be
easily released in the alveolar capil-
laries and exhaled from the lung.
3
phosphoric acid). The difference between the two types
of acids arises because H
2
CO
3
is in equilibrium with dis-
solved CO
2
, which is volatile and leaves the body by
way of the lungs. The nonvolatile or fixed acids are not
eliminated by the lungs. Instead, they are buffered by
body proteins or extracellular buffers, such as HCO
3
–
,
and then eliminated by the kidney.
Carbon Dioxide and Bicarbonate Production.
Carbon
dioxide, which is the end product of aerobic metabolism,
is transported in the circulation as a dissolved gas (i.e.,
PCO
2
), as the HCO
3
–
ion, or as CO
2
bound to hemo-
globin in carbaminohemoglobin (see understanding car-
bon dioxide transport). Collectively, dissolved CO
2
and
HCO
3
–
account for approximately 77% of the CO
2
that
is transported in the extracellular fluid; the remaining
CO
2
travels as carbaminohemoglobin.
2
Although CO
2
is a gas and not an acid, a small percentage of the gas
combines with water to form the weak H
2
CO
3
acid.
The reaction that generates H
2
CO
3
from CO
2
and water
is catalyzed by an enzyme called
carbonic anhydrase
,
which is present in large quantities in red blood cells,
renal tubular cells, and other tissues in the body. (see
Understanding: Carbon Dioxide Transport)
Because it is almost impossible to measure H
2
CO
3
,
carbon dioxide measurements are commonly used
when calculating pH. The H
2
CO
3
content of the
blood can be calculated by multiplying the partial
pressure of CO
2
(PCO
2
) by its solubility coefficient,
which is 0.03. This means that the concentration of
H
2
CO
3
in the arterial blood, which normally has a
PCO
2
of approximately 40 mm Hg, is 1.20 mEq/L
(40 × 0.03 = 1.20), and that for venous blood, which
normally has a PCO
2
of approximately 45 mm Hg, is
1.35 mEq/L.
Production of Nonvolatile Acids and Bases.
The
metabolism of dietary proteins and other substances
results in the generation of nonvolatile acids and bases.
4
For example, the metabolism of sulfur-containing amino
acids (e.g.,methonine and cysteine) results in the produc-
tion of
sulfuric acid
; of arginine and lysine,
hydrochloric
acid
; and of nucleic acids,
phosphoric acid
. Incomplete
oxidation of glucose results in the formation of
lactic
acid,
and incomplete oxidation of fats, the production
of
ketoacids
. The major source of bases is the metabo-
lism of amino acids such as aspartate and glutamate and
the metabolism of certain organic anions (e.g., citrate,
lactate, acetate).
Calculation of pH
The serum pH can be calculated using an equation
called the
Henderson-Hasselbalch equation.
This
equation uses the dissociation constant for the bicar-
bonate buffer system (which is 6.1) plus the log of
the HCO
3
–
- to - PCO
2
(used as a measure of H
2
CO
3
)
ratio (normally 20:1) to determine the pH (i.e.,
pH = 6.1 + log of 20 = 7.4). Because the ratio is used,
a change in either HCO
3
–
or PCO
2
will have little or
no effect on pH as long as there is an accompanying
change in PCO
2
and HCO
3
–
(Fig. 8-17). The pH will
decrease when the ratio decreases and increase when
the ratio increases.
Regulation of pH
The pH of body fluids is regulated by three major mecha-
nisms: (1) chemical buffer systems in body fluids, which
immediately combine with excess acids or bases to pre-
vent large changes in pH; (2) the lungs, which control
Transport
(
text continued from page 191
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