C h a p t e r 8
Disorders of Fluid, Electrolyte, and Acid–Base Balance
191
Acid–Base Balance
Metabolic activities of the body require precise regula-
tion of acid–base balance as reflected in the pH of the
ECF, which is normally maintained within a very narrow
range of 7.35 to 7.45.
1–3
Membrane excitability, enzyme
systems, and chemical reactions all depend on the pH
being regulated within a narrow physiologic range to
function in an optimal way.
Acid–Base Chemistry
Acids and bases have their own chemical properties and
definitions. An
acid
is defined as a compound that can
dissociate and release a hydrogen (H
+
) ion and a
base
as
a compound that can accept or combine with H
+
.
1–3
For
example, hydrochloric acid (HCl) dissociates in water to
form H
+
and Cl
–
ions. The bicarbonate ion (HCO
3
–
) is a
base because it can combine with H
+
to form carbonic
acid (H
2
CO
3
). Most of the body’s acids and bases are
weak; the most important are H
2
CO
3
, which is a weak
acid derived from carbon dioxide (CO
2
), and HCO
3
–
,
which is a weak base.
The concentration of H
+
in body fluids is low com-
pared with other ions. For example, the Na
+
is present
at a concentration approximately 3.5 million times that
of the H
+
. Because it is cumbersome to work with such
a small number, the H
+
concentration is commonly
expressed in terms of the
pH
. Specifically, pH repre-
sents the negative logarithm (log
10
) of the H
+
concen-
tration expressed in mEq/L. Since the pH is inversely
related to the H
+
concentration, a low pH indicates a
high concentration of H
+
and a high pH a low concen-
tration of H
+
.
Acid and Base Production
Acids are continuously generated as by-products of met-
abolic processes (Fig. 8-16). Physiologically, these acids
fall into two groups:
volatile
H
2
CO
3
and all other
non-
volatile
or
fixed acids
(e.g., sulfuric, hydrochloric, and
■■
Magnesium, which is the second most abundant
ICF cation, acts as a cofactor in many intracellular
enzyme reactions and is required for cellular
energy metabolism, functioning of the Na
+
/K
+
-
ATPase membrane pump, nerve conduction, ion
transport, and potassium and calcium channel
activity. Hypomagnesemia produces a decrease in
serum calcium due to suppression of PTH release
and a decrease in serum potassium due to renal
wasting, both of which contribute to an increase
in neuromuscular exitability. Hypermagnesemia
causes neuromuscular dysfunction with
hyporeflexia, muscle weakness, and confusion.
■■
Parathyroid hormone disorders impact both
calcium and phosphate homeostasis. Acute
hypoparathyroidism causes hypocalcemia,
manifested by signs of increased neuromuscular
excitability such as muscle cramps and tetany.
Chronic hypoparathyroidism is manifested by
lethargy and fatigue. Hyperparathyroidism can
occur as a primary disorder causing elevated
serum calcium levels and increased urinary
excretion of both calcium and phosphorus,
which provides the potential for development of
kidney stones. Secondary hyperparathyroidism,
which associated with chronic kidney disease,
exerts its effects on bone, causing renal
osteodystrophies.
Food intake
Digestion
Absorption
Cell metabolism
of food
Sulfate
Phosphate
Chloride
Sulfate
Phosphate
Chloride
Sulfate
Phosphate
Chloride
Bound by
body buffer
bases
New
Excreted
Excreted
(combined with
urinary
buffer bases)
Extracellular
fluid
[HCO
3
-
]
Extracellular
fluid
[HCO
3
-
]
Chemical
buffering
Respiratory
response
Renal
response
CO
2
CO
2
CO
2
H
+
H
+
HCO
3
-
H
+
FIGURE 8-16.
The maintenance of normal blood pH by
chemical buffers, the respiratory system, and the kidneys. On a
mixed diet, pH is threatened by the production of strong acids
(sulfuric, hydrochloric, and phosphoric) mainly as the result
of protein metabolism.These strong acids are buffered in the
body by chemical buffer bases such as extracellular fluid (ECF)
bicarbonate (HCO
3
–
).The kidney eliminates hydrogen ions (H
+
)
combined with urinary buffers and anions in the urine. At the
same time, they add new HCO
3
–
to the ECF to replace the HCO
3
–
consumed in buffering strong acids.The respiratory system
disposes of carbon dioxide (CO
2
). (From Rhoades RA, Bell DR.
Medical Physiology: Principles of Clinical Medicine. 4th ed.
Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams &
Wilkins; 2013:454.)
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text continues on page 193
)
