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C HAP TE R 7 / Fluid and Electrolyte and Acid–Base Balance and Imbalance 167

Table 7-15 ■ ROLE OF BUFFERS WITH RESPECT TO AN ACID OR BASE LOAD
Role with Base
Buffer Role with Carbonic Acid Load Role with Metabolic Acid Load (Bicarbonate) Load
Extracellular bicarbonate Not effective Major role (immediate action) Not effective
Other extracellular buffers Minor role (immediate action) Minor role (immediate action) Minor role (immediate action)
Intracellular buffers Major role (10–30 minutes) Important role (2–4 hours) Important role (hours)
Bone buffers Probably not important Important role (2–4 hours) Important role (hours)

A buffer system cannot buffer its own acid. Thus, the bicarbonate If the urine were to become too acidic, it could damage the
buffer system cannot buffer carbonic acid. The carbonic acid that cells that line the urinary tract. Fortunately, the urine does not be-
is produced by cells (as CO 2 and H 2 O) is buffered primarily by come dangerously acidic because the hydrogen ions in the renal
intracellular buffers. The bicarbonate buffer system is a major tubules are buffered by the urine buffers or combine chemically
buffer for metabolic acids. Table 7-15 summarizes the role of with ammonia. Ammonia (NH 3 ) is produced by renal tubular
buffers with respect to acid or base loads. cells and then diffuses into the tubular fluid. 135 Hydrogen ions
combine with ammonia in the tubular fluid to produce ammo-

Acid Excretion nium ions (NH 4 ). Because ammonium ions are charged parti-
Even though the buffers minimize pH changes while acid is pro- cles, they cannot cross the cell membranes to enter the blood; thus,
duced, they have a limited capacity. Therefore, acid excretion they are “trapped” in the renal tubular fluid and excreted in the
mechanisms are necessary to maintain acid–base balance. The urine. An increase of acid in the body (decreased pH) causes the
body has two acid excretion methods: the lungs excrete carbonic production of more ammonia, which facilitates renal excretion of
acid and the kidneys excrete metabolic acids. acid. This process begins within 2 hours but takes several days to
be maximally effective. 3
Role of the Lungs. The lungs excrete carbonic acid in the
Thus, the kidneys have several mechanisms that result in the
form of carbon dioxide and water. They cannot excrete metabolic
excretion of metabolic acids produced by cellular metabolism.
acids. When alveolar ventilation increases (increased rate and
These mechanisms can be adjusted to excrete more acid or less
depth of ventilation), more carbonic acid is excreted. Conversely,
acid, thereby maintaining the bicarbonate ion concentration
when alveolar ventilation decreases, less carbonic acid is excreted.
within normal limits. Changes in renal function with normal ag-
Because carbonic acid essentially is carbon dioxide and water, the
ing cause older adults to excrete an acid load more slowly than
body actually senses and regulates the partial pressure of carbon
younger adults.
dioxide (Pa CO2 ).
If carbonic acid begins to accumulate (increased Pa CO2 ),
chemoreceptors in the medulla and carotid and aortic bodies are Summary of Acid–Base Balance
stimulated by the increased Pa CO2 and decreased pH. 135 The re-
Cellular metabolism produces carbonic acid and metabolic acids.
sulting increased alveolar ventilation causes excretion of the excess
These acids must be excreted to maintain normal acid–base bal-
carbonic acid. Similarly, if too little carbonic acid is present (de-
ance. Buffers in all body fluids act to minimize changes in pH due
creased Pa CO2 ), the chemoreceptors are less stimulated, and alve-
to an acid load or a bicarbonate (base) load. Carbonic acid is ex-
olar ventilation decreases somewhat to retain carbonic acid in the
creted by the lungs; increases or decreases in alveolar ventilation
body. Hypoxia, sensed by the carotid chemoreceptors, stimulates
regulate the amount of carbonic acid excretion. The Pa CO2 is the
alveolar ventilation and may override the suppression of ventila-
clinical indicator of carbonic acid. Metabolic acids are excreted by
tion from decreased Pa CO2 . In a healthy person, alveolar ventila-
the kidneys, which can excrete more or less acid as needed. The
tion changes rapidly in response to changes in Pa CO2 , and thus
plasma bicarbonate ion concentration (or total CO 2 ) is the clinical
carbonic acid is excreted at a rate effective in maintaining 138
indicator of the amount of metabolic acid. Table 7-16 summa-
acid–base balance.
rizes the physiologic responses that maintain acid–base balance.
Role of the Kidneys. The kidneys excrete metabolic acids.
They cannot excrete carbonic acid. The renal epithelial cells that
line the proximal tubules secrete hydrogen ions into the renal tu- ACID–BASE IMBALANCES
bular fluid and reabsorb bicarbonate ions in the process. 136 Bicar-
bonate is the major extracellular buffer of metabolic acids. There- Acid–base imbalances occur when the capacity of the buffers to
fore, the bicarbonate ion concentration indicates how much modulate pH changes is exceeded. Two terms are important in un-
metabolic acid is present. A decreased serum bicarbonate concen- derstanding the physiologic responses to acid–base imbalances.
tration indicates increased amounts of metabolic acid. When the Correction of the imbalance occurs when the original problem is
proximal tubular cells secrete hydrogen ions that are eventually ex- fixed so that the pH, Pa CO2 , and plasma bicarbonate ion concen-
creted in the urine, they reabsorb bicarbonate ions, replenishing tration can return to normal. 138,139 Compensation for an acid–base
the bicarbonate ions that were used in buffering. Hydrogen ions imbalance restores the pH toward normal, but does not correct the
also are secreted into the renal tubular fluid by cells that line the problem that originally caused the imbalance. 139 In many cases, an
distal tubules and collecting ducts and these cells also can secrete acid–base imbalance persists long enough that compensatory physi-
bicarbonate into the tubular fluid or reabsorb it into the blood. 137 ologic processes occur. A partially compensated acid–base imbalance