823
CHAPTER TWENTY-ONE
Water, Electrolyte, and Acid-Base Balance
the concentration of bicarbonate ions in body fl
uids. These
mechanisms also help balance the sulfuric acid, phosphoric
acid, and various organic acids that appear in body fl
uids as
by-products of metabolic processes.
The metabolism of certain amino acids, for example,
produces sulfuric and phosphoric acids. Consequently, a diet
high in proteins may form excess acid. The kidneys compen-
sate by altering the tubular secretion of hydrogen ions, thus
resisting a shift in the pH of body fl
uids
(f g. 21.11)
. Once
hydrogen ions are secreted, phosphates F
ltered into the fl
uid
of the renal tubule buffer them, aided by ammonia (NH
3
).
Through deamination of certain amino acids, the cells of
the renal tubules produce ammonia, which diffuses readily
through cell membranes and enters the renal tubule. When
increase in the hydrogen ion concentration of body fl uids is
prolonged, the renal tubules increase ammonia production.
Ammonia is a weak base, so it can accept hydrogen ions to
form
ammonium ions
(NH
4
+
):
H
+
+ NH
3
NH
4
+
Cell membranes are impermeable to ammonium ions,
which are trapped in the renal tubule as they form and are
excreted with the urine. This mechanism helps to transport
excess hydrogen ions to the outside and helps prevent the
urine from becoming too acidic.
Time Course of Hydrogen Ion Regulation
The various regulators of hydrogen ion concentration operate
at different rates. Acid-base buffers function rapidly and can
convert strong acids or bases into weak acids or bases almost
immediately. ±or this reason, these chemical buffer systems
are called the body’s
f
rst line oF deFense
against shifts in pH.
Physiological buffer systems, such as the respiratory
and renal mechanisms, function more slowly and constitute
the
second line oF deFense.
The respiratory mechanism may
require several minutes to begin resisting a change in pH,
and the renal mechanism may require one to three days to
regulate a changing hydrogen ion concentration
(f g. 21.12)
.
tion of hydrogen ions increases, and the pH of the internal
environment drops (see chapter 19, pp. 757–758). Such an
increasing concentration of carbon dioxide in the central ner-
vous system and the subsequent increase in hydrogen ion
concentration in the cerebrospinal fl
uid stimulate chemosen-
sitive areas in the respiratory center.
In response, the respiratory center increases the depth
and rate of breathing so that the lungs excrete more carbon
dioxide. Hydrogen ion concentration in body fl uids returns
toward normal, because the released carbon dioxide is in
equilibrium with carbonic acid and CO
2
(f
g. 21.10)
:
CO
2
+ H
2
O
H
2
CO
3
H
+
+ HCO
3
Conversely, if body cells are less active, concentrations
of carbon dioxide and hydrogen ions in body fl uids remain
low. Breathing rate and depth fall. This increases the carbon
dioxide level in the body fl uids, returning pH to normal. If
the pH drops below normal, the respiratory center is stimu-
lated to increase the rate and depth of breathing.
Activity of the respiratory center, therefore, changes
in response to shifts in the pH of the body fl
uids, reducing
these shifts to a minimum. Most of the hydrogen ions in the
body fl
uids originate from carbonic acid produced when car-
bon dioxide reacts with water, so the respiratory regulation
of hydrogen ion concentration is important.
Renal Excretion of Hydrogen Ions
Nephrons help regulate the hydrogen ion concentration of
body fl uids by excreting hydrogen ions in the urine. Recall
from chapter 20 (p. 793) that the epithelial cells lining the
proximal and distal convoluted tubules and the collecting
ducts secrete these ions into the tubular fl
uid. The tubular
secretion of hydrogen ions is linked to tubular reabsorption
of bicarbonate ions. In this way, the kidneys also regulate
More CO
2
is eliminated through lungs
Rate and depth of breathing increase
Respiratory center is stimulated
H
2
CO
3
releases H
+
CO
2
reacts with H
2
O to produce H
2
CO
3
Cells increase production of CO
2
FIGURE 21.10
An increase in carbon dioxide
elimination follows an increase in carbon dioxide
production.
Concentration of H
+
in body fluids returns
toward normal
Increased metabolism
of amino acids
Increased formation
of sulfuric acid and
phosphoric acid
High intake of proteins
Increased concentration
of H
+
in urine
Increased secretion
of H
+
into fluid of
renal tubules
Increased concentration
of H
+
in body fluids
FIGURE 21.11
If the concentration of hydrogen ions in body F
uids increases, the renal
tubules increase their secretion of hydrogen ions into the urine.
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