Chapter 26
Fluid, Electrolyte, and Acid-Base Balance
1005
26
and strong bases are converted to weak bases:
NaOH
1
NaH
2
PO
4
S
Na
2
HPO
4
1
H
2
O
strong base
weak acid
weak base
water
Because the phosphate buffer system is present in low concen-
trations in the ECF (approximately one-sixth that of the bicar-
bonate buffer system), it is relatively unimportant for buffering
blood plasma. However, it is a very effective buffer in urine and
in ICF, where phosphate concentrations are usually higher.
Protein Buffer System
Proteins in plasma and in cells are the body’s
protein buffer sys-
tem
. In fact, at least three-quarters of all the buffering power of
body fluids resides in cells, and most of this reflects the powerful
buffering activity of intracellular proteins.
As described in Chapter 2, proteins are polymers of amino
acids. Some of the linked amino acids have exposed groups of
atoms called
carboxyl groups
(—COOH), organic acid groups
that release H
1
when the pH begins to rise:
R—COOH
S
R—COO
2
1
H
1
(Note that R indicates the rest of the organic molecule, which
contains many atoms.)
Other amino acids have exposed groups that can act as bases
and accept H
1
. For example, an exposed —NH
2
group can bind
with a hydrogen ion, becoming —NH
3
1
:
R—NH
2
1
H
1
S
R—NH
3
1
Because this binding removes free hydrogen ions from the so-
lution, it prevents the solution from becoming too acidic. Conse-
quently, a single protein molecule can function reversibly as either
an acid or a base depending on the pH of its environment. Molecules
with this ability are called
amphoteric molecules
(am
0
fo-ter
9
ik).
the buffer systems actually buffer one another, so the
entire
buffer system resists any dri±s in pH.
Bicarbonate Buffer System
Te
bicarbonate buffer system
is a mixture of carbonic acid
(H
2
CO
3
) and its salt, sodium bicarbonate (NaHCO
3
, a weak
base), in the same solution. Although it also buffers the ICF, it is
the
only
important ECF buffer.
Carbonic acid, a weak acid, does not dissociate to any great
extent in neutral or acidic solutions. When a strong acid such
as HCl is added to this buffer system, the existing carbonic acid
remains intact. However, the bicarbonate ions of the salt act as
weak bases to tie up the H
1
released by the stronger acid (HCl),
forming
more
carbonic acid:
HCl
1
NaHCO
3
S
H
2
CO
3
1
NaCl
strong acid
weak base
weak acid
salt
Because it is converted to the weak acid H
2
CO
3
, HCl lowers the
pH of the solution only slightly.
When a strong base such as sodium hydroxide (NaOH) is
added to the same buffer solution, a weak base such as sodium
bicarbonate (NaHCO
3
) does not dissociate further under the al-
kaline conditions. However, the added base forces the carbonic
acid to dissociate further, donating more H
1
to tie up the OH
2
released by the strong base:
NaOH
1
H
2
CO
3
S
NaHCO
3
1
H
2
O
strong base
weak acid
weak base
water
Te net result is that a weak base (NaHCO
3
) replaces a strong
base (NaOH), so the pH of the solution rises very little.
Although the bicarbonate salt in the example is sodium bi-
carbonate, other bicarbonate salts function in the same way be-
cause HCO
3
2
is the important ion, not the cation it is paired
with. In cells, where little Na
1
is present, potassium and mag-
nesium bicarbonates are part of the bicarbonate buffer system.
Te buffering power of this type of system is directly related
to the concentrations of the buffering substances. If enough acid
enters the blood so that all the available HCO
3
2
ions, o±en re-
ferred to as the
alkaline reserve
, are tied up, the buffer system
becomes ineffective and blood pH changes.
Te bicarbonate ion concentration in the ECF is normally
around 25 mEq/L and is closely regulated by the kidneys. Te
concentration of H
2
CO
3
is just over 1 mEq/L but the supply
of H
2
CO
3
(which comes from the CO
2
released during cellular
respiration) is almost limitless, so obtaining that member of the
buffer pair is usually not a problem. Te H
2
CO
3
content of the
blood is subject to respiratory controls.
Phosphate Buffer System
Te operation of the
phosphate buffer system
is nearly identical
to that of the bicarbonate buffer. Te components of the phosphate
system are the sodium salts of dihydrogen phosphate (H
2
PO
4
2
)
and monohydrogen phosphate (HPO
4
2
2
). NaH
2
PO
4
acts as a weak
acid. Na
2
HPO
4
, with one less hydrogen atom, acts as a weak base.
Again, H
1
released by strong acids is tied up in weak acids:
HCl
1
Na
2
HPO
4
S
NaH
2
PO
4
1
NaCl
strong acid
weak base
weak acid
salt
(a) A strong acid such as
HCI dissociates
completely into its ions.
(b) A weak acid such as
H
2
CO
3
does
not
dissociate completely.
H
2
CO
3
HCO
H
+
H
2
CO
3
H
2
CO
3
H
+
HCO
H
+
CI
CI
CI
CI
CI
CI
CI
H
+
H
+
H
+
H
+
H
+
H
+
H
2
CO
3
H
2
CO
3
HCI
3
3
Figure 26.11
Dissociation of strong and weak acids in
water.
HCl: hydrochloric acid; H
2
CO
3
: carbonic acid. Undissociated
molecules are shown in colored ovals.
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