SECTION
1.2
Buffers
9
present in several different isoenzyme forms. The most
prevalent red blood cell isoenzyme of carbonic anhydrase
is type I (CAI). Zinc ion, held by coordinate covalent link-
age by three imidazole groups of three histidine residues,
is involved in the catalytic mechanism of carbonic anhy-
drase. Water bound to zinc ion reacts with CO
2
bound to
the nearby catalytic site of carbonic anhydrase to produce
H
2
CO
3
. The action of carbonic anhydrase is essential for
a number of metabolic functions. Some include the for-
mation of H+ ion in stomach parietal cells (Chapter 12),
in bone resorption by the osteoclasts (Chapter 37) and
reclamation of HCO
3
in renal tubule cells (Chapter 39).
In osteoclasts and in the renal tubule cells, the isoenzyme
CAII catalyzes the hydration reaction of CO
2
. A deficiency
of CAII caused by an autosomal recessive disorder con-
sists of osteopetrosis (marble bone disease), renal tubular
acidosis, and cerebral calcification.
The diffusion of CO
2
from venous blood into the alveoli
is facilitated by a pressure gradient of CO
2
between the ve-
nous blood (45 mm Hg) and the alveoli (40 mm Hg) and by
the high permeability of the pulmonary membrane to CO
2
.
Blood leaving the lungs has a
Pco2
of about 40 mm Hg;
thus, essentially complete equilibration occurs between
alveolar C 0
2
and blood CO
2
.
Blood Buffer Calculations
Carbonic acid has the following pK' values:
H
2
CO
3
H+ + HCO^
pK'i = 3.8
(1.9)
HCO“
C 02
f
+ H+
pK
'2
= 10.2
(1.10)
It is apparent from the pK' values that neither equilib-
rium can serve as a buffer system at the physiological pH
of 7.4. However, carbonic acid (the proton donor) is in
equilibrium with dissolved CO
2
, which in turn is in equi-
librium with gaseous CO
2
:
[H2CO3]
Taking antilogarithms,
1.3
[HC0
3
]
20
proton acceptor
[H
2
CO
3
]
1
proton donor
This ratio is large because the pH is greater than the pK'
(see Table 1-3). At pH 7.4, the bicarbonate system is a good
buffer toward acid (i.e., it can neutralize large amounts of
acid) but a poor buffer for alkali. However, blood H
2
CO
3
is in rapid equilibrium with a relatively large (about
1 0 0 0
times as much) reservoir of cellular CO
2
and can function
as an effective buffer against increases in alkalinity. The
HCO
3 7
H
2
CO
3
ratio in blood is coupled to the partial pres-
sure of CO
2
, i.e., to the metabolic production of CO
2
and
to the loss of CO
2
during respiration. In the equilibrium
expression for the bicarbonate-carbonic acid buffer sys-
tem at pH 7.4, the carbonic acid term can be replaced by a
pressure term because the carbonic acid concentration is
proportional to
Pc
o
2
in the blood.
pH = 6.1 + log
[HCOJ]
a Pcch
(1.15)
where
a,
a proportionality constant, is defined by the equa-
tion
[H
2
CO
3
] =a/>co
2
(1.16)
The numerical value of
a
depends on the
solvent
and the temperature. For normal plasma at 37°C,
a ~
0.0301 mmol of dissolved CO
2
per liter of plasma per
mm Hg of CO
2
pressure. Assuming that 37°C (310 K)
is approximately the same as standard temperature, 25°C
(298 K), and that [H
2
CO
3
] in Equation (1.16) includes
both
carbonic acid and dissolved CO
2
, the value of
a
can
be derived from two facts:
H
2
O + C
0 2
(aqueous) ^ H
2
CO
3
(1.11)
The hydration reaction (1.11), coupled with the first
dissociation of carbonic acid (1.9), produces an apparent
pK' of
6
.1 for bicarbonate formation. Thus, the summation
of Equations (1.9) and (1.11) yields
H20 + C 0
2
^ H + + HCO^
(1.12)
,
[HCO-] [H+]
pK' (apparent) = L
= 6.1
(1.13)
The ratio of HCO
3
to H
2
C 0
3
at a physiological pH of 7.4
can be calculated by using the Henderson-Hasselbalch
relationship:
rH C crl
7.4 = 6.1 + log
------- (1.14)
ë [H
2
CO
3
]
'
'
1. At 37°C and 760 mm Hg of CO
2
pressure, 521 mL of
CO
2
will dissolve per liter of normal plasma;
2. At standard temperature and pressure, 1 mol of dry
CO
2
occupies a volume of 22.26 L (not 22.4 L since
CO
2
is not an ideal gas).
521 mL CO
2
/L of plasma
760 mm Hg x 22.26 mL/mmol of CO
2
mmol CO
2
/L of plasma
= 0.0301------------ - ------ --------
mm Hg of CO
2
pressure
The equation form of the Henderson-Hasselbalch ex-
pression [Equation (1.15)] can be further modified by
substituting another expression for the bicarbonate term.
When excess strong acid is added to plasma, CO
2
is sto-
ichiometrically released from dissolved CO
2
, carbonic
