192
C. ALBERS
oxygenated or deoxygenated hemoglobin (0,Hb and Hb, respectively).
Since carbonic anhydrase is not present in the plasma, the first reaction
is of minor importance. The other two reactions take place within the
red cell. From the law of mass action it is apparent that the higher the
concentration of hemoglobin, the higher the concentration of bicarbonate; hence, the CO, combining power of the blood. At the low body
temperatures of most fish a small fraction of the bicarbonate further
dissociates into carbonate ions and hydrogen ions. Because of the presence of ion pairs like NaCO:,- ( Siggaard-Andersen, 1963) it is impossible
at the present time to obtain quantitative information about the carbonate concentration from the few data available. The cations for the formation of bicarbonate are made available by the ampholytic dissociation
of the protein component of the hemoglobin, The functional group involved is the imidazole, the dissociation of which is strongly affected
by the oxygenation of the hemoglobin (see below).
Another chemical reaction between C 0 2 and hemoglobin depends on
some free a-amino groups, which at the alkaline side of the isoelectric
point form so-called carbamino hemoglobin according to the equation
HbNHi + CO1 HbNCOO- + H+
Since an increase in the concentration of CO, on the left side of the
equation is always associated with an increase in the concentration of
H at the right side of the equation, the equilibrium of the reaction is
hardly affected by the CO, tension. It is, however, strongly dependent
on the oxygenation of the hemoglobin. Although in mammals the carbamino hemoglobin is only a small fraction of the absolute amount of
the total CO., it plays an important role in the changes of the CO- concentration associated with the delivery of CO, from the tissues to the
capillary blood. This may be seen from the following example: In human beings arterial blood contains an average 22 mmoles/liter of total
CO,, which is made up of 1.25 mmoles/liter of physically dissolved CO,
(or 5.7% of the total CO,), 19.65 mmoles/liter of bicarbonate (or 89.3%
of the total), and 1.1 mmoles/liter of cxbamino hemoglobin (or 5.0%
of the total CO,). When this blood passes through a capillary it takes
up 2.2 mmoles/liter of C02, the venous blood having 24.2 mmoles/liter
of total CO,. The arteriovenous difference of 2.2 mmoles/liter is partitioned into 0.18 mmoles/liter of physically dissolved CO, (or 8.2% of
the arteriovenous difference), 1.39 mmoles/liter of bicarbonate (or 63%
of the arteriovenous difference ), and 0.63 mmoles/liter of carbamino
hemoglobin (or 28.8% of the arteriovenous difference), The changes in
carbamino hemoglobin therefore account for almost one-third of the
C. ALBERS
oxygenated or deoxygenated hemoglobin (0,Hb and Hb, respectively).
Since carbonic anhydrase is not present in the plasma, the first reaction
is of minor importance. The other two reactions take place within the
red cell. From the law of mass action it is apparent that the higher the
concentration of hemoglobin, the higher the concentration of bicarbonate; hence, the CO, combining power of the blood. At the low body
temperatures of most fish a small fraction of the bicarbonate further
dissociates into carbonate ions and hydrogen ions. Because of the presence of ion pairs like NaCO:,- ( Siggaard-Andersen, 1963) it is impossible
at the present time to obtain quantitative information about the carbonate concentration from the few data available. The cations for the formation of bicarbonate are made available by the ampholytic dissociation
of the protein component of the hemoglobin, The functional group involved is the imidazole, the dissociation of which is strongly affected
by the oxygenation of the hemoglobin (see below).
Another chemical reaction between C 0 2 and hemoglobin depends on
some free a-amino groups, which at the alkaline side of the isoelectric
point form so-called carbamino hemoglobin according to the equation
HbNHi + CO1 HbNCOO- + H+
Since an increase in the concentration of CO, on the left side of the
equation is always associated with an increase in the concentration of
H at the right side of the equation, the equilibrium of the reaction is
hardly affected by the CO, tension. It is, however, strongly dependent
on the oxygenation of the hemoglobin. Although in mammals the carbamino hemoglobin is only a small fraction of the absolute amount of
the total CO., it plays an important role in the changes of the CO- concentration associated with the delivery of CO, from the tissues to the
capillary blood. This may be seen from the following example: In human beings arterial blood contains an average 22 mmoles/liter of total
CO,, which is made up of 1.25 mmoles/liter of physically dissolved CO,
(or 5.7% of the total CO,), 19.65 mmoles/liter of bicarbonate (or 89.3%
of the total), and 1.1 mmoles/liter of cxbamino hemoglobin (or 5.0%
of the total CO,). When this blood passes through a capillary it takes
up 2.2 mmoles/liter of C02, the venous blood having 24.2 mmoles/liter
of total CO,. The arteriovenous difference of 2.2 mmoles/liter is partitioned into 0.18 mmoles/liter of physically dissolved CO, (or 8.2% of
the arteriovenous difference), 1.39 mmoles/liter of bicarbonate (or 63%
of the arteriovenous difference ), and 0.63 mmoles/liter of carbamino
hemoglobin (or 28.8% of the arteriovenous difference), The changes in
carbamino hemoglobin therefore account for almost one-third of the