  In the physiology of oxygen-hemoglobin binding, an important role is played
by the influence of $\text{H}^+$ and $\text{CO}_2$ on the affinity of
hemoglobin for $\text{O}_2$. Here we extend the allosteric model of hemoglobin
to include these effects. We assume purely allosteric modulation, i.e., that
the modulatory effects of $\text{H}^+$ and $\text{CO}_2$ on oxygen binding
occur only because of their influence on the T $\leftrightarrow$ R transition,
in which all four subunits of the hemoglobin molecule participate
simultaneously. We assume, moreover, that these modulatory influences occur
only through the interaction of $\text{H}^+$ and $\text{CO}_2$ with the amino
group at the N-terminal of each of the four polypeptide chains of the
hemoglobin molecule. We fit the model to experimental data and obtain
reasonable agreement with the observed shifts in oxygen-hemoglobin binding that
occur when the concentrations of $\text{H}^+$ and $\text{CO}_2$ are changed.
