Spider plot of the dependence of plasma pH on changes in the three independent variables ([SID+], PCO2, and [ATOT]) and five rate constants (solubility of CO2 in plasma [S], apparent equilibrium constant [K'1], effective equilibrium dissociation constant [Ka], apparent equilibrium dissociation constant for HCO3- [K'3], and ion product of water [K'w]) of Stewart's strong ion model. The spider plot is obtained by systematically varying one input variable while holding the remaining input variables at their normal values for human plasma. The influence of S and K'1 on plasma pH cannot be separated from that of PCO2, inasmuch as the three factors always appear as one expression. Large changes in two factors (K'3 and K'w) do not change plasma pH. [ATOT], total concentration of weak acids; PCO2, partial CO2 tension; SID+, strong ion difference. Reproduced with permission from Constable .