Graph of independent variables (PCO2, [SID+] and [ATOT]) versus pH. Published values were used for the rate constants Ka, K'w, K'1, K3, and SCO2. Point A represents [SID+] = 45 mEq/l and [ATOT] = 20 mEq/l, and point B represents [SID+] = 40 mEq/l and [ATOT] = 20 mEq/l. In moving from point A to point B, ΔSID+ = AB = base excess. However, if [ATOT] decreases from 20 to 10 mEq/l (point C), then AC ≠ SID+ ≠ base excess. [ATOT], total concentration of weak acids; PCO2, partial CO2 tension; SCO2, CO2 solubility; SID+, strong ion difference. Reproduced with permission from Corey .