The strong ion approach provides a quantitative physicochemical method for describing the mechanism for an acid-base disturbance. The approach requires species-specific values for the total concentration of plasma nonvolatile buffers (A(tot)) and the effective dissociation constant for plasma nonvolatile buffers (K(a)), but these values have not been determined for human plasma. Accordingly, the purpose of this study was to calculate accurate A(tot) and K(a) values using data obtained from in vitro strong ion titration and CO(2) tonometry. The calculated values for A(tot) (24.1 mmol/l) and K(a) (1.05 x 10(-7)) were significantly (P < 0.05) different from the experimentally determined values for horse plasma and differed from the empirically assumed values for human plasma (A(tot) = 19.0 meq/l and K(a) = 3.0 x 10(-7)). The derivatives of pH with respect to the three independent variables [strong ion difference (SID), PCO(2), and A(tot)] of the strong ion approach were calculated as follows: dpH/dSID(+) = [1 + 10(pK(a)-pH)](2)/(2.303 x [SPCO(2)10(pH-pK'(1)[1 + 10(pK(a)-pH](2) + A(tot)10(pK(a)-PH]]; dpH/dPCO(2) = S10(-pK'(1)/[2.303[A(tot)10(pH)(10(pH + 10(pK(a))(-2) - SID(+)10(-pH)]], dpH/dA(tot) = -1/[2.303[SPCO(2)10(pH-pK'(1) + SID(+)10(pK(a)-pH)]], where S is solubility of CO(2) in plasma. The derivatives provide a useful method for calculating the effect of independent changes in SID(+), PCO(2), and A(tot) on plasma pH. The calculated values for A(tot) and K(a) should facilitate application of the strong ion approach to acid-base disturbances in humans.