Fig. 4.

Determinants of [H⁺] in brain compartments. The putative pathogenesis of excessive acidosis during brain ischemia may be more readily defined if the [H⁺] and how it is established can be understood in important brain compartments such as the interstitial space or glia and neuronal intracellular space. In biological fluids [H⁺] is completely determined by the strong ion difference ([SID]), P_CO2, and total weak acid concentration ([A_tot]) (Stewart, 1981, 1983). It is important to recognize that [SID], P_CO2, and [A_tot] are three independent variables that completely define [H⁺] and other dependent variables, [OH⁻], [ ${\text{CO}}_3^{2-}$], [A⁻], [HA], and [ ${\text{HCO}}_3^{-}$]. In brain trans-membrane ion fluxes influence [H⁺] in a brain compartment through changes in [SID] while physicochemical H⁺ buffers which are thought to remain confined to viable cells influence [H⁺] through changes in [A_tot]. To accurately describe [H⁺] in a given compartment, equations 1 through 8 must be solved simultaneously.