Acid-base balance evaluation according to the Henderson-Hasselbalch equation enables to assess what is a contribution of respiratory (pCO2) and/or non-respiratory (metabolic, HCO3 –) components to the acid-base balance status. A new approach to acid-base balance evaluation according to Stewart-Fencl, which is based on a detail physical- -chemical analysis of body fluids, shows that metabolic acid-base balance disorders are characterized not only by [HCO3 –]. According to this concept the independent variables (variables which can change primarily and independently of each other) must be taken into account. The abnormality of concentration of one or more independent variable(s) determines the pH of a solution. The independent variables are: (1) The strong ion difference (SID); (2) The total concentration of nonvolatile weak acids [Atot –]; (3) In agreement with the Henderson-Hasselbalch concept it is also pCO2. Traditional evaluation of acid-base balance disorders is based on the pH of body fluids (though pH may be within a normal range ever if several acid-base balance disturbances are present). In order to maintain this traditional view and to simultaneously respect the Stewart-Fencl principle, we invented a new equation, which uses only the independent variables to define the pH of body fluids. This analysis shows that for a given value of pCO2, the pH of body fluids is determined by a difference between SID and [Atot –]. An evaluation of the individual components of this equation enables to detect, which of the independent variables (or combination of the independent variables) is deviated from a normal range and therefore which one (or ones) is a cause of the acid-base balance disorder. At the end of this paper we give examples of a practical application of this equation.

        Key words: acid-base balance, Henderson-Hasselbalch equation, Stewart-Fencl principle.