Two definitions of the base excess (BE) curve are known. The first definition is a geometrical one. We call the second definition the physicochemical one. These definitions are only valid if the water shift between red blood cells and plasma caused by pH alteration is neglected. We show that under this condition the BE curve cannot be calculated according to the mentioned definition. On the contrary we get a well-defined BE curve by calculation only if water shift is taken into account. In that case we must modify the physicochemical definition. The modified equations redefining the BE curve are derived. Furthermore the equivalence of both the geometrical definition and the modified physicochemical definition is shown. Using the modified definition the BE curve is calculated and compared to the curve measured by SIGGARD-ANDERSEN.