The buffer capacity and buffer systems of human whole saliva measured without loss of CO2

The buffer capacity of unstimulated (UWS) and stimulated (SWS) whole-mouth saliva involves three major buffer systems. The aim was to determine the buffer capacity of UWS and SWS at specific pH in the interval from pH 7.5 down to pH 3.0. The contribution of each of the buffer systems was also determined under conditions resembling those in the mouth. UWS and SWS were collected from 20 healthy volunteers; the saliva was collected under paraffin oil in order to avoid loss of CO2. The buffer capacity of UWS and SWS in samples with and without bicarbonate (HCO−3) and CO2 were measured at various pH by acid titration in a closed system at 36°C. The mean concentrations of the buffer systems in UWS (mean flow rate 0.55 ml/min) were 4.4 mmol/l HCO−3, 4.5 mmol/l phosphate (of which 1.3 mmol/l was present in the form of HPO2−4), 1876 μg/ml protein; the saliva pH was 6.8 and the Pco2 29.3 mmHg. The corresponding mean concentrations in SWS (mean flow rate 1.66 ml/min) were 9.7 mmol/l HCO−3, 3.8 mmol/l phosphate (of which 1.9 mmol/l was present in the form of HPO2−4), 1955 μg/ml protein; pH 7.2 and Pco2 25.7 mmHg. The highest buffer capacity of UWS and SWS was 6.0 and 8.5 mmol H+/(l saliva∗pH unit) at pH 6.25, respectively. At saliva pH in the range from pH 7 down to pH 5, the following had significant impact on buffer capacity: the HCO−3 concentration (p<0.001), the flow rate (p<0.01), and the pH of the saliva (p<0.05). At acidic pH in the range from pH 5 down to pH 4, however, only the protein concentration had a significant impact on buffer capacity (p<0.01).