Calculation of the buffering capacity of bicarbonate in the rumen and in vitro.

We describe a model to calculate the buffering capacity of bicarbonate in the rumen. The addition of NaHCO3 results in the release of CO2 from solution and eventually from the rumen via eructation. This process directly neutralizes ruminal acidity. The degree to which the process continues depends on the partial pressure of CO2 in the gas phase, the pH, and a constant (7.74), according to the Henderson-Hasselbalch equation: pH = 7.74 + log([HCO3-]/pressure of CO2 in atmospheres). The addition of NaHCO3 to buffer solutions and ruminal fluid under high pressure of CO2 increased pH as predicted. The buffering capacity of ruminal fluid under CO2 was greater at low pH than was previously determined by titration in air. In contrast, in vitro systems in which CO2 is not permitted to escape may result in reduced buffering capacity. In vitro systems in which excess CO2 may escape (under N2 gas pressure) may result in uncontrolled pH elevation. Dilution of ruminal fluid under constant pressure of CO2 decreased ruminal pH as predicted by the model. The pH under different pressures at equilibrium and the buffering capacity are easily calculated for in vitro and in vivo systems.