The Nature of the Carbon Dioxide Titration Curve in the Normal Dog*

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Abstract
Acute changes in extracellular fluid composition were studied in unanesthetized dogs subjected to whole body titration with CO2. Each animal was exposed over a 6-hour period to 3 concentrations of CO2 that produced arterial CO2 tensions ranging from approximately 30 to 140 mm Hg. The data demonstrate that increasing degrees of hypercapnia induce a curvilinear rise in extracellular HCO3 concentration, the increment in HC03 diminishing as pCO2 is elevated. On the other hand, changes in pCO2 induced linear changes in H+ ion concentration over the entire range of CO2 tensions studied. The mean increase in H+ was 0.77 moles per L for every millimeter increment in pCO2. Reversing the sequence of exposure, i.e., beginning the experiment with the most severe degree of hypercapnia, did not alter this relationship. We suggest that whole body titration with CO2 may provide a useful means for detecting experimentally induced changes in whole body buffer characteristics.