The Response of Extracellular Hydrogen Ion Concentration to Graded Degrees of Chronic Hypercapnia: The Physiologic Limits of the Defense of pH*

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Abstract
Studies have been carried out in 10 normal dogs in order to characterize the response of acid-base equilibrium to chronic, stepwise increases in arterial CO2 tension. At each concentration of CO2 (7, 11, 17%) the animals were exposed for a period of at least 5 days to allow extracellular composition to reach a new steady state. The data demonstrate that every increment of PCO2 induced a rise in renal acid excretion and a marked rise in plasma bicarbonate concentration, but at no level of CO2 tensions was the pH of the extracellular fluid restored to normal. A notable finding was the strikingly linear relationship between hydrogen ion concentration and PCO2, each millimeter of increment in carbon dioxide tension inducing an average rise of 0.32 nmoles per L of hydrogen ion concentration. On the basis of this finding it has been suggested that a feedback mechanism senstive to hydrogen ion concentration may govern the addition of bicarbonate to the extracellular fluid by the kidneys during chronic hypercapnia. The response curves for plasma hydrogen ion and bicarbonate concentrations, when expressed in terms of 95% significance bands, indicate that a relatively narrow range of concentrations defines the anticipated physiologic response to a given degree of chronic hypercapnia. The possible application of such significance bands in delineating metabolic acid-base disturbances in chronic respiratory acidosis has been considered.