Analysis of Bicarbonate Concentration in Human Blood Plasma at Steady State in Vivo

Address for Correspondence:Masaji Mochizuki, Minami-11, Nishi-20, 4-23 Chuo-Ku, Sapporo 064-0811, Japan The bicarbonate concentration in plasma , [HCO3], has a PCO2-dependent component, [HCO3]*, and a metabolic component, [HCO3]. The [HCO3]* in elderly patients and volunteers was expressed by an exponential function of PCO2 obtained by analysis of the venous-arterial (a-v) difference in [HCO3]. The slope of [HCO3]* against PCO2 was about 25% higher than that measured in oxygenated and deoxygenated blood . The regression functions of constituent ions were all linear versus [HCO3]* and [HCO3] over the physiological PCO2 range. The relationship between PCO2 and [HCO3]* agreed well with that calculated from the CO2 reaction rates on the active site of carbonic anhydrase using the Michaelis-Menten equation. At steady state the a-v difference in [HCO3] is proportional to that in O2 content ([O2]) as estimated from the respiratory quotient (RQ). Thus, [HCO3]* inevitably implied the Haldane effect component of [HCO3] ([HCO3]HE). However, [HCO3] measured in oxygenated and deoxygenated blood was free from the change in [O2] or the Haldane effect. The relationship between [HCO3]HE and [HCO3]* could be evaluated from the and difference in [HCO3], and the effect of RQ on [HCO3]HE was clarified. For analysing the ionic concentrations in plasma at steady state and the acid-base status, the equation for [HCO3]* was considered indispensable.