Rate of Elimination of Labeled Carbon Dioxide From the Body

Abstract

When C14-labeled bicarbonate is injected intravenously about 10% of it is exhaled within 30 seconds by the lungs before it is mixed with body CO2. Labeled CO2 that has escaped elimination by the lungs is mixed with body CO2 in a complex manner. The CO2 in those tissues with a high blood flow equilibrates very rapidly with that of the blood. Resting muscle and skin need at least 5 minutes for equilibration with the CO2 of the blood. Calculations of body CO2 pool based on the assumption of complete instantaneous equilibrium between blood and tissues are not valid. The manner of elimination of labeled CO2 produced in metabolism is complex and does not simulate that following intravenous or intraperitoneal injection of labeled bicarbonate.