Determination of rates of absorption in the blood by integration of rates of appearance and disappearance

Summary

We measured the changing concentrations in the arterial blood of a labeled substance that had been placed in the stomach or the small intestine. From these data a curve of the rate of appearance in the blood was obtained. Secondly, we determined the rate at which the same material disappeared from the blood when it was injected into the blood stream instantaneously; this was the disappearance curve. The two rates were determined simultaneously by use of tritiated water (THO) for intravenous injection, to give the disappearance curve, and by the use of heavy water (D₂O) for absorption, to give the appearance curve. From these two curves or rates—by their separation or “integration”—we obtain a measure of the actual rate at which the labeled material moved from the contents of the alimentary canal to the blood stream.