STUDIES IN CALCIUM METABOLISM. THE FATE OF INTRAVENOUSLY INJECTED RADIOCALCIUM IN HUMAN BEINGS 123

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Abstract
Over 90% of injected radiocalcium (Ca45) disappeared from the bloodstream of 9 boys and one adult within 20 minutes, at a rate of 19% per minute in the boys. Following an initially faster rate, the Ca45 disappeared more slowly from the bloodstream of the 21-year-old adult than from the blood of the boys (mean age: 12.6 years). The specific activities of urine and serum were equal. 1.5 to 2 times as much Ca45 was excreted via the urine than via the stools. The excretion of Ca45 in the feces indicated that probably no more than 120 mg endogenous Ca was excreted via the stools in these subjects. All subjects retained appreciable quantities of the injected Ca45. The "biological half-life" of Ca45 in the adult was 260 days in the period 18 to 59 days following injection.