Comparison of Rate of Absorption of Labeled Sodium and Water From Upper Small Intestine of Healthy Human Beings

¹ From the Section of Medicine, the Section of Physiology and the Section of Biophysics and Biophysical Research, Mayo Clinic and Mayo Foundation, Rochester, Minnesota

The rate of absorption of isotopically labeled water (D₂O) and sodium (Na²²) from the small intestine was determined in eight healthy adult human beings. The tests were made in the morning after an overnight fast. The isotopes were placed in the small intestine via a tube which had been swallowed and then passed into the third portion of the duodenum. The rate of appearance of the isotopes in the arterial blood was obtained by determination of their concentration in samples of arterial blood which were drawn in rapid succession. The actual rate of passage of the labeled substances from the contents of the intestine into the blood stream was calculated by integration of their rates of appearance in the arterial blood during their absorption with their previously determined rates of disappearance from the arterial blood when they had been given quickly intravenously. Labeled water uniformly passed more rapidly than Na²² from the contents of the small intestine into the blood stream. Both were absorbed rapidly. The mean initial rate of absorption of D₂O was about 20%/min. of that administered and the mean rate of absorption of radiosodium was about 10%/min. The mean time required for absorption of 50 and 67% was about 4 and 7 minutes, respectively, for D₂O and 7 and 10 minutes, respectively, for radiosodium.

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