Some differences between duodenal and ileal sorption

¹ Mayo Clinic and Mayo Foundation, Rochester, Minnesota

Previous studies have shown that large exchanges of water and sodium occur in both directions across the mucosa of the duodenum between the contents and the blood as if to develop and maintain equilibrium with the plasma. In contrast, in the ileum the exchanges are directed to absorption of the contents rather than equilibration. These findings indicate differences in function of the mucosa of the two sites. To illuminate these, isosmotic and hypertonic solutions of a slowly absorbed substance (mannitol) were placed in duodenal and ileal segments of the small bowels of dogs. The changes in the composition of these solutions and the bidirectional fluxes of sodium and water have been determined. Although mannitol was absorbed by the two sites at equal rates and the osmolality of the contents of both regions changed in a parallel fashion, the composition of the contents in the two sites underwent quite different alterations. In the duodenum the volume was unchanged if the solution was isosmotic and increased if it was initially hypertonic, while in the ileum it decreased in both instances. Sodium always entered the duodenal contents in large quantities, while little appeared in the ileal contents.