The permeability of the epithelium to small ions and water-soluble organic molecules is greater in the duodenum and jejunum than it is in the ileum, which would indicate that there are larger and more numerous water-filled channels high in the small bowel. The variation in absorptive capacity can also, in part, be explained by the higher surface area per unit length in the upper intestine compared with the lower part. A few studies have been carried out to compare the absorption of drugs in the upper and lower small intestine. Human perfusion studies have demonstrated a proximal-to-distal small intestinal absorption
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