Intestinal structure and function related to toxicology.

The study of toxic effects on small intestinal function is complicated by the integration of the activity of the small intestine with the activities of other regions of the GI tract. Also, the barrier and portal functions of the intestine are not as clearly defined as sometimes assumed. The intestinal surface functions as a barrier to the ingress of large quantities of large water soluble molecules. Lipidic substances enter the body quite readily as do small water-soluble molecules. The small intestinal surface is more a portal than a barrier, with its portal functions divided between nonspecific diffusional entry, which depends on physical properties and electric charge, and entry by specific membrane transport, which depends upon chemical structure. The implications of these properties of the small intestine for toxicological studies are stressed.