The small intestine is divided into three segments: duodenum, jejunum, and ileum. Each species has a small intestine adapted to their particular diet. The lumen surface of the small intestine is covered by millions of small projections called villi. Furthermore, metabolism takes place in the gut lumen before the compounds enter the portal vein.

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The structure of the intestinal wall is not consistent throughout the entire small intestine. The duodenum contains a thick wall, with deeply folded mucous membrane and duodenal digestive glands. The foldings are called the ‘folds of Kerckring’. They are well developed both in the duodenum and in the jejunum. The jejunum has a thicker wall and is more vasculated than the duodenum. Furthermore, it has larger and more villi than the ileum. The ileum has more lymphatic follicles than elsewhere in the intestine. The size and number of villi throughout the intestine varies upon diet and between populations.

Figure. Histology of the small intestine a three-dimensional view of layers of the small intestine showing villi (Tortora and Grabowski, 1996).

There have been relatively few studies done on pre- and postnatal maturation of intestinal motility (Heimann, 1980). In infants, intestinal motor activity occurs less frequently than in adults, with a different pattern of rhythmic peristaltic activity (Radde, 1985). In general, intestinal peristalsis is irregular and partially dependent on food intake and feeding habits (Heimann, 1980).
Another important difference between neonates and older individuals is the type and degree of bacterial colonisation of the gut. At birth the intestine is virtually sterile and a rapid colonisation occurs with a flora that is different in breast-fed and formula-fed infants (Raddle, 1985). A further change in bacterial flora occurs at the time of weaning (4-6 months) which is important for the hydrolysis of compounds that are conjugated and secreted in the bile so that unconjugated compound can be absorbed by the intestinal epithelium (e.g. conjugated bile acids). On the other hand, the neonatal gut is capable of converting glucuronides, excreted into the gastrointestinal tract from the bile, to their unconjugated and hence enterohepatic reabsorbable form by the presence of ß-glucuronidase. This enzyme is absent in the adult gut.