20.4: Small Intestine
The coiled tube of the small intestine is subdivided into three regions. From proximal (at the stomach) to distal, these are the duodenum, jejunum, and ileum (Figure \(\PageIndex{1}\)).
Histology
The wall of the small intestine is composed of the same four layers typically present in the alimentary system. However, three features of the mucosa and submucosa are unique. These features, which increase the absorptive surface area of the small intestine more than 600-fold, include circular folds, villi, and microvilli ( Figure 20.9 ). These adaptations are most abundant in the proximal two-thirds of the small intestine, where the majority of absorption occurs.