SMALL INTESTINE

The small intestine, which includes the duodenum, jejunum, and ileum, receives chyme from the stomach, bile from the liver, and digestive enzymes from the pancreas. Here, nutrients are hydrolyzed into an absorbable form; they are absorbed and transferred to blood and lymphatic capillaries. Undigested material is moved to the large intestine by peristalsis. The word small refers to diameter, not length: the small intestine is longer and narrower than the large intestine.
A. General Structure: The walls of the small intestine have the same layers as do the rest of the tract. A series of permanent folds, the plicae circulares (valves of Kerckring), composed of both submucosa and mucosa, extend into the lumen and increase the surface area about 3-fold. The main distinguishing features of the small intestine (as viewed through the microscope) are in the composition and organization of the mucosa.

B. Mucosa of the Small Intestine: This consists of simple columnar epithelium with goblet cells, underlain by a lamina propria and separated from the submucosa by a muscularis mucosae.