The Duodenum

The duodenum is the shortest part, measuring about 25.4 cm (10 in). It begins at the pyloric sphincter, curves around the head of the pancreas in a C-shape, and then joins the jejunum. 

It is divided into four parts: the superior, descending, horizontal, and ascending segments. 

The hepatopancreatic ampulla (ampulla of Vater) is located in the wall of the duodenum. Here, the common bile duct and the main pancreatic duct join and empty into the duodenum through the major duodenal papilla. 

The hepatopancreatic sphincter (sphincter of Oddi) regulates the flow of bile and pancreatic juice into the intestine.

Histology of the Small Intestine

The wall of the small intestine has the same four layers found throughout the alimentary canal. However, the mucosa and submucosa contain three specialized features that greatly increase the surface area for absorption: circular folds, villi, and microvilli. Together, these structures increase the absorptive surface area by more than 600-fold, with the greatest concentration in the proximal two-thirds of the small intestine, where most absorption occurs.

Circular folds (plicae circulares) are deep folds of the mucosa and submucosa that extend from the proximal duodenum to the middle of the ileum. They slow the movement of chyme by causing it to spiral through the intestine, giving more time for digestion and nutrient absorption.

Villi are small, finger-like projections about 0.5-1 mm long that cover the circular folds and give the intestinal lining a fuzzy appearance. There are about 20-40 villi per square millimeter. Each villus contains connective tissue, smooth muscle, a network of blood capillaries, and a central lymphatic vessel called a lacteal. Sugars and amino acids are absorbed into the blood capillaries, while lipid digestion products enter the lacteals and are transported through the lymphatic system before reaching the bloodstream.

Microvilli are microscopic projections on the apical surface of the absorptive epithelial cells. Together they form the brush border, which contains enzymes that complete the digestion of carbohydrates and proteins. With approximately 200 million microvilli per square millimeter, they greatly increase the surface area available for absorption.

Between the villi are intestinal glands (crypts of Lieberkühn), which secrete intestinal juice, a slightly alkaline mixture of water and mucus with a pH of 7.4-7.8. About 0.95-1.9 L of intestinal juice is produced each day in response to intestinal distension or irritation by chyme. In the duodenum, Brunner's (duodenal) glands secrete bicarbonate-rich alkaline mucus that neutralizes acidic chyme entering from the stomach.

The small intestine also contains mucosa-associated lymphoid tissue (MALT). In the distal ileum, clusters of lymphatic tissue called Peyer's patches help prevent bacteria from entering the bloodstream. They are most prominent in young people and become less distinct with age.

Mechanical Digestion

Mechanical digestion in the small intestine occurs mainly through segmentation and migrating motility complexes, rather than the mixing waves seen in the stomach. During segmentation, circular smooth muscle contracts rhythmically, mixing chyme with digestive juices and pressing it against the intestinal wall to promote absorption instead of moving it forward. Segmentation occurs most rapidly in the duodenum (about 12 contractions per minute) and slows to about 8 contractions per minute in the ileum.

As absorption is completed and the intestine becomes less distended, migrating motility complexes replace segmentation. These peristaltic waves begin in the duodenum and slowly move chyme toward the ileum over 90-120 minutes.

The ileocecal valve regulates the movement of chyme into the large intestine. Increased motility in the ileum causes the valve to relax, allowing chyme to enter the cecum. This process is stimulated by the gastroileal reflex and the hormone gastrin. After chyme passes through, pressure from the large intestine closes the valve to prevent backflow. Normally, chyme leaves the small intestine within 3-5 hours.

Chemical Digestion

Most protein and carbohydrate digestion, which begins in the stomach, is completed in the small intestine with the help of intestinal juice and pancreatic juice. Most lipid digestion also occurs in the small intestine through the action of bile and pancreatic lipase.

Intestinal juice and pancreatic juice provide a fluid medium that supports digestion and absorption. The small intestine is also the primary site for water absorption by osmosis. Unlike the stomach, digestive enzymes in the small intestine are found not only in the lumen but also attached to the brush border of the microvilli, where they complete the digestion of carbohydrates and proteins.

For efficient digestion, chyme must enter the duodenum slowly and in small amounts. This allows time for acidic chyme to be neutralized, mixed with bile and pancreatic juice, and properly digested without overwhelming the small intestine.