Anatomy of the Small Intestine

The small intestine is the largest part of the gastrointestinal tract. Three sections make up the small intestine, the duodenum, which is about a foot long and closest to the stomach, the jejunum is 5 to 8 feet long and the ileum, which is between 16 and 20 feet long.

Brunner's glands in the duodenum secrete an alkaline mucus that neutralizes the highly acidic chyme (semi-liquefied food) from the stomach. Without this alkalization of the chyme, the duodenum and small intestine would quickly become ulcerated. The vagus nerve, and direct food stimulation in the stomach, trigger the Brunner's gland secretions. Globlet cells in the mucosa also produce mucus. The duodenum is more prone to ulceration than the stomach due to less mucus protection. A deficiency of pancreatic enzymes, and/or stress that causes sympathetic nervous system inhibition of enzyme secretions, can lead to duodenal ulcers.

The duodenum is where the majority of digestive enzyme secretion takes place. In the duodenum the enzyme sucrase breaks down sucrose into glucose and fructose, the enzyme maltase changes maltose into glucose and lactase, another enzyme breaks down lactose into glucose and galactose. People who are lactose intolerant are usually deficient in the enzyme lactase. The enzyme peptidase breaks down peptides from protein into amino acids and lipase changes fat into glycerol and fatty acids.

Bile is received in the small intestine from the liver and gall bladder through the bile duct. Bile reduces the surface tension between water and fat globules. This allows them to be acted upon by lipase. Upon hormonal signals from pancreozymin and other hormones produced in the small intestine, and neural signals from the vagus nerve, the pancreas produces and delivers lipase, amylase, trypsin, chymotrypsin and sodium bicarbonate to the small intestine.