It is the first and shortest segment of the small intestine.
It receives partially digested food (known as chyme) from the stomach and plays a vital role in the chemical digestion of the chyme in preparation for absorption in the small intestine.
Many chemical secretions from the pancreas, liver, and gallbladder mix with chyme in the duodenum to facilitate chemical digestion.
Located below the stomach, the duodenum is a hollow C-shaped tube 10-12 inches (25-30 cm) long. The duodenum is a part of the gastrointestinal (GI) tract, attached to the pyloric sphincter of the stomach at its upper end and the jejunum of the small intestine at its lower end.
The pancreas, liver, and gallbladder release their digestive secretions into the duodenum through a hole known as the ampulla of Vater, which is located roughly in the middle of the duodenum on the left side.
The walls of the duodenum are made up of four layers of tissue that are consistent with the structure of the rest of the gastrointestinal tract:
- The innermost layer, the mucosa, lines the inner surface of the duodenum and is in contact with the chyme that passes through the intestinal lumen. It is made of a simple columnar epithelial tissue with microvilli on its surface to increase its surface area and improve nutrient absorption.
- Surrounding the mucosa layer is the submucosa, a layer of connective tissue that supports the other layers of tissue. Many blood vessels and nerves pass through the submucosa, while protein fibers give strength and elasticity to the duodenum.
- Surrounding the submucosa is the muscular layer that contains the smooth muscle tissue of the duodenum. Contractions of the muscularis mix the chyme and propel it through the duodenum into the rest of the small intestine.
- Lastly, the serosa is the outermost layer of the duodenum that acts as the outer skin of the intestine. The serous membrane made of simple squamous epithelium provides a smooth, slippery surface to avoid friction between the duodenum and surrounding organs.
After being stored and mixed with hydrochloric acid in the stomach for about 30 to 60 minutes, the chyme slowly enters the duodenum through the pyloric sphincter.
Next, Brunner’s glands in the mucosa of the duodenum secrete an alkaline mucus containing a high concentration of bicarbonate ions to neutralize the hydrochloric acid present in the chyme.
This alkaline mucus protects the walls of the duodenum and helps the chyme reach a pH conducive to chemical digestion in the small intestine.
Upon reaching the ampulla of Vater in the middle of the duodenum, the chyme mixes with bile from the liver and gallbladder, as well as the pancreatic juice produced by the pancreas. These secretions complete the chemical digestion process that began in the mouth and stomach by breaking complex macromolecules into their basic units.
The bile produced in the liver and stored in the gallbladder acts as an emulsifier, breaking down lipids into smaller globules to increase their surface area.
Pancreatic juice contains many enzymes to break down carbohydrates, lipids, proteins, and nucleic acids into their monomeric subunits. For example, pancreatic lipase breaks triglycerides, or fats, into glycerol and fatty acids that can be absorbed into the bloodstream by the intestinal wall.
Slow waves of smooth muscle contraction known as peristalsis flow through the gastrointestinal tract to push the chyme through the duodenum. Each wave begins in the stomach and pushes the chyme a short distance into the jejunum.
It takes many peristaltic contractions over the course of an hour for the chyme to travel the entire length of the duodenum. Small regional contractions of the intestinal wall, known as cleavages, help mix the chyme with the digestive secretions in the duodenum and increase the rate of digestion.
The cleavages also increase the contact of the chyme with the cells of the mucosa to increase the absorption of nutrients through the intestinal wall.