The duodenum is a short portion of the small intestine connecting it to the stomach. It is about 10 inches (25 cm) long, while the entire small intestine measures about 20 feet (6.5 meters). This structure begins with the duodenal bulb, bordered by the pyloric sphincter that marks the lower end of the stomach, and is connected by the ligament of Treitz to the diaphragm before leading into the next portion of the small intestine, the jejunum.
There are four distinct parts to the duodenum, the first three forming a "C" shape. It begins with the superior duodenum, which extends from the pyloric sphincter laterally to the right and posteriorly for about 2 inches (5 cm). The next section, the descending duodenum, is the middle of the "C". The inferior duodenum passes laterally to the left, and the last portion, the ascending duodenum, joins the jejunum at the duodenojejunal flexure.
Though the portion is such a tiny fraction of the small intestine, it is the site of most of the breakdown of the food passing through it. It is lined with Brunner's glands, which secrete an alkaline mucus that supports the intestinal enzymes and aids in the absorption of nutrients. The pancreatic duct, which introduces bile and pancreatic juice into the small intestine, is directly connected to the descending duodenum. Pancreatic juice contains enzymes that help break down food, while bile aids in the digestion and absorption of fats. This part of the small intestine is responsible for secreting hormones that trigger the pancreatic duct to release pancreatic juice and bile.
The duodenum also serves to neutralize the acidity of the chyme that exits the stomach, an intermediate product in the digestive process. Both the Brunner's glands and the pancreatic duct secrete alkaline fluids to temper the acidity of the chyme. In addition, the mucus secreted by the Brunner's glands helps protect the duodenum from the acidity, making it much less sensitive than the rest of the small intestine to the material. This allows it to help protect the rest of the small intestine by neutralizing the chyme to some extent before it passes into the jejunum.