The duodenum, the shortest part of the small intestine at about 10 to 12 inches long, is the first part of the lower gastrointestinal tract. This hallow jointed tube connects the stomach to the jejunum. The anterior part of the duodenum called the duodenal bulb is attached to that part of the stomach called the pyloric sphincter and the end part of the duodenum called the ligament of Treitz connects to the jejunum. This ligament of Treitz is a suspensory muscle that attaches the duodenum to the diaphragm. The duodenum is the most fixed part of the small intestine.
The duodenum has four parts. The first three parts are the superior duodenum, the descending duodenum and the inferior duodenum forms like a letter “C” and the last part known is the ascending duodenum. Only the superior duodenum is covered by the peritoneum. The rest of the duodenum is retroperitoneal and therefore immobile.
Most chemical digestion happens in the duodenum. The duodenal epithelium releases secretin and cholecystokinin when the pylorus opens and gastric chyme is released into the duodenum. This signals the liver and gallbladder to release digestive enzymes like trypsin, lipase and amylase through the pancreatic duct and the common bile duct which are directly connected to the duodenum. The Brunner’s glands that lines the duodenum secrete alkaline mucus that neutralizes the acidity of the chyme coming from the stomach. This protects the duodenum from too much acidity and in consequence the rest of the small intestine because the duodenum then passes a less acidic chyme to the jejunum. This mucus from the Brunner’s glands helps in the absorption of the nutrients as well. The duodenum is also the main site for iron absorption.