Duodenums

The duodenum is the first section of the small intestine, forming a roughly C-shaped tube about 25 to 30 centimeters long. It begins at the pyloric sphincter of the stomach and ends at the duodenojejunal flexure, where it transitions to the jejunum. The term duodenum is from Latin, meaning twelve, referring to its approximate length in finger-breadths. In common usage the plural is usually duodena; duodenums is rarely used outside some nontechnical contexts.

Anatomically, the duodenum is largely retroperitoneal except for its proximal part, which is intraperitoneal. It is divided into four parts: the superior (first) part, the descending (second) part where the major and minor duodenal papillae permit entry of bile, pancreatic juice, and other secretions, the horizontal (third) part, and the ascending (fourth) part. Its position and connections place it at the curve of the stomach and adjacent to the head of the pancreas. The major duodenal papilla opens into the second part via the common channel of the bile and pancreatic ducts; the minor papilla allows drainage from an accessory pancreatic duct.

The duodenum receives chyme from the stomach and is the primary site where chyme is neutralized and mixed with bile and pancreatic enzymes. This enables chemical digestion of fats, proteins, and carbohydrates. The mucosa features circular folds (plicae circulares), villi, and Brunner’s glands in the submucosa that secrete alkaline mucus to protect and lubricate the mucosa.

Blood supply comes from anterior and posterior superior pancreaticoduodenal arteries (from the gastroduodenal artery) and inferior pancreaticoduodenal arteries (from the superior mesenteric artery). Innervation is via sympathetic and parasympathetic fibers from celiac and superior mesenteric plexuses. Clinically significant conditions include duodenal ulcers, pancreatitis involving the surrounding structures, and congenital anomalies such as annular pancreas.