Duodenum

Duodenum

The duodenum is the first, shortest and most fixed part of the small intestine. It is a C-shaped tube about 25 cm long that wraps around the head of the pancreas and receives bile and pancreatic juice at the major duodenal papilla.

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Overview

Length≈25 cm

Parts (approx.)1st 5 cm · 2nd 7–10 cm · 3rd 6–8 cm · 4th 5 cm

Vertebral levelsL1 → L3 → L2

The duodenum runs from the pylorus to the duodenojejunal flexure. Its name comes from its length of about twelve fingerbreadths (duodecim). Apart from its first 2–2.5 cm it is secondarily retroperitoneal. The duodenum is where the stomach's acidic chyme is neutralised and mixed with bile and pancreatic enzymes. Its chemosensors and enteroendocrine cells (S cells for secretin, I cells for CCK, K cells for GIP, M cells for motilin) provide the feedback that controls gastric emptying and pancreaticobiliary secretion.

The first part has a smooth-walled proximal ampulla (duodenal cap or bulb), the site of most duodenal ulcers. Plicae circulares start in the second part. The major duodenal papilla, where the hepatopancreatic ampulla opens, lies on the posteromedial wall of the descending part. The minor duodenal papilla, for the accessory pancreatic duct, lies about 2 cm above it. The junction of foregut and midgut lies just distal to the major papilla. This is why the duodenum has a double arterial supply, from both the coeliac trunk and the SMA.

Duodenal (Brunner) glands in the submucosa, which are characteristic of the duodenum, secrete alkaline mucus (pH 8–9) that protects the mucosa from acid. Enteropeptidase in the brush border activates trypsinogen. The duodenum is also the main site of iron absorption and of active calcium absorption.

Location: Upper abdomen at L1–L3, curving around the head of the pancreas. The superior part lies at the transpyloric plane, L1.

Parts & subdivisions

Superior (1st) part
About 5 cm, at L1. The proximal 2–2.5 cm (ampulla or cap) is intraperitoneal and mobile, attached to the hepatoduodenal ligament above and the greater omentum below. Posteriorly lie the gastroduodenal artery, bile duct, portal vein and IVC.
Descending (2nd) part
About 7–10 cm, running down at L1–L3 to the right of the vertebrae. It curves around the pancreatic head, has the right kidney behind and is crossed in front by the transverse colon. Contains the major and minor duodenal papillae.
Horizontal (3rd) part
About 6–8 cm at L3. Crosses the IVC and aorta, passing between the aorta behind and the SMA and SMV in front.
Ascending (4th) part
About 5 cm. Rises to the left of the aorta to L2 and the duodenojejunal flexure, which is held up by the suspensory muscle of the duodenum (ligament of Treitz).
Duodenal (Brunner) glands
Submucosal glands secreting alkaline mucus. They are a histological hallmark of the duodenum.
Major and minor duodenal papillae
Openings of the hepatopancreatic ampulla (major) and of the accessory pancreatic duct (minor, about 2 cm proximal).

Anatomy

Innervation: Vagus nerve (parasympathetic) and greater and lesser splanchnic nerves via the coeliac and superior mesenteric plexuses (sympathetic).
Arterial supply: Superior pancreaticoduodenal arteries (anterior and posterior, from the gastroduodenal artery → common hepatic → coeliac trunk) anastomose with the inferior pancreaticoduodenal artery (from the SMA). The supraduodenal artery supplies the cap.
Venous drainage: Pancreaticoduodenal veins drain to the hepatic portal vein and the superior mesenteric vein.
Lymphatic drainage: Pancreaticoduodenal nodes drain to the coeliac nodes (anterior and upper) and the superior mesenteric nodes.
Relations: 1st part: liver and gallbladder in front; bile duct, gastroduodenal artery and portal vein behind. 2nd part: transverse colon in front; right kidney, renal hilum and ureter behind; pancreatic head medially. 3rd part: SMA and SMV in front; IVC, aorta and right psoas behind. 4th part: left psoas and left sympathetic trunk behind.
Histology: Broad, leaf-like villi; submucosal Brunner glands; numerous enteroendocrine cells.
Development: Develops from the terminal foregut and the proximal midgut. In weeks 5–8 the lumen is temporarily occluded by proliferating epithelium and then recanalises.

Function

Receives chyme, bile and pancreatic juice and neutralises the acid.
Main site of iron and active calcium absorption; folate and other nutrients are also absorbed here.
Hormonal control of the stomach, pancreas and gallbladder (secretin, CCK, GIP, motilin).
Activates pancreatic zymogens through brush-border enteropeptidase.

Clinical significance

Duodenal ulcer

Usually in the anterior or posterior wall of the cap. Mostly caused by H. pylori or NSAIDs. Epigastric pain is eased by eating. A posterior ulcer can erode the gastroduodenal artery, causing major haemorrhage. An anterior ulcer can perforate, producing free air under the diaphragm and peritonitis.

Duodenal atresia

Failure of recanalisation. Causes bilious vomiting in the first days of life (when the atresia is distal to the papilla), polyhydramnios, and a "double bubble" on radiographs. About a third of affected infants have Down syndrome.

Superior mesenteric artery syndrome

When the aortomesenteric angle narrows, usually after rapid weight loss, the 3rd part of the duodenum is compressed between the SMA and the aorta. Patients have postprandial pain and vomiting.

Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.