# Duodenum Anatomy Beyond the Basics

> Duodenum parts and relations, major and minor papillae, ligament of Treitz, SMA syndrome and ulcers for MBBS Anatomy exams.

- Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/duodenum
- Exam / course: MBBS · Subject: Anatomy
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Duodenum Anatomy Beyond the Basics", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/duodenum

## Direct answer

Twenty-five centimetres long and C-shaped around the head of the pancreas, the duodenum runs from the pylorus to the duodenojejunal flexure in four parts of very different character. The first part alone is peritoneal and mobile — which is why ulcers here perforate or bleed dramatically — while the second part carries the major papilla 8 to 10 cm beyond the pylorus, the third is clamped between aorta and superior mesenteric vessels, and the fourth is suspended by the ligament of Treitz, the landmark that divides upper from lower gastrointestinal bleeding on endoscopy.

## What you must remember

- **First part (about 5 cm):** the "duodenal cap", peritoneal and mobile; the gastroduodenal artery lies behind it — a posterior ulcer erodes it and bleeds.
- **Second part (about 8 cm):** descends along the right of L1 to L3, retroperitoneal; the major papilla (ampulla of Vater) opens 8 to 10 cm from the pylorus, with the minor papilla of Santorini's duct about 2 cm proximal.
- **Third part (about 10 cm):** horizontal at L3, crossed anteriorly by the superior mesenteric artery and vein — compressed between them and the aorta in the superior mesenteric artery syndrome.
- **Fourth part (about 2.5 cm):** ascends to the left of the aorta and ends at the duodenojejunal flexure, suspended by the ligament of Treitz from the right crus of the diaphragm.
- **Blood supply:** superior and inferior pancreaticoduodenal arteries, arising from the coeliac territory and the superior mesenteric artery respectively — the anatomical watershed between foregut and midgut.
- **Embryological divide:** the junction of foregut and midgut lies at the major papilla, which is why this region assembles bile duct, pancreatic ducts and duodenum in one small crossroads.
- **Posterior relations of the second part:** the inferior vena cava and right renal vessels — relevant in retroperitoneal duodenal rupture.

## Walking through a bleeding ulcer

A 45-year-old man with a long history of burning epigastric pain presents with melaena and a haemoglobin of 7 g/dL. The endoscopist finds a ulcer on the posterior wall of the first part of the duodenal bulb. Behind this centimetre of bowel lies the gastroduodenal artery, separated from the lumen by little more than shared adventitia once the ulcer penetrates the muscle coat — penetration is the mechanism, haemorrhage the presentation, and endoscopic adrenaline injection or surgical ligation the treatment. Had the ulcer leaned anteriorly, it would have perforated into the peritoneal cavity under the liver, giving the pneumoperitoneum and right shoulder-tip pain of a perforated duodenal ulcer.

Two centimetres can decide the operation. Juxtapyloric ulcers behave like duodenal ulcers; ulcers just beyond the second part's papilla raise the question of Zollinger-Ellison syndrome, because gastrinoma's ulcers march past the usual duodenal territory. The third part, meanwhile, explains the asthenic teenager with vomiting after meals and weight loss: a narrow aortomesenteric angle compresses the duodenum against the aorta, the superior mesenteric artery syndrome, classically after rapid weight loss or body spica immobilisation.

## Where students slip

The ligament of Treitz is described as a peritoneal fold; it is a musculofibrous structure from the right crus of the diaphragm to the duodenojejunal flexure, and its failure in malrotation lets the duodenum and caecum share a common mesentery with midgut volvulus. The minor papilla is forgotten: it drains the accessory pancreatic duct of Santorini and matters in pancreas divisum. And students place the major papilla "in the third part" — it belongs to the posteromedial wall of the second part, reachable by the side-viewing duodenoscope of ERCP at 8 to 10 cm from the pylorus.

## Frequently asked questions

### Why does a posterior duodenal ulcer bleed torrentially?

It erodes the gastroduodenal artery lying directly behind the first part of the duodenum.

### Where does the major duodenal papilla open?

On the posteromedial wall of the second part, 8 to 10 cm beyond the pylorus, receiving the common bile duct and main pancreatic duct through the ampulla of Vater.

### What is the superior mesenteric artery syndrome?

Compression of the third part of the duodenum between the aorta behind and the superior mesenteric vessels in front, causing proximal obstruction in asthenic individuals or after rapid weight loss.

### What is the ligament of Treitz?

A musculofibrous band from the right crus of the diaphragm to the duodenojejunal flexure that suspends the duodenojejunal junction and marks the endoscope's dividing line of upper GI bleeding.

### How is the duodenum's blood supply arranged?

By the superior pancreaticoduodenal artery from the gastroduodenal (coeliac territory) and the inferior pancreaticoduodenal from the superior mesenteric artery — an anastomotic watershed between foregut and midgut.

### Which duodenal part is most injured in blunt abdominal trauma?

The second and third parts, being retroperitoneal, rupture with handlebar or seat-belt injury and present with retroperitoneal gas rather than free peritonitis.