By MRCSA in MRCS Part A — 04 Jul 2026

05 DUODENUM

Overview

The duodenum is the C-shaped first part of the small bowel, roughly 25 cm long, wrapped around the head of the pancreas. It is the transition zone between foregut and midgut, which makes it a favourite MRCS Part A topic — embryology, blood supply, anatomical relations and surgical pathology all converge here.

It is retroperitoneal except for the first 2–3 cm of D1 (the "duodenal cap"), which is intraperitoneal and mobile. The rest is fixed to the posterior abdominal wall, which explains why duodenal injury is easy to miss on laparotomy and why peptic ulcers in D1 behave very differently depending on whether they sit anteriorly or posteriorly.

[Image: C-shaped duodenum wrapping around pancreatic head, showing D1–D4, ampulla of Vater, and the foregut–midgut transition]

The four parts

➡ D1 (Superior) — From pylorus, runs right and slightly posterior at the level of L1. The first 2 cm is the intraperitoneal "duodenal cap" — the most common site for peptic ulcers.

➡ D2 (Descending) — Curves down from L1 to L3 on the right of the vertebral column. Contains the ampulla of Vater (major duodenal papilla) where the common bile duct and main pancreatic duct drain. The minor papilla (accessory pancreatic duct of Santorini) sits ~2 cm proximal.

➡ D3 (Horizontal/Inferior) — Crosses left at L3, anterior to the IVC and aorta. The SMA and SMV cross anteriorly — the basis of SMA syndrome.

➡ D4 (Ascending) — Ascends to the left of the aorta to L2, where it turns forward at the duodenojejunal flexure, anchored by the suspensory ligament of Treitz (a fibromuscular band from the right crus of the diaphragm).

Anterior and posterior relations

You will be asked these. Memorise the table.

Part	Anterior relations	Posterior relations
D1	Quadrate lobe of liver, gallbladder	Gastroduodenal artery, CBD, portal vein, IVC
D2	Transverse colon, small bowel loops	Right kidney & hilum, right ureter, right renal vessels, psoas
D3	SMA & SMV, small bowel loops	Aorta, IVC, right ureter, right psoas, right gonadal vessels
D4	Transverse mesocolon, small bowel	Left psoas, left sympathetic chain, left gonadal & renal vessels

👩‍⚕️ The classic trap: the right kidney is posterior to D2, NOT D3. D3 is anterior to the great vessels (aorta and IVC) — the kidneys sit lateral and higher.

Ampulla of Vater — the foregut–midgut watershed

The ampulla of Vater (hepatopancreatic ampulla) sits on the postero-medial wall of D2. It receives the common bile duct and main pancreatic duct, surrounded by the sphincter of Oddi, and drains onto the major duodenal papilla.

This single point matters for three reasons:

1. Embryological transition. Everything proximal to the ampulla is foregut; everything distal is midgut. This dictates blood supply, lymphatic drainage and autonomic innervation.

2. Blood supply watershed. Proximal duodenum gets the superior pancreaticoduodenal artery (from gastroduodenal → coeliac trunk). Distal duodenum gets the inferior pancreaticoduodenal artery (from SMA). The two anastomose around the head of pancreas.

3. Surgical landmark. A gallstone impacted at the ampulla causes obstructive jaundice ± pancreatitis. ERCP cannulates here. Sphincterotomy is performed here.

👩‍⚕️ Foregut = coeliac trunk + greater splanchnic nerves (T5–T9) + referred pain to epigastrium. Midgut = SMA + lesser splanchnic (T10–T11) + referred pain to periumbilical region. The ampulla is the switch.

Blood supply and venous drainage

Arterial:

- Proximal to ampulla → superior pancreaticoduodenal artery (anterior and posterior branches) from the gastroduodenal artery, a branch of the common hepatic artery off the coeliac trunk.

- Distal to ampulla → inferior pancreaticoduodenal artery from the SMA.

Venous: Mirrors the arterial supply but drains into the portal system — superior pancreaticoduodenal vein → portal vein directly; inferior pancreaticoduodenal vein → SMV → portal vein.

Lymphatics: Pancreaticoduodenal nodes → coeliac nodes (proximal) and superior mesenteric nodes (distal).

Embryology — annular pancreas

The pancreas forms from two buds off the foregut: a ventral bud (which gives the uncinate process and inferior head) and a dorsal bud (body, tail, superior head). The ventral bud normally rotates clockwise behind D2 to fuse with the dorsal bud.

If the ventral bud splits and one limb migrates the wrong way, a ring of pancreatic tissue encircles D2 — an annular pancreas. This presents in neonates with duodenal obstruction (bilious vomiting, "double bubble" on AXR) or in adults with recurrent pancreatitis or obstruction.

[Image: Annular pancreas encircling D2 — showing failed ventral bud rotation]

Clinical correlations (high-yield)

Peptic ulcer disease — D1 is the classic site.

- Anterior D1 ulcer perforates → free gas under the diaphragm, peritonitis. Surgical emergency, omental (Graham) patch repair.

- Posterior D1 ulcer erodes into the gastroduodenal artery → catastrophic upper GI bleed. No free gas — the ulcer bleeds rather than perforates because it is retroperitoneal.

👩‍⚕️ Anterior ulcers perforate, posterior ulcers bleed. This dichotomy is asked repeatedly.

SMA syndrome. D3 is sandwiched between the SMA anteriorly and the aorta posteriorly. Rapid weight loss depletes the fat pad in this angle, narrowing it from the normal 38–65° to <25° — the SMA compresses D3, causing post-prandial vomiting and weight loss. Classic in young women, post-spinal-surgery patients and anorexia nervosa.

Duodenal atresia. Failure of recanalisation of the duodenal lumen at 8–10 weeks → "double bubble" sign on AXR. Associated with trisomy 21 in ~30% of cases.

Ligament of Treitz marks the boundary between upper and lower GI bleeding — useful in classifying haematemesis vs melaena vs haematochezia.

[Image: MCQs banner]

Test yourself

Which part of the small bowel does an annular pancreas affect?

((2nd part of the duodenum::☑️ Ventral pancreatic bud fails to rotate and forms a ring around D2.))
((1st part of the duodenum::Common site for peptic ulcers, not annular pancreas.))
((3rd part of the duodenum::Site of SMA syndrome; crossed by SMA and SMV anteriorly.))
((Jejunum::Distal to the duodenojejunal flexure — embryologically midgut, unrelated to pancreatic buds.))

👩‍⚕️ Annular pancreas presents in neonates as duodenal obstruction with the "double bubble" sign.

Which structure is NOT in direct posterior relation to the 3rd part of the duodenum?

((Right kidney::☑️ Kidney is posterior to D2, not D3 — sits higher (T12–L3) and more laterally.))
((Right ureter::Posterior to D3 as it descends along the psoas major.))
((Aorta::D3 crosses anterior to the aorta at L3.))
((IVC::D3 lies directly anterior to the IVC.))

👩‍⚕️ Easy mark: D2 = kidney posteriorly; D3 = great vessels posteriorly.

A patient presents with a massive upper GI bleed from a posterior duodenal ulcer. Which artery is the most likely source?

((Gastroduodenal artery::☑️ Runs directly posterior to D1 — eroded by posterior ulcers, causing torrential bleeding.))
((Superior mesenteric artery::Supplies midgut distal to the ampulla, not D1.))
((Left gastric artery::Supplies the lesser curve of stomach, not the duodenum.))
((Splenic artery::Runs along the superior border of pancreas; not a duodenal relation.))

👩‍⚕️ Anterior D1 ulcer → perforates (free gas). Posterior D1 ulcer → bleeds (gastroduodenal artery).

The ampulla of Vater marks the embryological transition between which two regions?

((Foregut and midgut::☑️ Proximal to ampulla = foregut (coeliac); distal = midgut (SMA).))
((Midgut and hindgut::Transition occurs at the distal third of the transverse colon.))
((Foregut and hindgut::Non-adjacent embryological regions.))
((Pharyngeal gut and foregut::Transition is at the oesophagus, not duodenum.))

👩‍⚕️ The ampulla switches blood supply (coeliac → SMA), referred pain (epigastric → periumbilical) and lymphatic drainage.

A young woman with anorexia presents with post-prandial vomiting and weight loss. Imaging shows compression of D3 by an artery. Which artery is responsible?

((Superior mesenteric artery::☑️ SMA crosses anterior to D3; loss of fat pad narrows the aorto-mesenteric angle.))
((Gastroduodenal artery::Lies posterior to D1, not D3.))
((Inferior mesenteric artery::Crosses anterior to the left common iliac, supplying hindgut.))
((Coeliac trunk::Arises at T12, well above D3.))

👩‍⚕️ Normal aorto-mesenteric angle is 38–65°; SMA syndrome occurs when it narrows below 25°.

Which ligament marks the duodenojejunal flexure and is used clinically to distinguish upper from lower GI bleeding?

((Ligament of Treitz::☑️ Suspensory muscle of duodenum from right crus of diaphragm to D4.))
((Falciform ligament::Attaches liver to anterior abdominal wall — contains ligamentum teres.))
((Gastrosplenic ligament::Connects greater curve of stomach to spleen; carries short gastric vessels.))
((Hepatoduodenal ligament::Free edge of lesser omentum containing the portal triad.))

👩‍⚕️ Bleeding proximal to Treitz = upper GI (haematemesis/melaena); distal = lower GI.

Revision summary

- Four parts: D1 superior (L1), D2 descending (L1–L3), D3 horizontal (L3), D4 ascending (L2).

- Retroperitoneal except the first 2 cm of D1 (duodenal cap).

- D1 relations — anterior: liver/GB; posterior: gastroduodenal artery, CBD, portal vein.

- D2 relations — posterior: right kidney, contains the ampulla of Vater.

- D3 relations — anterior: SMA/SMV; posterior: aorta, IVC.

- D4 ends at the DJ flexure, anchored by the ligament of Treitz.

- Ampulla = foregut/midgut transition. Proximal: coeliac → gastroduodenal → superior pancreaticoduodenal. Distal: SMA → inferior pancreaticoduodenal.

- Anterior D1 ulcer perforates; posterior D1 ulcer bleeds (gastroduodenal artery).

- Annular pancreas = failed ventral bud rotation → D2 obstruction; "double bubble".

- SMA syndrome = D3 compression between SMA and aorta, classically in rapid weight loss.

- Duodenal atresia → double bubble, associated with trisomy 21.