Duodenum — Parts, Relations & Arterial Supply

The duodenum is the shortest (≈25 cm), widest and most fixed part of the small intestine. It is a C-shaped, mostly retroperitoneal structure curving around the head of the pancreas, receiving bile and pancreatic secretions, and is a favourite of NEET PG anatomy because its four parts each carry distinct relations, peritoneal cover, vertebral levels and a watershed blood supply.

Definition & general features

The duodenum extends from the pylorus of the stomach (at the level of L1, the transpyloric plane) to the duodenojejunal (DJ) flexure (at L2). It is roughly 25 cm (≈10 inches) long, hence "duodenum" (from duodeni = twelve finger-breadths). It is curved into a "C" or horseshoe that hugs the head of the pancreas in its concavity.

A key conceptual point: the first 2.5 cm of the first part (the duodenal cap / ampulla) is intraperitoneal and mobile — it is connected to the liver by the hepatoduodenal ligament and to the stomach by the lesser omentum. The rest of the duodenum is retroperitoneal (secondarily retroperitoneal) and therefore relatively fixed.

High-yield: Only the proximal 2.5 cm of the 1st part is intraperitoneal/mobile. The remaining duodenum is retroperitoneal — this explains why duodenal injuries and ulcers tend to involve adjacent retroperitoneal structures (pancreas, IVC).

The four parts — vertebral levels, length, direction

Part	Name & length	Direction	Vertebral level	Peritoneal status
1st	Superior (5 cm)	Upward, backward, to the right	L1	Proximal 2.5 cm intraperitoneal; rest retroperitoneal
2nd	Descending (7.5 cm)	Downward	L1 → L3 (right of midline)	Retroperitoneal
3rd	Horizontal / inferior (10 cm)	Right to left, transversely	L3	Retroperitoneal
4th	Ascending (2.5 cm)	Upward, then forward at DJ flexure	L3 → L2	Retroperitoneal

Mnemonic for lengths: "Counting 1-2-3-4 → 5, 7½ (i.e. 7-8), 10, 2½", or simply remember the 3rd part is the longest (10 cm) and the 4th is the shortest (2.5 cm).

High-yield: The duodenojejunal flexure lies at the level of L2, to the left of the midline, and is held up by the suspensory muscle of the duodenum (ligament of Treitz) — a fibromuscular band from the right crus of the diaphragm / connective tissue around the coeliac trunk and SMA. It is the surgical landmark dividing the upper and lower GI tract for bleeding (haematemesis vs melena/PR bleed).

Part-by-part relations

First part (superior / "duodenal cap")

The proximal mobile portion is the commonest site of duodenal (peptic) ulcers.

• Anterior: quadrate lobe of liver, gallbladder.
• Posterior: gastroduodenal artery, common bile duct, portal vein, and the inferior vena cava.
• Superior: epiploic foramen (of Winslow), neck of gallbladder.
• Inferior: head/neck of pancreas.

High-yield: A posterior duodenal (1st part) ulcer erodes the gastroduodenal artery → massive upper GI haemorrhage. An anterior duodenal ulcer tends to perforate into the peritoneal cavity. Mnemonic: "posterior bleeds, anterior perforates."

Second part (descending)

This is the part that receives the bile and pancreatic ducts — heavily tested.

• Crossed anteriorly by the transverse colon / its mesentery; related to the liver and gallbladder above, and coils of small intestine below.
• Posterior: hilum of right kidney, right renal vessels, right ureter, right psoas.
• Medial: head of pancreas and the ducts.
• The bile duct and main pancreatic duct (of Wirsung) unite to form the hepatopancreatic ampulla (of Vater), which opens onto the major duodenal papilla on the posteromedial wall, about 8–10 cm from the pylorus, guarded by the sphincter of Oddi.
• The accessory pancreatic duct (of Santorini) opens at the minor duodenal papilla, about 2 cm above (proximal to) the major papilla.

High-yield: The major duodenal papilla (ampulla of Vater) marks the junction of the foregut and midgut — clinically the watershed for blood supply and the transition in arterial origin (coeliac vs SMA).

Third part (horizontal / inferior)

Longest part; runs transversely across the great vessels.

• Anterior: superior mesenteric vessels (SMA & SMV) and the root of the mesentery; coils of jejunum.
• Posterior: inferior vena cava, abdominal aorta, right ureter, right gonadal vessels, right psoas.
• Superior: head/uncinate process of pancreas.

High-yield: The third part is clamped anteriorly by the SMA and posteriorly by the aorta. When the angle between the SMA and aorta narrows, the duodenum is compressed → Superior Mesenteric Artery (SMA) syndrome (a.k.a. Wilkie syndrome / cast syndrome).

Fourth part (ascending)

Ascends on the left of the aorta to L2, then turns forward as the DJ flexure.

• Anterior: transverse colon, stomach.
• Posterior: left psoas, left sympathetic chain, left renal/gonadal vessels.
• Left: left kidney and left ureter.

Superior mesenteric artery (SMA) syndrome

Pathophysiology flow: Loss of retroperitoneal/mesenteric fat → narrowing of the aortomesenteric angle → compression of the 3rd part of duodenum between SMA (anterior) and aorta (posterior) → high (proximal) duodenal obstruction.

Stepwise: Weight loss / catabolic state → ↓ fat pad → SMA angle narrows (normal 38°–65° → <25°) and aortomesenteric distance shrinks (normal 10–28 mm → <8 mm) → bilious vomiting + post-prandial epigastric pain → relief on left lateral / knee-chest / prone position.

• Causes: rapid weight loss, anorexia nervosa, burns (cast syndrome after body cast / spinal surgery for scoliosis), prolonged bed rest, malabsorption.
• Investigation of choice: CT angiography / contrast-enhanced CT (measures aortomesenteric angle and distance); upper GI barium shows an abrupt vertical "cut-off" at the 3rd part.
• Management: nutritional rebuilding to restore the fat pad; nasojejunal feeding; surgery (duodenojejunostomy, or Strong's procedure = division of ligament of Treitz to lower the duodenum) if conservative fails.

Parameter	Normal	SMA syndrome
Aortomesenteric angle	38°–65°	< 25° (often < 20°)
Aortomesenteric distance	10–28 mm	< 8 mm
Obstruction site	—	3rd part of duodenum
Relief position	—	Left lateral / knee-chest / prone

Arterial supply — the foregut/midgut watershed

The duodenum derives from both foregut and midgut, with the division at the level of the major duodenal papilla (ampulla of Vater) in the 2nd part. This dual embryological origin dictates a dual blood supply.

Supply flow:

• Coeliac trunk → common hepatic artery → gastroduodenal artery → superior pancreaticoduodenal artery (anterior + posterior branches) → supplies proximal duodenum (above the papilla) — the foregut part.
• Superior mesenteric artery → inferior pancreaticoduodenal artery (anterior + posterior branches) → supplies distal duodenum (below the papilla) — the midgut part.

The superior and inferior pancreaticoduodenal arteries anastomose in the groove between the duodenum and head of pancreas, forming the important pancreaticoduodenal arcades — one of the connections between the coeliac and SMA territories.

Segment	Embryological origin	Arterial supply	Parent vessel
Duodenum above ampulla	Foregut	Superior pancreaticoduodenal a.	Gastroduodenal a. (coeliac)
Duodenum below ampulla	Midgut	Inferior pancreaticoduodenal a.	Superior mesenteric a.

High-yield: The gastroduodenal artery is the single most tested vessel — it lies posterior to the 1st part of the duodenum and is eroded by posterior duodenal ulcers. It is a branch of the common hepatic artery, not the SMA.

Venous drainage: veins follow the arteries and drain into the portal venous system — superior pancreaticoduodenal vein → portal vein; inferior pancreaticoduodenal vein → SMV. Lymphatics drain to pancreaticoduodenal nodes → coeliac and superior mesenteric nodes.

Nerve supply

• Sympathetic: from coeliac and superior mesenteric plexuses (T9–T10, greater & lesser splanchnic). Referred pain of duodenal pathology → epigastrium.
• Parasympathetic: vagus (secretomotor, increases motility).

Histology (quick exam points)

• Brunner's glands are the hallmark of the duodenum — submucosal mucous (alkaline) glands that secrete bicarbonate-rich mucus to neutralise gastric acid. Their presence in submucosa distinguishes duodenum from jejunum/ileum.
• Tall plicae circulares (valves of Kerckring), villi present.

High-yield: Brunner's glands = submucosa of duodenum. If a histology slide shows submucosal glands neutralising acid → duodenum.

Clinical features & complications

• Duodenal ulcer: epigastric pain relieved by food, classically 2–3 hours after meals and at night (contrast with gastric ulcer, where pain is aggravated by food). Strongly linked to H. pylori (>90%) and NSAID use; hyperacidity-driven.
• Posterior perforation/erosion → gastroduodenal artery bleed (haematemesis/melena) or penetration into pancreas → may mimic pancreatitis with back pain.
• Anterior perforation → pneumoperitoneum (gas under diaphragm on erect chest X-ray).
• Duodenal atresia (congenital) → "double-bubble" sign on X-ray, bilious vomiting in a neonate, association with Down syndrome (trisomy 21) and polyhydramnios. Obstruction is usually distal to the ampulla (bilious).
• SMA syndrome as above.
• Periampullary / duodenal carcinoma → obstructive jaundice, occult bleed.

Diagnosis & investigation of choice

• Peptic/duodenal ulcer, masses, bleeding: Upper GI endoscopy (OGD) is the investigation of choice — diagnostic + biopsy + therapeutic (clipping/injection of bleeding vessel).
• H. pylori: urea breath test, rapid urease (CLO) test on biopsy, stool antigen.
• Perforation: erect chest/abdomen X-ray (free gas); CT if equivocal.
• SMA syndrome: CT angiography (gold standard for angle/distance) ± barium meal.
• Duodenal atresia: plain abdominal X-ray (double bubble).

Management / drug of choice

• Duodenal ulcer: Proton pump inhibitors (PPIs) are the drug of choice for acid suppression and healing; H. pylori eradication with triple therapy = PPI + amoxicillin + clarithromycin for 14 days (or sequential/quadruple regimens where resistance is high). Stop NSAIDs.
• Bleeding ulcer: endoscopic haemostasis (clips, adrenaline injection, thermal) + IV PPI; surgery (under-running of gastroduodenal artery) if endoscopy fails.
• Perforation: resuscitation, IV PPI, antibiotics, surgical repair (omental/Graham patch).
• Duodenal atresia: duodenoduodenostomy.

Key differentials

Feature	Duodenal ulcer	Gastric ulcer
Pain vs food	Relieved by food	Worsened by food
Timing	2–3 h post-meal, night pain	Soon after eating
Weight	Maintained (eats to relieve)	Loss (avoids eating)
Malignant risk	Rare	Must biopsy to exclude Ca
Acid	High/normal	Normal/low

Other differentials for the clinical scenarios: pancreatitis (penetrating posterior ulcer), gastric outlet obstruction, biliary colic (RUQ), and for high duodenal obstruction in young thin patients consider SMA syndrome vs annular pancreas vs malrotation with Ladd's bands.

Recently asked / exam angle

• "Structure posterior to the first part of the duodenum eroded in posterior duodenal ulcer?" → Gastroduodenal artery (also CBD, portal vein, IVC posteriorly).
• "Which artery crosses anterior to the 3rd part of duodenum?" → Superior mesenteric artery (cause of SMA syndrome). The structure posterior to the 3rd part is the aorta/IVC.
• "Opening of the bile duct and main pancreatic duct is in which part?" → Posteromedial wall of the 2nd (descending) part, at the major duodenal papilla / ampulla of Vater, ~8–10 cm from pylorus.
• "Ligament of Treitz is attached to?" → DJ flexure; landmark dividing upper vs lower GI bleed; at L2, left of midline.
• "Duodenum derived from foregut and midgut — junction at?" → Major duodenal papilla (opening of bile duct).
• "Double bubble sign with Down syndrome?" → Duodenal atresia.
• "Submucosal glands secreting alkaline mucus?" → Brunner's glands (duodenum).
• "Strong's operation / duodenojejunostomy is done for?" → SMA (Wilkie) syndrome.
• "Most mobile / intraperitoneal part of duodenum?" → Proximal 2.5 cm of 1st part (duodenal cap).

Rapid revision

1. Duodenum = ~25 cm, C-shaped, from pylorus (L1) to DJ flexure (L2); only proximal 2.5 cm of 1st part is intraperitoneal.
2. Four parts: 1st = 5 cm (superior, ulcer site), 2nd = 7.5 cm (descending, ducts open here), 3rd = 10 cm (horizontal, crossed by SMA), 4th = 2.5 cm (ascending).
3. Gastroduodenal artery lies posterior to the 1st part → posterior ulcer bleeds; anterior ulcer perforates.
4. Bile duct + main pancreatic duct (Wirsung) → ampulla of Vater → major papilla on posteromedial 2nd part, guarded by sphincter of Oddi; accessory duct (Santorini) at minor papilla 2 cm proximal.
5. SMA (anterior) and aorta (posterior) sandwich the 3rd part → SMA/Wilkie syndrome when angle <25° / distance <8 mm.
6. DJ flexure held by suspensory muscle of duodenum (ligament of Treitz) at L2, left of midline — divides upper/lower GI bleed.
7. Dual blood supply: above ampulla = superior pancreaticoduodenal (from gastroduodenal/coeliac, foregut); below = inferior pancreaticoduodenal (from SMA, midgut).
8. Foregut–midgut junction is at the major duodenal papilla.
9. Brunner's glands (submucosa) secrete alkaline mucus — duodenum's histological signature.
10. Duodenal ulcer pain is relieved by food, occurs at night, linked to H. pylori; treat with PPI + H. pylori eradication.
11. Double-bubble sign = duodenal atresia, associated with Down syndrome and polyhydramnios.
12. Posterior relations of 2nd part: right kidney hilum, right renal vessels, right ureter; of 3rd part: IVC and aorta.