Duodenal Anatomy

The duodenum is the first part of the small intestine (3-8.5 m), followed by the jejunum and ileum (in that order); it is also the widest, shortest (25 cm), and most predictably located part of the small intestine. ^[3] The duodenum is a C-shaped or horseshoe-shaped structure that lies in the upper abdomen near the midline between the level of the first and third lumbar vertebrae in the supine position (see the image below). ^{[1, 2]} The duodenum's anatomical positioning is primarily retroperitoneal, except for the proximal 2.5 cm, which is intraperitoneal. It extends from the termination of the pylorus of the stomach to the duodenojejunal (DJ) junction, where it transitions to the jejunum. ^[3]

Stomach and duodenum, coronal section.

The duodenum plays a pivotal role in digestion by receiving chyme from the stomach and mixing it with bile (from the gallbladder) and pancreatic enzymes. It neutralizes gastric acid through bicarbonate-rich secretions. It also facilitates water and nutrient (vitamins, minerals, carbohydrates, fats, and proteins) absorption. ^{[3, 4]}  

The pylorus of the stomach (at L1 level) leads to the duodenum, which has the following four parts:

• The first (superior) part, or duodenal bulb or cap (5 cm), starts at the termination of the pylorus of the stomach and ends at the superior duodenal flexure. ^[3] It is connected to the undersurface of the liver (porta hepatis) by the hepatoduodenal ligament (HDL), containing the proper hepatic artery, hepatic portal vein (HPV), and common bile duct (CBD); the quadrate lobe (segment IV) of the liver and the gallbladder are in front, whereas the CBD, HPV, and the gastroduodenal artery (GDA) are behind the first part of the duodenum. The first 2-3 cm of the duodenum is called the duodenal bulb (duodenal cap). It is lined by relatively smooth mucosa and readily distends on insufflation during endoscopy. ^[3] The first part transitions from being intraperitoneal (first 2.5 cm) to retroperitoneal. ^[5]
• The second (descending) part (about 8 cm long) extends from L1 to L3 vertebral levels. ^[3] It starts at the superior duodenal flexure and runs inferiorly in a gentle curve that is convex toward the right side of the spine. It is covered by peritoneum only on its proximal anterior surface. ^[3] The descending part has an upper and a lower genu (flexure); the transverse mesocolon and transverse colon are in front, and the right kidney and inferior vena cava (IVC) are behind it; the head of the pancreas lies in the concavity of the duodenal C at the level of L2 vertebra, while the right colic flexure is cephalad and lateral. ^[3]
• The third (horizontal) part (about 10 cm long) starts at the inferior duodenal flexure. ^[3] It runs from the right side of the inferior border of L3 and passes to the left and slightly superiorly, anterior to the IVC and abdominal aorta, with the superior mesenteric vessels (the vein on the right and the artery on the left) in front of it.
• The fourth (ascending) part (2.5 cm), begins to the left of the abdominal aorta, runs superiorly and laterally to the level of the superior border of L2, then turns sharply anteroinferiorly at the DJ flexure to become continuous with the jejunum. ^[3] The DJ flexure is an important landmark for locating the inferior mesenteric vein radiologically or surgically. ^[3]

The DJ junction or flexure is an abrupt turn at the level of L2 vertebra (see the image below); it is identified during surgery by the inferior mesenteric vein (IMV), which lies to its immediate left. The DJ junction is attached to the posterior abdominal wall by the suspensory ligament of the duodenum or the ligament of Treitz; many fossae (paraduodenal, DJ) are found around this junction.

Stomach and duodenum, coronal section.

Except for its first part, the duodenum is largely retroperitoneal and therefore fixed; it has no mesentery and is covered by peritoneum only on its anterior surface.

The first part of the duodenum divides the CBD into supraduodenal (in the HDL), retroduodenal, and infraduodenal (retropancreatic) parts; the terminal part of the CBD is intraduodenal (intramural) as it traverses the wall of the duodenum to open into its lumen (see Endoscopic anatomy).

Endoscopic anatomy

The terminal part of the CBD is joined by the terminal part of the pancreatic duct in the pancreatic head to form a common channel (called the biliopancreatic ampulla when dilated), which runs through the medial duodenal wall and opens on the dome of the major duodenal papilla, a nipple-like projection on the medial wall of the middle segment of the second part (C loop) of the duodenum. The site of the greater duodenal papilla marks the junction of the embryologic foregut and midgut.

Both ampulla and papilla are eponymously related to Vater. The greater duodenal papilla is covered by a semicircular hood-like mucosal fold superiorly. A smooth muscle sphincter (of Oddi) is present around the common channel of the CBD and the pancreatic duct and prevents reflux of duodenal juice into the two ducts.

Aspects such as the weak duodenal wall, the sphincter of Oddi, and the risks for wide excisions should be considered during surgical and endoscopic procedures in this area such as sphincterotomies. ^[6]

Blood supply

The duodenal bulb (first part) is supplied by the right gastric artery and right gastro-omental (gastroepiploic) artery.

The duodenum (C loop) shares its blood supply very intimately with the head of the pancreas, which lies in its concavity.

The celiac trunk (axis) arises as a branch from the anterior surface of the abdominal aorta at the level of T12-L1. It has a short length (about 1 cm) and trifurcates into the common hepatic artery (CHA), the splenic artery, and the left gastric artery. The CHA runs toward right on the superior border of the proximal body. The superior mesenteric artery (SMA) comes off as the second anterior branch from the abdominal aorta (the inferior mesenteric artery is the third anterior branch) just below the origin of the celiac trunk at the level of L1 behind the neck of the pancreas and descends in front of the third (horizontal) part of the duodenum to enter the small bowel mesentery.

The GDA, a branch of the CHA, runs down behind the first part of the duodenum in front of the neck of the pancreas and gives off the posterior superior pancreaticoduodenal artery before dividing into the right gastro-omental (gastroepiploic) artery and the anterior superior pancreaticoduodenal artery (ASPDA). The GDA also gives off the small supraduodenal artery (of Wilkie).

The inferior pancreaticoduodenal artery (IPDA) arises from the SMA and bifurcates into anterior and posterior branches. Anterior and posterior branches of the SPDA and IPDA join each other and form anterior and posterior pancreaticoduodenal arcades in the anterior and posterior pancreaticoduodenal grooves, supplying small branches to the first, second, and third parts of the duodenum (vasa recta duodeni) and to the head and uncinate process of the pancreas.

Veins accompany the SPDA and IPDA. Superior pancreaticoduodenal veins (SPDVs) drain into the HPV, and inferior pancreaticoduodenal veins drain into the superior mesenteric vein (SMV). The anterior SPDV may drain into the gastro-colic trunk of Henle.

The SMV lies to the right of the SMA in front of the third part of the duodenum. The union of the vertical SMV and the horizontal splenic vein (SV) forms the HPV behind the neck of the pancreas. The IMV lies to the immediate left of the DJ flexure and joins the junction of SV and SMV (or maybe SV or even SMV). The PV receives the SPDV, the right gastro-omental vein, the left gastric vein (LGV), and the right gastric vein (RGV) and runs up (superiorly) behind the first part of the duodenum in the HDL behind (posterior to) the CBD on the right and the proper hepatic artery (HA) on the left. The hepatic portal venous system (SV, SMV, and PV) has no valves.

Nerves

The duodenum is supplied with parasympathetic nerves by hepatic and celiac branches of the anterior and posterior vagi, respectively. Sympathetic innervation is provided by postganglionic fibers originating from the celiac plexus, which itself receives preganglionic input from thoracic spinal segments T5-T9 via the greater splanchnic nerves. ^[7] Additionally, the enteric nervous system contributes to local control through the submucosal (Meissner) and myenteric (Auerbach) plexuses, which regulate mucosal function and peristalsis. ^[8]

Lymphatic drainage

Lymphatics from the duodenum drain into pancreaticoduodenal, supra- and infrapyloric, superior mesenteric, mesocolic and celiac lymph nodes. The distal duodenum drains into the superior mesenteric lymph nodes. Additional drainage pathways include the suprapyloric and infrapyloric lymph nodes, with all lymph eventually converging at the cisterna chyli via the celiac trunk. This system plays a critical role in immune defense and lipid transport. ^[9] Lymphatic drainage to lateral aortic nodes has also been described. ^[3]

The wall of the duodenum contains the same four layers that are seen in the remainder of the small bowel — namely, the mucosa (lined with columnar epithelium, containing lamina propria and muscularis mucosa), the submucosa, the muscularis propria (with inner circular and outer longitudinal layers), and the serosa (only on its anterior surface). The duodenal mucosa is characterized by the presence of Brunner's glands, which secrete mucous. Endocrine cells in the duodenal wall produce cholecystokinin (CCK) and secretin.

Mucosa:

• The mucosa is lined with simple columnar epithelium and features villi and crypts of Lieberkühn, which increase the surface area for absorption. The lamina propria, composed of loose connective tissue, supports the epithelium and contains immune cells. The muscularis mucosa is a thin layer of smooth muscle that separates the mucosa from the submucosa. It consists of inner circular and outer longitudinal layers of smooth muscle, facilitating local movements of the mucosa. ^{[3, 12]}
• Unique to the duodenum, Brunner's glands are found in the submucosa but extend into the mucosa near their openings into crypts. These glands secrete an alkaline mucus rich in bicarbonate to neutralize acidic chyme from the stomach and protect the epithelium. ^[10]

Submucosa:

• The submucosa contains loose connective tissue, blood vessels, lymphatics, and Meissner's plexus, which regulates glandular secretions. ^{[3, 12]}
• Brunner's glands are a hallmark feature of this layer in the duodenum. Their secretions not only neutralize gastric acid but also provide an optimal pH for pancreatic enzyme activity. ^[11]

Muscularis Propria:

• This layer consists of two smooth muscle layers: an inner circular layer and an outer longitudinal layer. These layers coordinate peristalsis to propel digested material through the intestinal lumen. ^[12]

Serosa:

• The serosa covers only the anterior surface of the duodenum, as much of it is retroperitoneal. It consists of a thin layer of connective tissue covered by the mesothelium. ^[12]

Specialized Features

Brunner's Glands:

• These branched acinotubular glands are predominantly located in the proximal duodenum and decrease in number distally. They secrete mucus-containing bicarbonate ions, epidermal growth factor (which inhibits gastric acid secretion), and antimicrobial peptides, contributing to mucosal protection, digestion, and gut microbiome regulation. ^{[10, 13]}

Enteroendocrine Cells: Scattered within the epithelium, these cells release hormones such as: ^[14]

• CCK - Secreted by I cells in response to fats and proteins, it stimulates bile release from the gallbladder and pancreatic enzyme secretion
• Secretin - Produced by S-cells in response to acidic chyme, it stimulates bicarbonate secretion from the pancreas and bile ducts while inhibiting gastric acid secretion into the stomach.

The following anomalies may be noted:

• Duodenal atresia (stenosis) is associated with Down syndrome and manifests as neonatal gastric outlet obstruction; a radiograph of the abdomen reveals a double bubble (one in the fundus of the stomach and the other in the first part of the duodenum).
• In malrotation of gut, the DJ flexure lies to the right side (instead of normal left) of the spine. A Ladd's band runs across the duodenum and obstructs it; patients present clinically with signs of duodenal obstruction.
• A perivaterian (around the ampulla/papilla of Vater) duodenal diverticulum may be present on the medial wall of the duodenum; it makes an endoscopic papillotomy difficult due to the risk for perforation.

The following considerations should also be taken into account:

• In blunt abdominal trauma, duodenal injuries are often associated with pancreatic injuries.
• A deformed duodenal bulb (on barium swallow and on endoscopy) can occur in chronic (healed) peptic (duodenal) ulcers.
• Peptic ulcer perforation occurs on the anterior wall of the first part of the duodenum.
• Bleeding in the duodenal ulcer comes from the GDA, which runs vertically down behind the first part of the duodenum; a duodenotomy is performed on the anterior wall of the first part of the duodenum to control the vessel from inside the duodenum.
• Pyloroplasty involves a longitudinal incision in the stomach (antrum) and the first part of the duodenum as well; the incision is then closed transversely.
• An inflamed gallbladder can be adherent to the first part of the duodenum.
• Gall bladder cancer can infiltrate the first part of the duodenum and cause gastric outlet obstruction.
• Duodenal (gastric outlet) obstruction is present in most patients with pancreatic head cancer.
• The first part of the duodenum is mobile and can be used for a biliary-enteric anastomosis (choledochoduodenostomy).
• With a side-viewing (flexible) endoscope in the second part of the duodenum, the CBD and the pancreatic duct can be cannulated through the papilla, and radiographs can be obtained after injection of radiologic contrast medium (endoscopic retrograde cholangiopancreatography).
• Papilla may bleed after endoscopic papillotomy; control of bleeding involves duodenotomy (in the second part) and transduodenal sphincteroplasty.
• Internal hernias can occur into the fossae around the DJ flexure.
• The Cattell-Braasch maneuver is downward (inferior) mobilization of the hepatic flexure of the colon and the right transverse colon before mobilization of the duodenum.
• Kocherization is anterior and leftward mobilization of the second part (C loop) of the duodenum after incision of the parietal peritoneum on its right aspect; the IVC and left renal vein are encountered posteriorly. Kocherization of the duodenum exposes the aortocaval area for an aortocaval lymph node biopsy.
• Periampullary cancers include those of the lower CBD, ampulla, pancreatic head, and duodenum (including the papilla) within 1-2 cm of the ampulla.
• Duodenal resection alone, without pancreatic resection, is technically difficult because of intimate sharing of blood supply.
• Pancreaticoduodenectomy is required for the resection of duodenal cancer because of the shared blood supply between the second part (C loop) of the duodenum and the head of the pancreas.
• The duodenum is at risk for intraoperative injury during right hemicolectomy and right nephrectomy.