Abdomen:Hollow viscus:Duodenum

The **duodenum** is the first part of the small intestine and is the continuation of the stomach. It is a 20-30cm C-shaped hollow viscus predominantly on the right hand side of the vertebral column. It lies at the level of L1-3 and the convexity of the duodenum usually encompasses the head of the pancreas.

The duodenum begins at the duodenal bulb and ends at the ligament of Treitz, where it continues as the jejunum (this is often called the duodenojejunal or D-J flexure). It is composed of four distinct parts (superior, descending, horizontal and ascending), which is 2, 3, 4 and 1 inches in length.

Segments

**First part (D1)**

The first (superior) part of the duodenum runs from the pylorus. It passes laterally to the right, superiorly and posteriorly, for approximately 5cm, before making a sharp curve inferiorly into the superior duodenal flexure. The initial segment is called the the duodenal cap (smooth walled, with no plicae circulares). It is intraperitoneal and 2-3cm long, between the lesser and greater omentum (forms the lower boundary of the epiploic foramen). The rest are retroperitoneal. Relations are:

* anteriorly - gallbladder, liver
* posteriorly - common bile duct, portal vein, gastroduodneal artery
* superiorly - epiploic foramen
* inferiorly - pancreatic head

**Second part (D2)**

The second (descending) part of the duodenum begins at the superior duodenal flexure. It passes inferiorly to the lower border of vertebral body L3, over the hilum of the right kidney, before turning sharply to the inferior duodenal flexure. Relations are:

* anteriorly - transverse mesocolon (crosses the D2)
* posteriorly - right ureter, right adrenal gland, hilum of the right kidney
* superiorly - liver, gallbladder (variable)
* inferiorly - loops of jejunum
* laterally - ascending colon, hepatic flexure, right kidney
* medially - pancreatic head

The pancreatic duct and common bile duct enter D2 via the major duodenal papilla (ampulla of Vater) at its posteromedial wall. 2 cm proximal is the minor duodenal papilla, the entrance for the accessory pancreatic duct. The junction between the embryological foregut and midgut lies just below the major duodenal papilla.

**Third part (D3)**

The third (inferior/horizontal) part of the duodenum begins at the inferior duodenal flexure and passes transversely to the left, crossing the vertebral column. It lies in the right infracolic compartment. Relations are:

* anteriorly - small bowel mesentery root
* posteriorly - right psoas muscle, right ureter, gonadal vessels, aorta and the beginning of IMA, and IVC
* superiorly - pancreatic head
* inferiorly - loops of jejunum

**Fourth part (D4)**

The fourth (ascending) part passes superiorly, either anterior to, or to the right of, the aorta, until it reaches the inferior border of the body of the pancreas. Then, it curves anteriorly and terminates at the duodenojejunal flexure where it joins the jejunum. The duodenojejunal flexure is surrounded by a peritoneal fold containing muscle fibres: the ligament of Treitz (a thin band of connective tissue that run from the the right crus of the diaphragm to the connective tissue around the coeliac trunk). Relations:

* superiorly - stomach
* inferiorly - loops of jejunum
* posteriorly - left psoas muscle, left lumbar sympathetic trunk, aorta

Blood supply

* arterial supply:
	+ duodenal cap (first 2.5cm) - right gastric artery, right gastroepiploic artery
	+ remaining D1 to mid-D2 - superior pancreaticodudenal artery (branch of gastroduodenal artery)
	+ mid-D2 to ligament of Trietz - inferior pancreaticoduodenal arteries (branch of SMA)
* venous drainage:
	+ duodenal cap (first 2.5cm) - prepyloric vein (drains to portal vein)
	+ remaining duodenum - superior pancreaticoduodenal vein (drains to portal vein) and inferior pancreaticoduodenal vein (drains to inferior mesenteric vein)

Nerve supply

* sympathetic nerve fibres via coelic and superior mesenteric trunks
* parasympathetic nerve fibres via anterior and posterior vagal trunks

Lymphatic supply

* pancreaticoduodenal nodes that drain:
	+ distally to superior mesenteric nodes
	+ proximally to coeliac nodes

Variant anatomy

* duodenal diverticulum - most commonly occurs in D2 or D3
* duodenal duplication - most commonly occurs at the medial wall of D2 or D3; appear as a cystic structure that does not communicate with the lumen