The duodenum, the proximal portion of the small intestine, begins at the pylorus and terminates at the duodenojejunal flexure. Deeply placed in the epigastric and umbilical regions of the abdomen, it curves round the head of the pancreas and is shaped like the letter ‘C’ (Fig. 4.49). Unlike the remainder of the small intestine, the duodenum is mostly retroperitoneal and therefore relatively immobile. The duodenal lumen receives bile and pancreatic secretions via the bile duct and the pancreatic ducts.
Fig. 4.49 Duodenum and some related structures. The liver and gall bladder have been slightly displaced.
Parts and structure
The duodenum is conventionally described as consisting of four parts (Fig. 4.49). The superior (first part) begins slightly to the right of the midline at the level of the first lumbar vertebra (on the transpyloric plane) and passes upwards, backwards and to the right. In clinical practice, its initial portion is sometimes termed the duodenal bulb or cap. The descending duodenum (second part) runs vertically to the level of the third lumbar vertebra (Fig. 4.41). The inferior or horizontal duodenum (third part) runs to the left across the midline, arching forwards across the inferior vena cava and aorta. The ascending duodenum (fourth part) slopes upwards and to the left and terminates at the level of the second lumbar vertebra by turning sharply forwards at the duodenojejunal flexure. Close to the pylorus the duodenal mucosa is smooth but in the second and subsequent parts of the organ, it is raised to form numerous circular folds, the plicae circulares (Fig. 4.50). The commonest site for duode- nal ulcers is in the first part.
The bile duct and main pancreatic duct approach the descending duodenum near its midpoint from the posteromedial aspect (Fig. 4.51). They usually pierce the duodenal wall in proximity and commonly open into a single chamber, the hepatopancreatic ampulla (of Vater). The ampulla raises a projection, the major duodenal papilla, on the internal aspect of the duodenum. Bile and pancreatic secretions enter the duodenal lumen through the tip of this papilla via a minute opening controlled by a ring of smooth muscle, the ampullary sphincter (of Oddi). Immediately above the major duodenal papilla, there is often a prominent mucosal fold forming a hood (Fig. 4.50), which may serve as a guide to the location of the papilla, particularly during endoscopic examinations. The pancreas usually possesses a second and smaller duct, the accessory pancreatic duct, which enters the descending duodenum at the minor duodenal papilla, about 2 cm proximal to the major papilla.
Most of the duodenum is retroperitoneal. However, the initial 2 cm have peritoneal relationships similar to the stomach in that the lesser and greater omenta attach, respectively, to the superior and inferior borders. This short segment is relatively mobile and lies immediately inferior to the omental foramen (Fig. 4.49). Posterior duodenal ulcers may erode the pancreas or gastroduodenal artery (Fig. 4.52).
Anterior relations of the proximal portion of the duodenum include the liver and gall bladder. Crossing in front of the descending duodenum are the transverse colon and mesocolon (Fig. 4.49), below which lie coils of jejunum and ileum. Running obliquely across the inferior duodenum are the superior mesenteric vessels (Fig. 4.49), contained in the root of the mesentery of the small intestine. Adjacent to the ascending duodenum are often folds of peritoneum forming paraduodenal recesses.
Posteriorly, the superior duodenum is related to the portal vein, the bile duct and the gastroduodenal artery (Fig. 4.52). The descending duodenum lies in front of the hilum of the right kidney and the right renal vessels while the inferior duodenum crosses the right ureter and gonadal vessels, the inferior vena cava, aorta and origin of the inferior mesenteric artery (Fig. 4.52). The ascending duodenum ascends in front of the left psoas muscle, the left gonadal and renal vessels and the inferior mesenteric vein.
Within the concavity of its C-shaped curve, all parts of the duodenum are related to the pancreas (Figs 4.49 & 4.52).
Fig. 4.52 Arterial supply and some relations of the duodenum. The superior duodenum has been displaced laterally to reveal the gastroduodenal artery, bile duct and portal vein.
The gastroduodenal branch of the common hepatic artery descends behind the superior duodenum and divides into right gastro-omental and superior pancreaticoduodenal branches (Fig. 4.52). The latter vessel, which is often duplicated, runs in the interval between the duodenum and head of the pancreas and supplies the portion of the duodenum proximal to the major papilla.
The remainder of the duodenum is supplied by the inferior pancreaticoduodenal branch of the superior mesenteric artery (Fig. 4.52), given off as the superior mesenteric artery emerges from between the neck and uncinate process of the pancreas. The inferior pancreaticoduodenal artery runs to the right between the duodenum and pancreas, supplying both structures and anastomosing with the superior pancreaticoduodenal artery. The veins draining the duodenum follow the arterial supply and terminate in the portal venous system.