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Anatomy and Physiology of the Duodenum
Although spanning only 20 to 30 cm, from the pylorus to the ligament of Treitz, the duodenum is the “gate” that controls the passage of food from the stomach to the jejunum. The name is derived from the Latin phrase intestinum duodenum digitorum , or “intestine of twelve digits.” This Latin phrase may have derived from the writings of the Greek physician Herophilus (334–280 bc ).
Although it is the shortest segment of small bowel, the duodenum is the initial site of contact for gastric secretions, bile, and digestive enzymes from the common bile duct and the pancreas. Thus it plays an important role in the regulation of digestion, absorption of essential micronutrients and macronutrients, and bowel motility. Its relationships to the major structures of the upper abdomen lend it to exposure during a large number of gastrointestinal (GI) surgical interventions. As such, it is important to understand both the structure and function of the duodenum as it relates to alimentary surgery.
Embryogenesis
In the adult the duodenum's position in the upper abdomen follows the normal development and rotation of the embryonic gut. This process starts at the beginning of the third week of embryonic development with primitive foregut demarcation from the midgut and hindgut. Early in the second month of gestation, the embryologic midgut migrates ventrally to descend into the yolk sac. The apex of this loop is marked by the omphalomesenteric duct (yolk sac) with an axis around the superior mesenteric artery, the proximal portion of the yolk sac's primitive blood supply (vitelline artery). Over the next several weeks, the midportion of the intestine elongates faster than the abdominal cavity expands, thereby enlarging the midgut loop, which continues to push into the umbilical cord. As this occurs, the midgut undergoes a counterclockwise 90-degree rotation around the superior mesenteric artery, forming “prearterial” and “postarterial” halves. Following this rotation, the cranial (“prearterial”) segment of midgut (future duodenum and proximal small bowel) lies to the right of the caudal (“postarterial”) segment (future colon). The cranial (“prearterial”) segment continues to elongate into the umbilical cord until the tenth gestational week, after which the midgut returns into the abdomen. The cranial limb migrates back into the abdomen first, causing the duodenum to pass behind the superior mesenteric artery. The caudal limb follows with the cecum and terminal ileum entering last. During its return, the midgut rotates another 180 degrees (total rotation = 270 degrees).
At the completion of these movements, the colon is situated anterior to the superior mesenteric artery, and the cecum is located at the level of the iliac crest. From the twelfth week of gestation until after birth, the colon elongates while the cecum remains in its original position. This colonic growth effectively produces an “ascent” of the hepatic flexure toward the right upper quadrant that seems like a cecal “descent.”
Anatomic relations between the duodenum, liver, and pancreas are also dictated by early development. The future duodenum lies between the transverse septum of the ventral mesentery (the future primordium of the liver, the bile ducts, and the ventral pancreatic bud) and the dorsal mesentery (the future dorsal pancreatic bud). After midgut rotation is complete, the hepatic parenchyma and the intestines proliferate and the dorsal and ventral pancreatic buds fuse, establishing the duodenum's final anatomic location ( Fig. 68.1 ).
Initially, the duodenum is composed of a single layer of endodermal cells surrounded by undifferentiated mesenchymal cells. By the end of the fourth week of gestation, the duodenal mucosa begins to proliferate along the ventral wall near the origin of the hepatic diverticulum. During midgut rotation, mesenchymal tissue beyond the first portion of the duodenum increases along the dorsal aspect of the duodenum, fixing it to the retroperitoneum beyond this point. During fixation, this dorsal mesentery transforms into an avascular plane of loose connective tissue known as the fascia of Treitz (not to be confused with the ligament of Treitz). This plane is entered when lifting the duodenum medially during a Kocher maneuver.
At the end of the third gestational week, the liver primordium, gallbladder, and biliary duct (both originating from the gallbladder bud) arise as a ventral outgrowth from the distal end of the foregut. Later (fifth gestational week) the connecting elements between the hepatic diverticulum and the duodenum form the bile duct and, ultimately, the cystic duct and the gallbladder. Around the ninth week of gestation, rapid hepatic growth occurs secondary to the hematopoietic function of the liver and the formation of multiple hepatic sinusoids. This hepatic growth, combined with the midgut's elongation, pushes the duodenum below the liver. At this time, the ventral mesentery produces the lesser omentum, the falciform ligament, and the hepatoduodenal ligament; these structures envelop the portal triad as it extends from the liver.
Other structures within the portal triad include the hepatic artery and the portal vein. Spatially, the portal vein is complexly related to the duodenum during the latter's development. The portal vein develops from the primitive paired vitelline veins that arise in the yolk sac and pass up the body stalk to enter the developing heart. Two extrahepatic cross-connections develop between the paired vessels: The cranial anastomosis lies behind the duodenum and the caudal anastomosis passes in front of the duodenum. Normally the cranial retroduodenal anastomosis persists as the portal vein and the caudal anastomosis disappears. This preduodenal caudal anastomosis can persist as the portal vein, leading to the rare congenital anomaly known as a preduodenal portal vein.
At the end of the fourth gestational week (see Fig. 68.1 ), the developing duodenum is joined by a dorsal pancreatic primordium. One week later the ventral pancreatic primordial bud arises at the base of the hepatic diverticulum. At the end of the sixth week, these two primordia fuse as the ventral pancreas migrates below and behind the dorsal pancreatic segment; these changes form portions of the adult pancreatic head and uncinate process. After fusion, the principal pancreatic ducts fuse, typically with the ventral duct fusing at the midportion of the dorsal pancreatic duct. The combination of the ventral duct and the fused mid- to distal dorsal duct becomes the duct of Wirsung. This duct connects to the common bile duct, contributing to the ampulla of Vater at the site of the common bile duct's entry into the duodenum. After fusion, the proximal portion of the dorsal pancreatic duct (the duct of Santorini) typically regresses as the duct of Wirsung assumes dominance. As the midgut expands and rotates, the duodenum achieves its final location along the retroperitoneum with the first and second portion located laterally while the remainder traverses inferiorly to the fused pancreatic primordia ( Fig. 68.2 ).
Failure of the pancreatic primordia to fuse results in a condition known as pancreatic divisum. In this situation the ducts of Wirsung and Santorini drain separately into the duodenum. Annular pancreas may also develop after anomalous pancreatic fusion; in this developmental anomaly, a thin flat band of normal pancreatic tissue surrounds the second portion of the duodenum and connects to either side of the pancreatic head. The ring that forms around the duodenum can cause duodenal stenosis. Although this anomaly is described in children, it may be entirely asymptomatic until discovered as an incidental finding on necropsy.
Gross Anatomy and Topographic Relations
The duodenum is divided into four segments: the duodenal bulb or cap; the second vertical or descending portion; the third horizontal or transverse portion; and the fourth oblique or ascending portion ( Fig. 68.3 ). The duodenum begins at the end of the gastric pylorus, in the plane of the first lumbar vertebra. Starting at the second portion, it descends in a C -shaped curve around the pancreatic head. The third part of the duodenum lies inferior to the superior mesenteric artery at the level of the second lumbar vertebra; it is situated in the angle formed by the superior mesenteric artery and the aorta where it crosses the midline to join the fourth duodenal segment and later the jejunum.
The duodenum is related anteriorly to the liver and gallbladder; superiorly to the epiploic foramen; laterally (second portion) and inferiorly (third portion) to the pancreatic head; and posteriorly to the common bile duct, portal vein, inferior vena cava, and gastroduodenal artery. It is separated laterally from the inferior vena cava by a small amount of connective tissue.
First Duodenal Segment (Approximately 5 cm Long)
The first portion of the duodenum passes superiorly from the gastric pylorus to the neck of the gallbladder. The proximal half, the duodenal bulb or cap, is mobile; the distal half is fixed. Most (90%) duodenal ulcers occur in the duodenal bulb. Clinically, the mobility of the duodenal bulb facilitates operations on the pylorus and duodenum, particularly after a Kocher maneuver. Its longitudinal muscle folds can be appreciated on upper endoscopy and used as a landmark prior to entering the second portion where transverse folds can be seen.
The hepatoduodenal portion of the lesser omentum attaches to the superior duodenal border within the initial 2.5 cm of this segment; the greater omentum attaches to this segment's inferior border. The distal 2.5 cm is covered by peritoneum anteriorly, resulting in the posterior surface closely contacting the portal triad and the gastroduodenal artery. This segment's relationship to the gastroduodenal artery explains the artery's susceptibility to bleeding when posterior peptic ulcers erode. When encountering this condition, surgeons should remember that the gastroduodenal artery arises 15 to 30 mm above the superior border of the first part of the duodenum, and the distance between the artery's origin and the pylorus can range from 5 to 50 mm. Finally, the duodenum's proximity to the gallbladder facilitates cholecystoduodenal fistulas and the passage of gallstones into the intestinal tract after severe bouts of cholecystitis.
Second Duodenal Segment (Approximately 7.5 cm Long)
This portion of the duodenum extends from the gallbladder neck to the upper border of L4. It joins the first portion of the duodenum on the right side of the first lumbar vertebra, behind the costal margin slightly superior and medial to the ninth costal cartilage's tip. Beyond this junction, the duodenum becomes a retroperitoneal structure through fusion of its lateral visceral peritoneum to the posterolateral abdominal wall. After forming an acute angle with the superior duodenal flexure, the second portion descends from the gallbladder with a loop that passes over the right renal hilum, the adrenal gland, the psoas major, and the edge of the inferior vena cava. Concurrently, it passes under the right hepatic lobe, the colonic hepatic flexure, and parts of the transverse colon and the jejunum. Peritoneal folds pass above and below this duodenal segment to form the mesocolon. The relationship between the duodenum, hepatic flexure, and mesocolon must be considered when mobilizing the hepatic flexure during surgical interventions on the proximal colon.
Medially, the pancreatic head is intimately related to the duodenal C-loop. The superior pancreaticoduodenal branch of the gastroduodenal artery runs in the groove between the two structures. At about the midpoint of the C-loop, the pancreaticobiliary tract opens into the papilla of Vater, on the second duodenal segment's concave posteromedial side.
Third Duodenal Segment (Approximately 12 to 13 cm Long)
The third portion of the duodenum extends from the right side of L3 or L4 to the left side of the aorta. As this segment passes from right to left across the midline anterior to the ureter, the psoas muscles, the inferior vena cava and the aorta, it remains posterior to the superior mesenteric vessels. Superiorly, the pancreatic head and uncinate processes are separated from this part of the duodenum by a groove containing the inferior pancreaticoduodenal artery. This segment ends to the left of the third or fourth lumbar vertebra, next to the root of the small intestine's mesentery.
Fourth Duodenal Segment (Approximately 2.5 cm Long)
The fourth portion of the duodenum starts at the left upper border of L2. After an upward and oblique ascent, it travels to the duodenojejunal angle at the root of the transverse mesocolon, approximately 4 cm below and medial to the ninth costal cartilage's tip. It then descends leftward to form the duodenojejunal flexure where the duodenum's suspensory ligament attaches to the mesentery (ligament of Treitz). This ligament, a remnant of the dorsal mesentery, extends from the duodenojejunal flexure to the right diaphragmatic crus. Its termination closely approximates the terminal part of the inferior mesenteric vein, the left ureter, and the left kidney.
Arterial Supply
The first portion of the duodenum is supplied by the posterior superior pancreaticoduodenal branch of the gastroduodenal artery, and variably by the supraduodenal and retroduodenal arteries (either separately or in variable combinations). In some patients, branches of the right gastric artery also supply the first centimeter of the duodenum. The gastroduodenal artery descends between the first part of the duodenum and the pancreatic head, terminating into the right gastroepiploic artery and the anterior superior pancreaticoduodenal artery. This rich periduodenal arterial anastomotic network often frustrates attempts to control bleeding from posterior duodenal ulcers.
The arterial supply to the remainder of the duodenum is derived from major arterial anastomoses between the celiac and superior mesenteric arterial circulations. As noted earlier, the anterior superior pancreaticoduodenal artery arises from the gastroduodenal artery on the pancreas' ventral surface. The posterior superior pancreaticoduodenal artery crosses in front of the common bile duct and then spirals posteriorly to the pancreatic head. The anterior and posterior inferior pancreaticoduodenal arteries arise from the superior mesenteric artery or its first jejunal branch, either separately or through a common origin. These two arteries split and run in posterior and anterior grooves between the descending and transverse portions of the duodenum and the pancreatic head, where they join to form continuous anterior and posterior arcades. Through these arterial arcades, the duodenum shares its blood supply with the proximal pancreas ( Fig. 68.4 ). As such, resection of either the duodenum or the pancreas alone is technically challenging and potentially hazardous.
Venous Supply
Pancreaticoduodenal veins parallel the pancreaticoduodenal arteries, accompanying them in anterior and posterior pancreaticoduodenal arcades ( Fig. 68.5 ). Surgeons usually encounter these veins superficial to their arterial analogues. The lower portion of the proximal duodenal bulb drains into the right gastroepiploic veins; the upper part drains into the portal vein or posterior superior pancreaticoduodenal vein via several suprapyloric veins. The posterior arcade ends in the portal vein above and the superior mesenteric vein below. The posterior superior pancreaticoduodenal vein may follow its companion artery anterior to the bile duct, although it usually runs behind the duct. This vein terminates inferiorly on the superior mesenteric vein's left border. Here it may be joined by a jejunal vein or by the anterior inferior pancreaticoduodenal vein.
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