Parts and their topography
The intestine is anatomically divided into four main parts:
- Superior part (pars superior). Length 3–5 cm, diameter about 4 cm (the widest section). Located between the pylorus and the superior duodenal flexure (flexura duodeni superior). Covered by peritoneum on 3/4 of its circumference, making it relatively mobile — a property utilized by surgeons when creating an anastomosis with the gastric stump. The non-peritoneal areas are located at the attachment sites of the hepatoduodenal ligament (lig. hepatoduodenale), the right part of the gastrocolic ligament, and the zone adjacent to the head of the pancreas. On radiographs, this section appears as a "gas bubble" and is called the ampulla (ampulla duodeni).
- Descending part (pars descendens). Length 9–12 cm. Extends from the superior flexure downwards to the inferior flexure (flexura duodeni inferior). It has limited mobility because the peritoneum covers only its anterolateral part. The posterior and medial surfaces lack a peritoneal covering. In relation to the transverse mesocolon (mesocolon), this segment crosses three levels: the upper section lies above the mesocolon, the middle section lies behind its root, and the lower section lies below it.
- Horizontal (inferior) part (pars horizontalis). Runs in a gentle arch from the inferior flexure to the duodenojejunal flexure (flexura duodenojejunalis). Located below the mesocolon. Covered anteriorly by the peritoneum (except for the area behind the root of the mesentery of the small intestine), while posteriorly it lies adjacent to the endoabdominal fascia (fascia endoabdominalis).
- Ascending part (pars ascendens). The terminal segment transitioning into the jejunum.
Projection onto the anterior abdominal wall
Boundaries of the duodenal projection onto the abdomen:
- Superior: horizontal line through the anterior ends of the 8th ribs.
- Inferior: horizontal line through the umbilicus.
- Left: vertical line 4 cm to the left of the midline.
- Right: vertical line 6–8 cm to the right of the midline.
The projection of the ampulla (duodenal bulb) is located 4–6 cm above the umbilicus, strictly along the midpoint of the width of the right rectus abdominis muscle. The fixation of the intestine to the posterior abdominal wall is accomplished by connective tissue structures. The most tightly fixed segment is the one wrapping around the head of the pancreas, while the least fixed are the ampulla and flexura duodenojejunalis.
Internal structure: duodenal papillae
The mucous membrane of the descending part features critical structures for digestion:
- Major duodenal papilla (papilla duodeni major, Vater's papilla). Located on the posteromedial wall (at the midpoint or the border of the middle and lower thirds). Its height varies from 0.2 to 2 cm, often covered by a proximal transverse mucosal fold. The orifice diameter is 2.0–4.5 mm. In 80% of cases, the common bile duct (ductus choledochus) and the pancreatic duct merge here to form a common ampulla. In 20% of cases, they open via two separate orifices.
- Minor duodenal papilla (papilla duodeni minor). Located 3–4 cm superior to the major papilla. The accessory pancreatic duct (ductus pancreaticus accessorius) opens here. It is patent in more than 30% of cases.
Blood supply, venous and lymphatic drainage
Blood supply is derived from two major vascular pools: the gastroduodenal artery (a. gastroduodenalis) and the superior mesenteric artery (a. mesenterica superior).
Superior and inferior pancreaticoduodenal arteries branch off from these main trunks and divide into anterior and posterior branches. The vessels run toward each other in the groove between the intestine and the head of the pancreas, anastomosing to form the anterior and posterior arterial arcades. Short, straight branches radiate from the arcades into the intestinal wall. An exception is the flexura duodenojejunalis, which additionally receives blood from the first jejunal artery.
Venous drainage follows the course of the arteries and empties into the portal vein system (v. portae). Lymph is collected into regional nodes located in the porta hepatis, along the margins of the pancreas, and at the root of the mesentery of the small intestine. Innervation is provided by the vagus nerves (n. vagus) and sympathetic fibers from the celiac, superior mesenteric, and other plexuses.
Briefly: answers to topical questions
Which organs are syntopically related to the posterior surface of the descending part of the duodenum?
The posterior surface of the descending part of the duodenum lacks a peritoneal covering and lies adjacent to the retroperitoneal fascia and the head of the pancreas.
Which ligaments and muscles form the fixation apparatus of the duodenojejunal flexure?
- Suspensory ligament of Treitz (lig. suspensorium duodeni): anchors the duodenojejunal flexure (flexura duodenojejunalis).
- Suspensory muscle of duodenum (m. suspensorius duodeni): a muscular and connective tissue band extending from the left crus of the diaphragm.
Which veins directly collect blood from the duodenum before it drains into the portal vein?
- The veins of the duodenum follow the course of the arteries and drain into the portal venous system.
- Direct tributaries into the portal vein trunk include the superior pancreaticoduodenal vein (v. pancreaticoduodenalis superior).
Which specific nerve plexuses provide sympathetic innervation to the duodenum?
- Innervation of the duodenum is provided by sensory, sympathetic, and parasympathetic fibers via periarterial plexuses.
- Sources of innervation include the celiac plexus (plexus coeliacus), superior mesenteric plexus, hepatic, superior and inferior gastric, and gastroduodenal plexuses.
The skeletotopy of the intestine can be remembered by its extreme points: it begins at the level of the upper margin of L1 (first lumbar vertebra) and descends to the level of L4.
Class and exam questions
Which part of the duodenum has the greatest mobility?
The superior part (ampulla) is the most mobile, as it is covered by the peritoneum on 3/4 of its circumference. The duodenojejunal flexure is also mobile. The remaining sections are firmly fixed to the posterior abdominal wall.
Where exactly does the common bile duct open?
It opens at the major duodenal (Vater's) papilla, located on the posteromedial wall of the descending part of the intestine. In 80% of cases, it merges with the pancreatic duct into a common ampulla.
How is the duodenum positioned relative to the transverse mesocolon?
The descending part crosses its root: the upper section lies above the mesocolon, the middle section lies behind its root, and the lower section along with the horizontal part lie below the mesocolon.
From which sources does the duodenojejunal flexure receive its blood supply?
It receives blood from the anterior and posterior inferior pancreaticoduodenal arteries, as well as from the first jejunal artery.
What else to study on this topic
- Detailed topography of intestinal segments entirely devoid of a peritoneal covering.
- Variants of biliary and pancreatic duct fusion (formation of a common ampulla).
- Anatomy of the anterior and posterior arterial arcades and their anastomoses.
- Ligamentous apparatus of the intestine and fixation of the flexura duodenojejunalis.
- Relationships of various intestinal segments to the retroperitoneal fascia.
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