Topography and Segments of the Organ
The duodenum begins at the stomach at the level of the 12th thoracic (ThXII) or 1st lumbar (LI) vertebra, located inferior to the liver and to the right of the vertebral column. Anatomically, it is divided into four consecutive parts:
- Superior part (pars superior). Runs from left to right and posteriorly. At its origin, it is somewhat dilated and forms the ampulla (duodenal bulb). It terminates at the superior duodenal flexure (flexura duodeni superior).
- Descending part (pars descendens). Descends vertically anterior to the right kidney to the level of LII–LIII, transitioning into the next part via the inferior duodenal flexure (flexura duodeni inferior).
- Horizontal part (pars horizontalis). Turns to the left, running almost horizontally, and crosses anterior to the inferior vena cava.
- Ascending part (pars ascendens). Runs obliquely upward anterior to the abdominal aorta. At the level of LI–LII, it transitions into the jejunum, forming the sharp duodenojejunal flexure (flexura duodenojejunalis). The posterior surface of this flexure is anchored by the suspensory muscle of the duodenum (m. suspensorius duodeni).
Along the course of the duodenum, minor narrowings occur: in the middle of the descending part (site of crossing by the right colic artery) and at the border of the horizontal and ascending parts (region crossed by the superior mesenteric vessels).
Mucosal Relief
The mucosa (tunica mucosa) forms folds whose shape varies by region. In the superior part, these are longitudinal folds, while in the descending and horizontal parts, they form permanent circular folds (plicae circulares), occupying half to two-thirds of the intestinal circumference.
On the medial wall of the lower half of the descending part, the longitudinal fold of the duodenum (plica longitudinalis duodeni) is located, measuring about 11 mm in length. At its distal end is the major duodenal papilla (papilla duodeni major). The common bile duct and the main pancreatic duct open through its apex into the intestinal lumen. Slightly superiorly, the minor duodenal papilla (papilla duodeni minor) may be present, where the accessory pancreatic duct opens.
Histological Structure and Peritoneal Relations
The initial segment of the superior part (about 2.5–5.0 cm) is covered by peritoneum on three sides. The remaining parts (descending and inferior) lie retroperitoneally and are covered externally by adventitia.
The muscular layer (tunica muscularis) is thicker here than in other parts of the small intestine (0.3–0.5 mm) and comprises an outer longitudinal and an inner circular layer.
The mucosa has a velvety appearance due to intestinal villi (villi intestinales), numbering up to 40 per 1 mm². They are leaf-shaped, 0.5–1.5 mm high, and 0.2–0.5 mm thick. A lymphatic capillary runs through the center of each villus, and blood vessels form a dense network.
Intestinal crypts containing the mouths of intestinal glands lie between the bases of the villi. The surface epithelium is simple columnar with goblet cells. A characteristic feature of the submucosal layer in this region is the presence of branched tubular duodenal glands (glandulae duodenales).
Blood Supply and Innervation
Arterial blood supply is derived from two major vascular beds forming pancreaticoduodenal arcades:
- From the celiac trunk (truncus coeliacus), giving rise to the superior pancreaticoduodenal arteries (anterior and posterior branches of the gastroduodenal artery).
- From the superior mesenteric artery (a. mesenterica superior), giving rise to the inferior pancreaticoduodenal arteries.
Venous drainage occurs via corresponding veins into the superior mesenteric vein and partially directly into the portal vein.
Innervation is provided by the autonomic nervous system. Sympathetic fibers originate from the celiac and superior mesenteric plexuses. Parasympathetic innervation is supplied by branches of the vagus nerve (n. vagus). The intramural nervous system is represented by the submucosal (plexus submucosus) and myenteric (plexus myentericus) plexuses.