Duodenal bulb

The duodenal bulb, also known as the duodenal cap or duodenal ampulla, is the dilated proximal segment of the superior (first) part of the duodenum immediately distal to the pylorus. It constitutes approximately the first 2 cm of the duodenum and extends from the pyloric sphincter to the narrower distal portion of the superior duodenum. Situated at the level of the L1 vertebra, it is directed upwards, backwards, and to the right.

Distinguishing features

The duodenal bulb differs from the remainder of the duodenum in several important respects. It is almost completely covered by peritoneum, making it relatively mobile, whereas most of the remaining duodenum is retroperitoneal and fixed. Internally, Its mucosal surface is smooth due to the absence of plicae circulares (circular folds), which become prominent in the distal duodenum and throughout much of the jejunum.

Relations

• Anteriorly, the duodenal bulb is related to the quadrate lobe of the liver, the gallbladder, and the anterior abdominal wall.

• Posteriorly, it is closely related to the gastroduodenal artery, common bile duct, and portal vein, relationships of considerable clinical significance.

• Inferiorly, it rests upon the head of the pancreas,

• while superiorly it is related to the hepatoduodenal ligament (containing portal triad) and the epiploic foramen.

Blood supply and venous drainage

The arterial supply of the duodenal bulb is derived from the superior pancreaticoduodenal artery, which is a branch of the gastroduodenal artery. Venous drainage accompanies the arterial supply and ultimately drains into the portal venous system through the pancreaticoduodenal and gastroduodenal veins.

Histological features

Histologically, the duodenal bulb is lined by simple columnar epithelium with intestinal villi and crypts. The submucosa contains abundant Brunner's glands which secrete alkaline mucus rich in bicarbonate. This secretion helps neutralise acidic gastric contents entering from the stomach, thereby protecting the duodenal mucosa and creating an optimal pH for pancreatic enzymatic activity. The absence of circular folds, together with its expanded lumen, makes the duodenal bulb readily recognisable during endoscopic and radiological investigations.

Clinical correlates

The duodenal bulb is the most common site of duodenal peptic ulceration. Because of its close anatomical relationships, the complications of ulcer disease depend on the direction of ulcer penetration. Posterior duodenal ulcers may erode the gastroduodenal artery, resulting in severe and potentially lifethreatening upper gastrointestinal haemorrhage. In contrast, anterior ulcers may perforate into the peritoneal cavity, leading to acute peritonitis.