The duodenal bulb is the initial, slightly widened segment of the duodenum, serving as the gateway between the stomach and the small intestine. It plays a key role in digestion by initiating the shift from the stomach’s acidic environment to the intestine’s alkaline one.
Precise Anatomical Location
The duodenal bulb is positioned at the beginning of the duodenum, the first segment of the small intestine. It starts immediately after the pyloric sphincter, the valve controlling the release of chyme from the stomach. This section is also known as the superior part of the duodenum and is typically only about 5 centimeters (2 inches) long.
This first duodenal segment is unique because it is the only part that is initially mobile and suspended by a mesentery, making it intraperitoneal for its first 2 to 3 centimeters. It sits high in the abdomen, posterior to the liver and the gallbladder, and superior to the head of the pancreas. The duodenum forms a C-shape around the pancreas, and the bulb is the initial upward curve of this formation.
Specialized Internal Structure
The internal structure of the duodenal bulb is adapted to function as the first line of defense against stomach acid. Unlike the rest of the small intestine, which features numerous internal folds called plicae circulares, the mucosal lining of the duodenal bulb is relatively smooth. This smooth surface allows the bulb to readily distend as it receives contents from the stomach.
A distinguishing feature of the duodenal bulb is the high concentration of specialized submucosal glands called Brunner’s glands. These tubular glands are situated deep within the submucosa, the layer beneath the mucosal lining, and are most numerous near the pylorus. Brunner’s glands are the defining anatomical characteristic of the duodenum, reflecting its protective role.
Essential Role in Digestion
The duodenal bulb’s primary job is to receive and neutralize the highly acidic chyme passed from the stomach through the pyloric sphincter. This acidic environment, caused by hydrochloric acid, would damage the unprotected intestinal lining. The first line of defense against this influx is the alkaline secretion produced by the Brunner’s glands.
These glands secrete a mucus-rich fluid containing a high concentration of bicarbonate. This alkaline fluid is expelled into the duodenal lumen, where it neutralizes the incoming stomach acid. This creates a protective layer of near-neutral pH adjacent to the epithelial cells and establishes the proper environment for digestive enzymes. The bulb also facilitates the initial mixing of the chyme with these protective secretions before the material moves into the rest of the duodenum for the introduction of bile and pancreatic juices.
Common Medical Conditions
Because the duodenal bulb is the first part of the intestine to encounter the high acid load from the stomach, it is vulnerable to damage. The most frequent condition affecting this area is the formation of a duodenal ulcer, a type of peptic ulcer. These open sores develop when aggressive factors, such as acid and the bacterium Helicobacter pylori (H. pylori), overwhelm the protective mucosal defenses.
Duodenal ulcers are more common than gastric ulcers and frequently occur in the bulb because the neutralization process is still underway in this proximal segment. If the ulcer grows deep enough, it can cause severe complications, such as bleeding if it erodes into the gastroduodenal artery, or perforation if it breaks through the intestinal wall. Inflammation of the duodenal lining, known as duodenitis, can also occur here, often caused by H. pylori infection and the use of nonsteroidal anti-inflammatory drugs (NSAIDs).