Duodenum

The duodenum is the first portion of the small intestine, connecting the stomach's pylorus to the jejunum. Roughly 25 centimeters (about 12 inches) long, it curves in a C-shape around the head of the pancreas. Anatomists divide it into four parts: the superior (first) part, the descending (second) part, the horizontal (third) part, and the ascending (fourth) part, which ends at the duodenojejunal flexure, anchored by the suspensory ligament of Treitz.

Functionally, the duodenum is where digestion accelerates. Acidic chyme entering from the stomach is neutralized by bicarbonate-rich secretions, including those from Brunner's glands, which are unique to the duodenum. In the descending part, the major duodenal papilla receives bile from the liver and gallbladder and enzymes from the pancreas via the hepatopancreatic ampulla (ampulla of Vater), enabling the breakdown of fats, proteins, and carbohydrates. The duodenum also releases key hormones, including secretin and cholecystokinin.

Its anatomy carries clinical weight. Most of the duodenum is retroperitoneal — only the first part is intraperitoneal, and that first part (the duodenal bulb) is the most common site of duodenal ulcers. The major papilla also marks the embryologic foregut–midgut boundary, which is why the proximal duodenum is supplied by celiac trunk branches and the distal duodenum by the superior mesenteric artery.

The duodenum is high-yield anatomy for medical licensing and admissions exams. USMLE Step 1 tests its parts, relations, blood supply, and ulcer sites, while the MCAT covers its role within the digestive system's structure and function.