The duodenum is the first part and "central processing unit" of the small bowel. It is the:

• Beginning of small bowel motor activity (peristalsis)
• Site of multiple hormones related to digestion
• Entrance site for liver bile and pancreatic enzymes that aid digestion
• Site of antibody production

It is easily approached for diagnostic testing but difficult to operate upon

Anatomy and Physiology

The duodenum connects the stomach to the jejunum, which is the second part of the small bowel (Figure 1)

• It is 20 cm (8 inches) in length and divided into four parts
• The duodenum is retroperitoneal (has peritoneum, the thin layer of tissue that lines the abdominal cavity) only on the anterior (front) side; fixed in location; wraps around the head, neck and body of the pancreas; and the superior mesenteric artery and vein (major blood vessels for the bowel) pass anterior to the 3rd portion
• The ampulla of Vater (entrance site into duodenum of joined common bile duct and pancreatic duct) enters the medial (towards the midline) side of the 2nd portion of the duodenum
• The accessory pancreatic duct enters slightly higher in the medial wall of the duodenum
• The ligament of Treitz (a supporting band of peritoneum and muscle fibers) marks the point between the duodenum and jejunum The wall of the small bowel (Figure 2) is made up of four layers:

Figure 1 - Anatomy of the stomach, duodenum, pancreas and jejunum. See text. © C. McKee