Chapter 64: Small Intestine Anatomy

The duodenum is the fixed, retroperitoneal segment, distinguished by its elongated C-shape, where the head of the pancreas lies within its concavity. It is divided into four parts, with the descending (second) part being critical as the site of entry for the common bile duct and pancreatic duct, usually through the major duodenal papilla. Notably, the horizontal (third) part passes anterior to the abdominal aorta and posterior to the superior mesenteric artery; excessive narrowing of this angle can result in duodenal obstruction known as superior mesenteric artery syndrome. The duodenum ends at the duodenojejunal flexure, stabilized by the suspensory ligament of the duodenum (ligament of Treitz). The jejunum and ileum, which are mobile segments suspended by the mesentery, are differentiated morphologically. The proximal jejunum is characterized by a thicker wall, a wider diameter, and fewer arterial arcades (one to three tiers) but longer straight arteries (vasa recta), and more numerous and pronounced circular folds (plicae circulares). In contrast, the distal ileum has a thinner wall, a narrower diameter, more arterial arcades (two to six tiers), shorter vasa recta, and contains the bulk of the aggregated lymphoid tissue known as Peyer’s patches. The small intestine dramatically increases its absorptive surface area through the circular folds, villi, and microvilli, facilitating the absorption of nutrients, fluid, and electrolytes, with specialized processes occurring in the terminal ileum for Vitamin B12 and conjugated bile acids. Structurally, the intestinal glands house Paneth cells, specialized epithelial cells that secrete antimicrobial proteins for mucosal immunity. Motility, including the Migrating Motor Complex (MMC) observed during fasting, is driven by pacemaker cells called Interstitial cells of Cajal (ICC), which are present even in transplanted, denervated intestine. The duodenal blood supply is robustly collateralized by the superior and inferior pancreaticoduodenal arteries. Venous drainage occurs primarily via the superior mesenteric vein. In terms of innervation, sympathetic input is vasoconstrictor and inhibitory to musculature. Pain arising from the duodenum (a foregut derivative) is referred to the epigastric region, while pain from the jejunum and ileum (midgut derivatives) is referred to the periumbilical region. Clinically, surgical resection leading to short bowel syndrome is defined by specific residual lengths, and proximal stomata, such as jejunostomies, are associated with a greater volume output and fluid loss compared to ileostomies.