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The small intestine is the portion of the alimentary tract extending from the pylorus to the cecum. The structure, function, and diseases of the duodenum are discussed in Chapter 23; the jejunum and ileum are described in this chapter.

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Gross Anatomy

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The small intestine in an adult is 5–6 m long from the ligament of Treitz to the ileocecal valve. The upper two fifths of the small intestine distal to the duodenum is termed the jejunum, and the lower three fifths is the ileum. There is no sharp demarcation between the jejunum and the ileum; however, as the intestine proceeds distally, the lumen narrows, the mesenteric vascular arcades become more complex, and the circular mucosal folds become shorter and fewer (Figure 29–1). In general, the jejunum resides in the left side of the peritoneal cavity, and the ileum occupies the pelvis and right lower quadrant.

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Figure 29–1.
Graphic Jump Location

Blood supply and luminal surface of the small bowel. The arterial arcades of the small intestine increase in number from one or two in the proximal jejunum to four or five in the distal ileum, a finding that helps to distinguish proximal from distal bowel at operation. Plicae circulares are more prominent in the jejunum.

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The small bowel is attached to the posterior abdominal wall by the mesentery, a reflection from the posterior parietal peritoneum. This peritoneal fold arises along a line originating just to the left of the midline and passing obliquely to the right lower quadrant. Although the mesentery joins the intestine along one side, the peritoneal layer of the mesentery envelops the bowel and is called the visceral peritoneum, or serosa.

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The mesentery contains fat, blood vessels, lymphatics, lymph nodes, and nerves. The arterial blood supply to the jejunum and ileum derives from the superior mesenteric artery. Branches within the mesentery anastomose to form arcades (Figure 29–1), and small straight arteries travel from these arcades to enter the mesenteric border of the gut. The antimesenteric border of the intestinal wall is less richly supplied with arterial blood than the mesenteric side, so when blood flow is impaired, the antimesenteric border becomes ischemic first. Venous blood from the small intestine drains into the superior mesenteric vein and then enters the liver through the portal vein.

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Submucosal lymphoid aggregates (Peyer patches) are much more numerous in the ileum than in the jejunum. Lymphatic channels within the mesentery drain through regional lymph nodes and terminate in the cisterna chyli.

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Parasympathetic nerves from the right vagus and sympathetic fibers from the greater and lesser splanchnic nerves reach the small intestine through the mesentery. Both types of autonomic nerves contain efferent and afferent fibers, but intestinal pain appears to be mediated by the sympathetic afferents only.

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Microscopic Anatomy

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