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The small intestine is the portion of the alimentary tract extending from the pylorus to the ileocecal valve, and consists of three segments: the duodenum, the jejunum, and the ileum. The anatomy, physiology, and pathology of the duodenum are discussed in Chapter 27.
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The jejunum begins at the ligament of Treitz. The jejunum and ileum are suspended on a mobile mesentery covered by visceral peritoneum that extends onto the external surface of the bowel to form the serosa. There is no sharp demarcation between the jejunum and the ileum, with the jejunum arbitrarily defined as the proximal two-fifths of the jejunoileal segment and the ileum as the distal three-fifths. As the intestine proceeds distally, the lumen narrows and the mesenteric vascular arcades become more complex.
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The mucosa is characterized by circular folds about 10 mm high, named plicae circulares (also called valvulae conniventes), that are tallest and most numerous in the distal duodenum and proximal jejunum, tapering from there until almost completely disappearing by the terminal ileum (Figure 33–1). These folds, combined with the presence of 20-40 villi per square millimeter on the surface of the valvulae conniventes, increase the absorptive surface area about eight times.
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The mesentery contains fat, blood vessels, lymphatic channels and nodes, and nerves. The jejunum and ileum are supplied by the superior mesenteric artery (SMA). Branches within the mesentery form vascular arcades, and small straight arteries from these arcades enter the mesenteric border of the gastrointestinal tract. Venous blood is drained through the superior mesenteric vein (SMV), which then joins the splenic vein behind the pancreas to form the portal vein.
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Lymphatic drainage is abundant. Lymphoid aggregates known as Peyer’s patches are present in the submucosa on the antimesenteric border along the distal ileum, and smaller follicles are evident throughout the remainder of the small intestine. The lymph is drained into the lymph nodes that follow the mesenteric arcade and then into the lymphatic vessels along the SMA. Eventually, the lymph is drained into the cisterna chyli that is located below the diaphragm.
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The innervation for the small intestine is both sympathetic (fibers from the greater and lesser splanchnic nerves) and parasympathetic (from the right vagus nerve). Although both types of autonomic nerves contain efferent and afferent fibers, only the sympathetic afferents appear to mediate intestinal pain.
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The wall of the small ...