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Small bowel (small intestine)

Normal anatomy


Reviewer: Hanni Gulwani, M.D. (see Reviewers page)
Revised: 13 December 2012, last major update August 2012
Copyright: (c) 2003-2012, PathologyOutlines.com, Inc.

General
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● Extends from gastric pylorus to ileocaecal valve
● 6 meters long, divided into duodenum, jejunum, ileum

Duodenum:
● 25 cm long, from pyloric sphincter to ligament of Treitz, mostly retroperitoneal, fixed in position
● Common bile duct (CBD) and pancreatic duct enter second part of duodenum posteromedially at ampulla of vater (eMedicine)

Jejunum:
● 240 cm long, 40% of remainder of bowel, begins at ligament of Treitz
● Has prominent circular mucosal folds (folds of kerckring) that increase absorptive surface

Ileum:
● 360 cm long, distal 60% of post-duodenal bowel
● Mucosa has transverse folds, prominent in proximal ileum, flat/absent at terminal ileum

Ileocecal valve:
● At end of small bowel
● 2 lip structure containing adipose tissue and lymphoid tissue

Lymph nodes:
● Duodenum drains to portal and pyloric nodes
● Jejunum and proximal ileum drain to mesenteric nodes and nodes around superior mesenteric artery, terminal ileum drains to ileocolic nodes
Lacteals are lymphatic channels in villi for chylomicrons

Intestinal immune system consists of:
● Peyer’s patches in ileum (ovoid lymphoid follicles, partly mucosal and partly submucosal, in antimesenteric side of terminal ileum)
● Small intestinal goblet cells, which deliver low molecular weight soluble antigens from intestinal lumen to CD103+ lamina propria dendritic cells, which regulates development of T cells (Nature 2012;483:345)
● M (membranous) cells, part of follicle associated epithelia (MALT) in small bowel and colon, which transfer antigen macromolecules from lumen to lymphocytes
● T cells, usually CD8+ and scattered in surface epithelium
● Lamina propria contains CD4+ T cells and B cells
● Mucosa associated lymphoid tissue: lymphoid nodules, mucosal lymphocytes, appendiceal lymphoid follicles and mesenteric nodes (Annu Rev Cell Dev Biol 2000;16:301, RIKEN Research Center for Allergy and Immunology

Neuromuscular function:
● Anterograde and retrograde peristalsis mixes food and promotes maximal contact of nutrients with mucosa
● Colonic peristalsis prolongs contact with mucosa
● Peristalsis is mediated via myenteric plexus and autonomic innervation (sympathetic-thoracolumbar, parasympathetic-vagal)
● Also through interstitial cells of Cajal (pacemaker cells) and smooth muscle cells
● Vagal receptors are abundant in duodenum and scattered throughout wall

Diagrams
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Duodenum


M cells

Gross images
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Ileum and terminal ileum

Micro images
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Peyer's patches


Lymph nodes: lacteals

End of Small bowel (small intestine) > Normal anatomy


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