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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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