Small bowel (small intestine)
General
Normal anatomy

Authors: Hanni Gulwani, M.D. (see Authors page)

Revised: 11 July 2016, last major update August 2012

Copyright: (c) 2003-2016, PathologyOutlines.com, Inc.

PubMed search: embryology[title] small bowel
Cite this page: Normal anatomy. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/smallbowelnormalanatomy.html. Accessed December 7th, 2016.
Definition / General
  • 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)

    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 / Tables

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Duodenum

Gross Images

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Ileum and terminal ileum

Micro Images

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Contributed by Dr. Grigory Demyashkin, I. M. Sechenov First Moscow State Medical University (Russia) - 6-8 week embryo:

Upper left: small intestine; bottom left: esophagus; center: stomach;top right: pancreas; right: liver

Small intestine, epithelium

Left: small intestine; bottom: stomach; center: gonad and kidney primary (mesonephros); right: symphysis



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Peyer's patches

Lymph nodes: lacteals

Contributed by Dr. Suhail Muzaffar, City Hospital (United Kingdom)