German Contact How to study Login Register

Register now and grab your free ultimate anatomy study guide!

Sidebar ebook trimmed

Small Intestine



The small intestine is a region of the alimentary canal that consists of the duodenum, the jejunum and the ileum. This article will highlight the main anatomical features of this part of the gastrointestinal tract and also mention some primary pathological causes of disease within this somatic system.

Recommended video: Organs of the digestive system
Anatomy and function of the main organs of the digestive system.

The Duodenum

The duodenum is the first of the three sections of the small intestine and it is also the shortest. It starts at the base of the stomach by the pyloric sphincter and wraps around the head of the pancreas in a C-shape. This first section is intraperitoneal, whereas the second to fourth parts are all retroperitoneal.

Duodenum - ventral view

These four distinctions within the duodenal tube are named according to the direction they take and are situated in the following order running along the canal:

Superior part of duodenum - ventral view

  • Descending part - It moves caudally on the right side of the spinal column between the first and third lumbar vertebrae and it is in this section on the posteromedial wall that the common bile duct and the pancreatic duct empty into the bowels through the sphincter of Oddi.

Descending part of duodenum - ventral view

  • Horizontal section - It is positioned at the level of the third lumbar vertebra and is bound anteriorly by the superior mesenteric artery, the superior mesenteric vein and the root of the mesentery. Posteriorly it faces the inferior vena cava and the aorta.

Horizontal part of duodenum - ventral view

  • Ascending part - It turns back towards the second lumbar vertebra, but this time on the left side of the spinal column, before curving anteriorly towards the duodenojejunal junction.

Ascending part of duodenum - ventral view

The Jejunum and The Ileum

The jejunum and the ileum together can reach a length of approximately six meters, with the jejunum consisting of two out of five parts and the ileum consisting of the other three. The jejunum starts at the duodenojejunal junction and the ileum continues up until the ileocecal junction. The anatomical differences between the ileum and the jejunum are that the jejunum is found mostly in the left upper quadrant (LUQ) of the abdomen, has a thick wall and dense vasculature, while the ileum is found in the right lower quadrant (RLQ) and has much thinner, less vascularized walls.

Jejunum - ventral view

There are also histological differences in this part of the bowel as each section has different cells and different functions. The most prominent microscopic finding that sets the ileum apart from the jejunum are the cellular accumulations known as Peyer’s patches.

Peyer's patch - histological slide

Lastly, the mesentery of the small intestine is worth noting, as it is a double layer of visceral peritoneum that attaches both the jejunum and the ileum to the posterior aspect of the abdominal wall. Its root follows an oblique line that runs caudally to the right from the ligament of Treitz, which supports the small intestine at the duodenojejunal junction, all the way to the ileocolic junction.


Clinical Aspects

The small intestine has a long list of potential pathological findings that have been grouped into categories based on their common cause:

  • Obstructive disorders including paralytic ileus, a hernia or volvulus are common but can become complicated.
  • Infectious diseases such as tapeworm, tropical sprue or giardiasis are rare but severe when left untreated. More common infections such as the adenovirus or salmonella are seen in the west.
  • Neoplastic growths may include gastrointestinal stromal tumors (GIST), lymphomas and sarcomas.
  • Developmental, congenital or genetic conditions include pyloric stenosis, duodenal atresia and gastroschisis.
  • Other conditions or a miscellaneous mixture of diseases that can affect the small intestine include crohn’s disease, coeliac disease, gastric dumping syndrome and irritable bowel syndrome to name but a few.
Get me the rest of this article for free
Create your account and you'll be able to see the rest of this article, plus videos and a quiz to help you memorize the information, all for free. You'll also get access to articles, videos, and quizzes about dozens of other anatomy systems.
Create your free account ➞
Show references


  • Frank H. Netter, MD, Atlas of Human Anatomy, Fifth Edition, Saunders - Elsevier, Chapter Abdomen, Subchapter 27 Viscera (Gut), Guide: Duodenum, Jejunum and Ileum, Pages 143 to 145.
  • Healthwise Staff: Small Intestine. Healthwise, Incorporated., accessed on 23/07/2014.
  • Organs - Small intestine. BBC., accessed on 23/07/2014.
  • Duodenum. Encyclopædia Britannica, Inc., accessed on 23/07/2014.
  • Ileum. Encyclopædia Britannica, Inc., accessed on 23/07/2014.


  • Dr. Alexandra Sieroslawska


  • Duodenum - ventral view - Samantha Zimmerman
  • Superior part of duodenum - ventral view - Irina Münstermann
  • Descending part of duodenum - ventral view - Irina Münstermann
  • Horizontal part of duodenum - ventral view - Irina Münstermann
  • Ascending part of duodenum - ventral view - Irina Münstermann
  • Jejunum - ventral view - Irina Münstermann
  • Peyer's patch - histological slide - Smart In Media
  • Mesentery - Paul Kim
© Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

Continue your learning

Article (You are here)
Other articles
Well done!

Register now and grab your free ultimate anatomy study guide!

Sidebar ebook trimmed
Create your free account.
Start learning anatomy in less than 60 seconds.