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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: Mesentery
Overview of the mesentery on anterior view of abdomen with greater omentum reflected and small intestine removed.

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. 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, descending, horizontal and ascending.

The superior part starts just at the side and slightly anterior to the first lumbar vertebra, it rises above the level of the pylorus of the stomach where it communicates with the liver via the hepatoduodenal ligament which is formed by the lesser omentum of the peritoneum. Behind this section sits the inferior vena cava, the portal vein and the common bile duct. The descending part 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. The horizontal section 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. Lastly, the ascending part of the duodenum 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.

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. 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. 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.


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

  1. Obstructive disorders including paralytic ileus, a hernia or volvulus are common but can become complicated.
  2. 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.
  3. Neoplastic growths may include gastrointestinal stromal tumors (GIST), lymphomas and sarcomas.
  4. Developmental, congenital or genetic conditions include pyloric stenosis, duodenal atresia and gastroschisis.
  5. 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.
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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.
  • Organs - Small intestine. BBC.
  • Duodenum. Encyclopædia Britannica, Inc.
  • Ileum. Encyclopædia Britannica, Inc.


  • Dr. Alexandra Sieroslawska


  • Duodenum and Jejunum (green) - Irina Münstermann
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