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Key Points

  • image The small intestine performs a diverse set of functions.

  • image Small bowel obstruction is one of the most common surgical diagnoses.

  • image Most cases of small bowel obstruction are due to adhesions from previous surgery and resolve with conservative management.

  • image Tumors and malignancies of the small bowel are rare and difficult to diagnose.

  • image If following surgical resection less than 200 cm of small bowel remains, patients are at risk of developing short bowel syndrome.


The small intestine is a remarkable and complex organ that is not only the principle site of nutrient digestion and absorption but also contains the body’s largest reservoir of immunologically active and hormone-producing cells. Hence, it can be conceptualized as the largest organ of the immune and endocrine systems.1 It achieves this diversity of action through unique image anatomical features, which provide it with a massive surface area, a diversity of cell types, and a complex neural network to coordinate these functions.

Despite the size and importance of the small intestine, diseases of this organ are relatively infrequent and can present diagnostic and therapeutic challenges. Despite introduction of novel imaging techniques such as capsule endoscopy and double balloon endoscopy, diagnostic tests lack sufficient ability to reliably assess the small bowel. Furthermore, few high-quality, controlled data on the efficacy of surgical therapies for small bowel diseases are available.

Therefore, sound clinical judgment and a thorough understanding of anatomy, physiology, and pathophysiology remain essential to the care of patients with suspected small bowel disorders.


The small intestine is a tubular structure that extends from the pylorus to the cecum. The estimated length varies depending on whether radiologic, surgical, or autopsy measurements are made. In the living, it is thought to measure 4 to 6 meters.2 The small intestine consists of three segments lying in series: the duodenum, the jejunum, and the ileum. The duodenum, the most proximal segment, lies in the retroperitoneum immediately adjacent to the head and inferior border of the body of the pancreas. The duodenum is demarcated from the stomach by the pylorus and from the jejunum by the ligament of Treitz. The jejunum and ileum lie within the peritoneal cavity and are tethered to the retroperitoneum by a broad-based mesentery. No distinct anatomical landmark demarcates the jejunum from the ileum; the proximal 40% of the jejunoileal segment is arbitrarily defined as the jejunum and the distal 60% as the ileum. The ileum is demarcated from the cecum by the ileocecal valve.

The small intestine contains internal mucosal folds known as plicae circulares or valvulae conniventes that are visible upon gross inspection. These folds are also visible radiographically and help in the distinction between small intestine and colon, which does not contain them, on abdominal radiographs. These folds are more prominent in the proximal intestine ...

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