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KEY POINTS

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  1. The small intestine performs a diverse set of functions.

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

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

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

  5. Small bowel may be the source of gastrointestinal bleeding, which may be difficult to diagnose.

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

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INTRODUCTION

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The small intestine is the raison d’être of the gastrointestinal tract as it is the principle site of nutrient digestion and absorption.1 The small intestine is also the body’s largest reservoir of immunologically active and hormone-producing cells and hence can be conceptualized as the largest organ of the immune and endocrine systems, respectively. It achieves this diversity of action through unique anatomic features, which provide it with a massive surface area, a diversity of cell types, and a complex neural network to coordinate these functions.

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Despite its size and importance, diseases of the small intestine are relatively infrequent and present diagnostic and therapeutic challenges. Treatments for common conditions such as postoperative ileus are hardly more effective than those used at the dawn of the last century. Mortality rates associated with acute mesenteric ischemia have not improved during the past 50 years.

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Despite introduction of novel imaging techniques such as capsule endoscopy and double balloon endoscopy, diagnostic tests lack sufficient predictive power to definitively guide clinical decision making for individual patients. Furthermore, few high-quality, controlled data on the efficacy of surgical therapies for small bowel diseases are available.

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

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GROSS ANATOMY

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The small intestine is a tubular structure that extends from the pylorus to the cecum. The estimated length of this structure varies depending on whether radiologic, surgical, or autopsy measurements are made. In the living, it is thought to measure 4 to 6 m.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 anatomic 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.

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