15 Small Intestine Diseases
Anatomy of the Small Intestine
Jejunum
Ileum
Microscopic Anatomy
• Submucosa: strongest layer, connective tissue, Meissner’s plexus (parasympathetic ganglion cells and neuronal network)
Innervation
• Parasympathetic: vagus
• Sympathetic
Vessels and Lymphatics
Arterial Supply
Venous Drainage
• Duodenal veins empty into splenic vein, superior mesenteric vein, and portal vein (which lies posterior to the first part).
Clinical Correlates
Intussusception
• Portion of bowel (intussusceptum) invaginates into an adjoining segment of bowel (intussuscipiens), causing obstruction.
Diverticular Disease
Meckel’s Ileal Diverticulum
• Can also include gastric tissue: symptomatic with ulcer occurring in opposite gut wall (due to acid secretion)
Cancer of the Small Intestine
• Most common malignant neoplasms: adenocarcinomas, carcinoid tumors, lymphomas, and gastrointestinal stroma tumors, all rare
• Carcinoid
Slowly growing tumor of enterochromaffin (argentaffin or Kulchitsky) cells, producing serotonin and bradykinin

• Gastrointestinal stromal tumors (GIST): most common GI mesenchymal neoplasm (1% of all), often associated with Kit gene mutation
Crohn’s Disease
Short-Bowel Syndrome
• Because of absorptive and vascular reserve capacity of small intestine, limited resection of bowel is generally associated with minimal morbidity.
• Extensive resection can result in short-bowel syndrome, with insufficient absorptive activity, intractable diarrhea, malnutrition, weight loss, and dehydration.