Small Intestine Parts
The small intestine is 6–7 m long and consists of the duodenum, jejunum, and ileum. Each segment has distinct anatomy, histology, and clinical significance.
✦ The Mnemonic
"Doctors Discover Hidden Anatomy; Just Imagine"
Duodenum (D1–D4) · Jejunum · Ileum — with distinct features at each level
Clinical Breakdown
The duodenum is entirely retroperitoneal except for D1 (which has a peritoneal 'cap'). The ampulla of Vater in D2 is the combined opening of the common bile duct and main pancreatic duct, surrounded by the sphincter of Oddi. Gallstone impaction at the ampulla causes simultaneous biliary and pancreatic obstruction.
Distinguishing jejunum from ileum on imaging: jejunum has a 'featureless' or 'stacked coins' appearance (tall circular folds, valvulae conniventes); ileum has a smoother outline with less prominent folds. The jejunum is in the left upper abdomen; the ileum tends to the right lower abdomen.
Meckel's diverticulum: remnant of the vitello-intestinal duct — the rule of 2s: 2% of population, 2 feet from ileocaecal valve, 2 cm long, 2 types of ectopic tissue (gastric most clinically significant, causing peptic ulceration and bleeding), presents within 2 years of life. May contain ectopic pancreatic or gastric mucosa.
⭐ Clinical Pearl
Coeliac disease: autoimmune enteropathy caused by gluten sensitivity — affects the duodenum and jejunum predominantly. Villous atrophy on biopsy leads to malabsorption. Anti-TTG antibodies are the screening test; duodenal biopsy (from D2) is confirmatory. Treatment: strict lifelong gluten-free diet.