The small intestine is the longest part of the digestive tract, a coiled tube about 6-7 m long that runs from the pylorus to the ileocaecal junction. It has three parts: the short fixed duodenum and the long mobile jejunum and ileum. Its lining is folded into circular folds, villi and microvilli, giving an enormous surface for absorption.
| Location | From the pylorus to the ileocaecal junction, with the duodenum curving around the pancreatic head and the jejunum and ileum filling the central and lower abdomen |
|---|---|
| Parts |
|
| Nerve Supply | Vagus nerve (parasympathetic); Greater and lesser splanchnic nerves via the coeliac and superior mesenteric plexuses (sympathetic); Enteric nervous system |
| Blood Supply | Gastroduodenal artery; Anterior and posterior superior pancreaticoduodenal arteries; Anterior and posterior inferior pancreaticoduodenal arteries; Jejunal and ileal arteries (from the superior mesenteric artery); Ileocolic artery |
| Venous drainage |
|
| Lymphatic drainage |
|
Completes digestion using pancreatic enzymes, bile and its own brush border enzymes, and absorbs most nutrients, water and electrolytes.
Small bowel obstruction, most often from adhesions or hernias, causes colicky central pain, vomiting, distension and absent passage of flatus. Visceral pain from the small intestine is felt around the umbilicus because the midgut is supplied by T8-T10 sympathetic fibres. Loss of large lengths after resection causes short bowel syndrome with malabsorption.
Blockage of the small intestine, usually from adhesions or hernia, causing colicky pain, vomiting and distension; strangulation is a surgical emergency.
Immune reaction to gluten that flattens the villi of the proximal small intestine, causing malabsorption, anaemia and diarrhoea.
Chronic transmural inflammatory bowel disease that can affect any part of the gut but most often the terminal ileum, causing strictures and fistulas.
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