Small intestine
Intestinum tenue
Also known as: small bowel
The small intestine is the longest part of the gut, running from the pylorus to the ileocaecal junction. Most chemical digestion and almost all nutrient absorption happen here, across a mucosa enlarged by folds, villi and microvilli.
Overview
The small intestine has three parts. The duodenum is about 25 cm long, fixed and mostly retroperitoneal, and curves in a C around the head of the pancreas. The jejunum is the proximal two-fifths of the mobile, mesenteric small bowel, and the ileum is the distal three-fifths. The jejunum and ileum are suspended by the mesentery and coil within the frame formed by the colon. Total length is about 6–7 m at autopsy but only about 3 m in life, because of muscle tone. The diameter is about 2.5–3 cm.
The absorptive surface is amplified at three levels. Plicae circulares give about 3× the surface, villi about 10× and microvilli about 20×, roughly a 600-fold increase over a smooth tube. Classic textbooks put the resulting area at about 200 m². More recent morphometric work suggests about 30 m², which is still an enormous interface.
The epithelium renews itself completely every 3–5 days from stem cells in the crypts. Specialisation varies along the length. Iron, calcium and folate are absorbed mainly in the duodenum and jejunum, and most macronutrients in the duodenum and jejunum. The terminal ileum has unique carriers for vitamin B12–intrinsic factor and for bile salts.
During a meal, motility is mainly segmentation, which mixes, with short peristaltic runs that propel. The slow-wave frequency falls from about 12/min in the duodenum to about 8/min in the ileum, so the net drift is aboral. Between meals the migrating motor complex sweeps the lumen clean. Chyme takes about 3–5 hours to pass through.
- Location
Central and lower abdomen. The duodenum lies around the pancreatic head at L1–L3. Jejunal loops lie mainly in the left upper and central abdomen; ileal loops lie mainly in the right lower abdomen and pelvis.
Parts & subdivisions
- C-shaped, about 25 cm, mostly retroperitoneal. Receives bile and pancreatic juice.
- Proximal two-fifths of the mesenteric small intestine. The main absorptive segment.
- Distal three-fifths. Absorbs B12 and bile salts. Contains Peyer patches.
- Permanent circular folds of mucosa and submucosa.
- Finger-like absorptive projections, with the regenerative glands between them.
- Controls entry of chyme into the caecum and limits reflux.
Anatomy
- Innervation
Parasympathetic: vagus nerve. Sympathetic: T8–T10 via the greater and lesser splanchnic nerves to the coeliac and superior mesenteric ganglia. Visceral pain is referred to the periumbilical region (T10). The enteric nervous system lies within the wall.
- Arterial supply
Duodenum: superior pancreaticoduodenal arteries (from the gastroduodenal artery, coeliac territory) and inferior pancreaticoduodenal arteries (from the SMA). Jejunum and ileum: 15–18 jejunal and ileal branches of the superior mesenteric artery, which form arterial arcades and then vasa recta. The terminal ileum also receives the ileocolic artery.
- Venous drainage
Superior mesenteric vein → hepatic portal vein. Duodenal veins drain to the portal vein and the SMV.
- Lymphatic drainage
Lacteals → mesenteric lymph nodes (often more than 100) → superior mesenteric nodes → intestinal lymph trunk → cisterna chyli.
- Histology
Mucosa with villi and crypts. The epithelium contains enterocytes, goblet cells (numbers increase distally), enteroendocrine cells, Paneth cells and stem cells. The submucosa contains Brunner glands in the duodenum and Peyer patches in the ileum. The muscularis externa has inner circular and outer longitudinal layers. Serosa covers the jejunum and ileum; the retroperitoneal duodenum has adventitia.
- Development
The duodenum distal to the major papilla, the jejunum and the ileum all come from the midgut. The proximal duodenum comes from the foregut. The midgut loop herniates into the umbilical cord in week 6, rotates 270° anticlockwise around the SMA, and returns by week 10.
Function
- Completes the digestion of carbohydrates, proteins and lipids using pancreatic and brush-border enzymes and bile.
- Absorbs nearly all nutrients, most vitamins and minerals, and about 7.5 L of water a day.
- Secretes about 1.8 L of alkaline intestinal juice and mucus.
- Endocrine regulation through CCK, secretin, GIP, motilin, GLP-1 and PYY, which control the stomach, pancreas, gallbladder and appetite.
- Immune surveillance through GALT, Peyer patches and secretory IgA.
Clinical significance
Small bowel obstruction
In developed countries adhesions after surgery are the most common cause, followed by hernias and tumours. Patients have colicky central pain, early vomiting (bilious if the obstruction is high), distension and absolute constipation. On imaging, dilated loops over 3 cm with valvulae conniventes crossing the whole lumen are typical (the "3-6-9 rule": small bowel 3 cm, colon 6 cm, caecum 9 cm).
Acute mesenteric ischaemia
Usually caused by an embolus, often from atrial fibrillation, lodging in the SMA just beyond the middle colic origin. The hallmark is severe pain out of proportion to abdominal signs, with raised lactate. It leads to bowel infarction unless revascularised quickly.
Short bowel syndrome
Extensive resection, for example for Crohn disease, volvulus or necrotising enterocolitis, leaves too little absorptive surface. The results are diarrhoea, malabsorption, dehydration and nutritional deficiency. Loss of the terminal ileum and ileocaecal valve is especially harmful.
Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.