Structure
The small intestine is several metres long in life, with the duodenum forming a short fixed first segment and the jejunum and ileum a long mobile remainder.
Duodenum
The duodenum is a C-shaped tube about 25 cm long that wraps around the head of the pancreas. It is divided into four parts.
| Part | Level | Key features |
|---|---|---|
| First (superior) | L1 | About 5 cm; its first 2 cm (the duodenal cap) is intraperitoneal and mobile; the rest is retroperitoneal |
| Second (descending) | L1 to L3 | About 7.5 cm; runs down right of the vertebral bodies; receives the bile and pancreatic ducts at the major papilla |
| Third (horizontal) | L3 | About 10 cm; crosses the IVC and aorta, with the superior mesenteric vessels passing in front |
| Fourth (ascending) | L3 to L2 | About 2.5 cm; ends at the duodenojejunal flexure, held by the suspensory muscle of the duodenum (ligament of Treitz) |
Jejunum and ileum
The jejunum forms roughly the proximal two-fifths and the ileum the distal three-fifths of the mobile small bowel. There is no sharp boundary, but the features change gradually.
| Feature | Jejunum | Ileum |
|---|---|---|
| Position | Upper left of the infracolic compartment | Lower right and pelvis |
| Wall and calibre | Thicker, wider, redder | Thinner, narrower, paler |
| Circular folds (plicae circulares) | Tall and closely packed | Low and sparse, absent distally |
| Arterial arcades | One or two, with long vasa recta | Many, with short vasa recta |
| Mesenteric fat | Less, leaving clear windows | More, extending to the gut wall |
| Lymphoid tissue | Scattered solitary follicles | Aggregated lymphoid follicles (Peyer's patches) |
The mesentery of the small intestine has a root about 15 cm long running from the duodenojejunal flexure, left of L2, down to the right sacroiliac joint.
Relations
The duodenum has fixed, surgically important relations, while the jejunum and ileum are mobile loops lying in front of the posterior abdominal wall and below the transverse mesocolon.
- First part: posteriorly the gastroduodenal artery, bile duct and portal vein; superiorly the epiploic foramen; anteriorly the gallbladder and liver.
- Second part: posteriorly the hilum of the right kidney; anteriorly the transverse colon crossing it; medially the head of the pancreas.
- Third part: sandwiched between the aorta behind and the superior mesenteric artery in front.
- Fourth part: lies left of the aorta, beside the left psoas and left sympathetic trunk.
The jejunum and ileum are covered in front by the greater omentum and are bordered by the ascending and descending colon on either side.
Blood supply and innervation
The small intestine proximal to the major duodenal papilla is foregut and is supplied through the coeliac trunk; everything distal is midgut and supplied by the superior mesenteric artery.
The first and second parts of the duodenum are supplied by the superior pancreaticoduodenal arteries from the gastroduodenal artery, which also gives the supraduodenal artery. The inferior pancreaticoduodenal artery from the SMA supplies the lower duodenum. The jejunum and ileum receive 15 or more jejunal and ileal branches from the left side of the SMA, which form arcades in the mesentery before giving straight vasa recta to the gut. The terminal ileum is also supplied by the ileal branch of the ileocolic artery.
Veins drain to the superior mesenteric vein and then the portal vein. Lymph from the lacteals passes through mesenteric nodes to the superior mesenteric nodes and on to the cisterna chyli. Vagal fibres increase motility and secretion; sympathetic fibres from the thoracic splanchnic nerves carry pain afferents, so midgut pain is felt around the umbilicus (T10 dermatome).
Clinical relevance
Small bowel anatomy explains the complications of peptic ulcers, the behaviour of obstruction, and the site of several classic diseases.
- Duodenal ulcer: a posterior ulcer in the first part may erode the gastroduodenal artery and bleed heavily; an anterior ulcer tends to perforate into the peritoneal cavity.
- Superior mesenteric artery syndrome: the third part of the duodenum is compressed between the SMA and the aorta, usually after rapid weight loss.
- Meckel's diverticulum: a remnant of the vitellointestinal duct on the antimesenteric border of the ileum; it may contain gastric or pancreatic tissue and cause bleeding, obstruction or inflammation.
- Crohn's disease: most often affects the terminal ileum, which absorbs vitamin B12 and bile salts.
- Gallstone ileus: a stone passing through a cholecystoduodenal fistula lodges at the narrow ileocaecal junction.
- Obstruction: adhesions and hernias are the common causes; distended loops with circular folds crossing the full width identify small bowel on a radiograph.