Intestinal Tract
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Direct answer
Twenty-five centimetres long and C-shaped, the duodenum curves around the head of the pancreas from the pylorus to the duodenojejunal flexure, suspended by the ligament of Treitz — the landmark that divides upper from lower gastrointestinal bleeding and defines malrotation on imaging. The midgut, from the ampulla to the proximal two-thirds of the transverse colon, belongs to the superior mesenteric artery; the hindgut, from there to the upper anal canal, to the inferior mesenteric artery. Between their territories lie the watershed points — the splenic flexure (Griffith's) and the rectosigmoid junction (Sudeck's) — where ischaemia strikes when perfusion fails.
What you must remember
- The first part of the duodenum is intraperitoneal and forms the bulb, seat of most duodenal ulcers; the second part receives the bile and pancreatic ducts at the ampulla of Vater on its posteromedial wall, 8 to 10 cm from the pylorus, with the minor papilla of Santorini 2 cm proximal.
- The third part is crossed anteriorly by the root of the mesentery and superior mesenteric vessels, which compress it in nutcracker-type obstruction; the fourth part ascends to the ligament of Treitz.
- Jejunum versus ileum: jejunum is wider with thicker walls, long vasa recta, one or two arcades, prominent valvulae conniventes, scant mesenteric fat; ileum is narrower with three to five arcades, short vasa recta, fatty mesentery and Peyer's patches on the antimesenteric border.
- Peyer's patches are the reservoir typhoid invades; the terminal ileum reabsorbs bile salts and vitamin B12, so its resection causes megaloblastic anaemia and gallstones.
- The appendix base lies where the three taeniae converge; McBurney's point, the junction of the lateral and middle thirds of the spinoumbilical line, marks the surface projection; retrocaecal is the commonest position, pelvic next.
- The colon shows taeniae coli, haustra and appendices epiploicae; the caecum is the widest part, hence the first to perforate in distal obstruction and a volvulus site alongside the sigmoid.
- Above the dentate line the anal canal is endodermal hindgut — superior rectal vessels to the portal system, visceral pain, lymph to inferior mesenteric nodes, painless internal haemorrhoids; below it is ectodermal proctodeum — inferior rectal vessels to the systemic circulation, somatic pain, lymph to superficial inguinal nodes, painful external haemorrhoids.
Appendicitis pain, explained anatomically
A young adult develops periumbilical ache, anorexia and vomiting, then pain shifting to the right iliac fossa with tenderness at McBurney's point. Early visceral pain is midgut pain: the appendix shares T10 afferents with the rest of the midgut, so the ache is referred to the T10 dermatome around the umbilicus. As inflammation touches the parietal peritoneum, somatic intercostal nerves (T12–L1) localise the pain exactly where the appendix lies — hence the shift. Position then predicts the examination: a retrocaecal appendix irritates psoas, giving a positive psoas sign and muted tenderness that delays diagnosis; a pelvic appendix produces tenderness only on rectal or vaginal examination, with diarrhoea and tenesmus from rectal irritation. When the stem adds a right iliac fossa mass after several days, the greater omentum has walled the perforation — an appendicular lump initially managed conservatively in standard Indian teaching, the "policeman" doing its work.
Where students slip
Watershed confusion is commonest: Griffith's point is the splenic flexure, junction of superior and inferior mesenteric territories and the usual site of ischaemic colitis; Sudeck's point is the rectosigmoid junction where the superior rectal artery's last branches meet internal iliac supply — candidates swap them or attribute both to the midgut. The second error is treating the dentate line as a detail: every property of haemorrhoids, fissures and tumours — pain, drainage, lymphatics, epithelium — flips across it, so fissures below the line cause exquisite somatic pain, the common answer for painful defaecation. Finally, do not localise vitamin B12 absorption to the jejunum — its receptors are confined to the terminal ileum.
Frequently asked questions
What is the clinical importance of the ligament of Treitz?
It marks the duodenojejunal flexure — the divider of upper from lower gastrointestinal bleeding and the anchor in malrotation.
Where do the bile and pancreatic ducts enter the duodenum?
At the ampulla of Vater on the posteromedial wall of the second part, 8 to 10 cm from the pylorus, guarded by the sphincter of Oddi.
What distinguishes jejunum from ileum macroscopically?
Jejunum: wider, thicker, prominent valvulae conniventes, few arcades, long vasa recta. Ileum: narrower, more arcades, short vasa recta, fatty mesentery, Peyer's patches.
Which arterial territories meet at the splenic flexure?
The middle colic branch of the superior mesenteric and the left colic branch of the inferior mesenteric artery — Griffith's watershed, the usual site of ischaemic colitis.
What is the commonest position of the appendix?
Retrocaecal, followed by pelvic, with the base constant where the taeniae converge on the caecum.
How does the dentate line change haemorrhoid behaviour?
Internal haemorrhoids above it drain to the portal superior rectal vein and are painless; external ones below drain to systemic veins under somatic innervation and hurt.