By MRCSA in MRCS Part A — 04 Jul 2026

51 SMALL BOWEL

# 52 SMALL BOWEL

Detailed notes

The small bowel runs roughly six metres from the pylorus to the ileocaecal valve and is where the bulk of digestion and absorption actually happen. It has three parts — duodenum, jejunum and ileum — but examiners rarely ask about the duodenum in this context (covered in its own lesson). Here we focus on the jejunoileal segment, which is intraperitoneal, mobile on a fan-shaped mesentery, and derived from the midgut (so it is supplied by the superior mesenteric artery).

The duodenojejunal flexure is fixed retroperitoneally and is suspended by the suspensory ligament of Treitz, a fibromuscular band that anchors it to the right crus of the diaphragm. This is the surgical landmark for the start of the jejunum and the divider between an "upper" and "lower" GI bleed.

Jejunum vs ileum

A favourite exam scenario: a surgeon at laparotomy reaches into the abdomen and pulls up a loop. How do they know if it's jejunum or ileum? They cannot see inside — they read the mesentery.

Feature	Jejunum	Ileum
Position	Proximal 2/5 (LUQ)	Distal 3/5 (RLQ)
Wall	Thick, deep red	Thinner, paler pink
Lumen	Wider	Narrower
Plicae circulares	Tall, numerous	Few, low or absent distally
Vasa recta	Long, straight	Short
Arterial arcades	Few (1–2 loops)	Many (3–5 loops)
Mesenteric fat	Sparse, "windows" between vessels	Encroaches onto the bowel wall
Peyer's patches	Few	Many (lymphoid aggregates)
Key absorption	Iron, folate, calcium, most nutrients	B12, bile salts (terminal ileum)

👩‍⚕️ The terminal ileum is the only site of vitamin B12 and bile salt reabsorption. Resect it (commonly for Crohn's) and you get megaloblastic anaemia plus cholesterol gallstones plus fat-soluble vitamin (A, D, E, K) deficiency.

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Blood supply and lymphatics

Everything from the second part of the duodenum to the proximal two-thirds of the transverse colon is midgut and supplied by the superior mesenteric artery (SMA). The SMA gives off jejunal and ileal branches that anastomose to form the arcades, from which vasa recta run straight to the bowel wall. Venous drainage is via the superior mesenteric vein into the portal system — so small-bowel carcinoid metastasises first to the liver.

Lymphatics are clinically distinct: each villus contains a central blind-ended lacteal that absorbs long-chain fatty acids as chyle. Lymph drains via mesenteric nodes to the cisterna chyli, then up the thoracic duct to the left subclavian vein. Damage to the cisterna or thoracic duct produces chylous ascites or chylothorax.

Histology and cell types

The mucosa is folded into villi and crypts to maximise absorptive surface (~200 m²). Five cell types are testable:

➡ Enterocytes — columnar absorptive cells with apical microvilli ("brush border"). Bear the disaccharidases and peptidases of terminal digestion.

➡ Goblet cells — secrete mucin; increase in number distally.

➡ Paneth cells — sit at the base of the crypts of Lieberkühn. Secrete lysozyme, defensins and TNF-α to regulate flora and protect crypt stem cells. Found only in the small bowel and proximal colon.

➡ Enteroendocrine cells — secrete CCK, secretin, motilin, GIP, somatostatin etc.

➡ M cells — flat epithelial cells overlying Peyer's patches in the ileum. Sample luminal antigen and present it to underlying lymphoid tissue. They are the gateway used by Salmonella and prions.

> Pearl: Crypts contain stem cells and Paneth cells; villi contain enterocytes and goblet cells. Coeliac flattens villi (atrophy) but the crypts compensate with hyperplasia.

Small bowel obstruction (SBO)

The most common indication for an emergency laparotomy in the UK. Causes, in order of frequency in adults:

1. Adhesions (~60%) — from previous abdominal/pelvic surgery

2. Hernia — particularly femoral and incisional

3. Malignancy — caecal or small-bowel tumour

4. Crohn's strictures, intussusception, volvulus, gallstone ileus, internal hernia

Clinical pattern: colicky central pain, early vomiting (bilious if proximal, faeculent if distal), abdominal distension, absolute constipation late. The more proximal the obstruction, the earlier the vomiting and the less the distension.

Plain film: dilated central loops >3 cm, valvulae conniventes crossing the full lumen (vs colonic haustra which do not). CT is now first-line — identifies the transition point and signs of strangulation (mesenteric oedema, free fluid, lack of bowel-wall enhancement, "whirl sign" in volvulus).

Management: "drip and suck" — NG tube, IV fluids, NBM, catheter and analgesia. Most adhesional obstructions resolve in 48–72 h. Surgery is mandatory for strangulation, closed-loop obstruction, hernia, or failure to settle.

👩‍⚕️ Gallstone ileus = ectopic stone lodged at the ileocaecal valve after eroding from gallbladder into duodenum. Rigler's triad on imaging: SBO + pneumobilia + ectopic gallstone. Classic exam question in an elderly woman.

Crohn's disease

Chronic transmural inflammation that can affect anywhere from mouth to anus, but favours the terminal ileum. Key features:

- Skip lesions with normal mucosa between

- Transmural inflammation → fistulae, abscesses, strictures

- Non-caseating granulomas (only 30% of biopsies)

- "Cobblestone" mucosa, deep linear ulcers, fissuring

- Smoking aggravates (opposite of UC)

- Surgery is not curative — ileocaecal resection is the most commonly performed operation

Extra-intestinal manifestations: erythema nodosum, pyoderma gangrenosum, episcleritis, large-joint arthritis, cholelithiasis (bile-salt loss), oxalate renal stones, and osteomalacia (vitamin D malabsorption). Primary sclerosing cholangitis is more typical of UC.

Coeliac disease

Immune reaction to gliadin in genetically susceptible individuals (HLA-DQ2/DQ8). The proximal small bowel bears the brunt — duodenum and jejunum.

- Histology: villous atrophy + crypt hyperplasia + intraepithelial lymphocytosis

- Serology: anti-tTG (first line), anti-endomysial (EMA), anti-deamidated gliadin peptide. Always check IgA — coeliacs are often IgA deficient and tTG will be falsely negative.

- Duodenal biopsy is gold standard (must be on a gluten-containing diet)

- Treatment: lifelong gluten-free diet

- Complications: iron/folate deficiency, osteoporosis, infertility, enteropathy-associated T-cell lymphoma (EATL), small-bowel adenocarcinoma

Meckel's diverticulum

A true diverticulum — a persistent remnant of the vitellointestinal (omphalomesenteric) duct on the antimesenteric border of the ileum. Supplied by a persistent vitelline artery off the ileocolic.

Rule of 2s:

- 2% of the population

- 2 feet (~60 cm) proximal to the ileocaecal valve

- 2 inches long

- Presents under age 2 (when symptomatic)

- 2:1 male predominance

- 2 ectopic tissue types: gastric and pancreatic

Classic presentation: painless lower-GI bleeding in a young child — ectopic gastric mucosa secretes acid, ulcerating the adjacent ileum. Diagnosed by technetium-99m pertechnetate scan ("Meckel's scan") which is taken up by gastric mucosa. Can also cause obstruction (as the lead point of an intussusception) or mimic appendicitis.

Intussusception

Telescoping of one segment of bowel into the next, usually ileocolic.

	Child	Adult
Lead point	Lymphoid hyperplasia (often post-viral)	Pathological — tumour, polyp, Meckel's
Classic features	Currant-jelly stool, "sausage" RUQ mass, drawing up legs	SBO picture
USS sign	Target / doughnut sign	Same
Management	Air (or contrast) enema reduction; surgery if fails	Surgical resection (lead point may be malignant)

Small bowel tumours

The small bowel is remarkably resistant to cancer despite its surface area — high turnover, IgA, fast transit. When tumours occur, expect:

➡ Carcinoid (neuroendocrine) tumour — most common primary, usually terminal ileum or appendix. Secretes serotonin, kinins, histamine. Carcinoid syndrome (flushing, diarrhoea, bronchospasm, right-sided valvular disease) only occurs once there are liver metastases — otherwise the liver inactivates serotonin via first-pass. Diagnosed by urinary 5-HIAA.

➡ Adenocarcinoma — rare; risk increased by Crohn's and coeliac. Usually duodenal/jejunal.

➡ Lymphoma — ileal; coeliac-associated EATL is the classic.

➡ GIST (gastrointestinal stromal tumour) — arises from interstitial cells of Cajal; c-KIT (CD117) positive; treated with imatinib.

[Image: MCQs banner]

Test yourself

Where are Paneth cells located in the gastrointestinal tract?

((Base of the crypts of Lieberkühn::☑️ Paneth cells sit deep in small-bowel crypts, secreting lysozyme and defensins.))
((Tips of intestinal villi::Villus tips contain mature absorptive enterocytes and goblet cells.))
((Gastric antrum::Houses G cells (gastrin) and mucous cells, not Paneth cells.))
((Lamina propria::Connective-tissue layer with immune cells, not epithelium.))

👩‍⚕️ Paneth cells protect the crypt stem-cell niche — a classic histology stem.

During laparotomy, how is the jejunum distinguished from the ileum?

((Longer vasa recta and fewer arcades::☑️ Jejunum has long straight vasa recta off 1–2 arcades; ileum has short vasa recta off many arcades.))
((More lymphoid tissue::Peyer's patches are denser in the ileum, not the jejunum.))
((Narrower lumen::Jejunum has a wider lumen and thicker wall.))
((Less peritoneal covering::Both are intraperitoneal on a mesentery.))

👩‍⚕️ "Long vasa recta, fat-free windows, thick red wall" = jejunum.

Which artery supplies the jejunum and ileum?

((Superior mesenteric artery::☑️ Midgut blood supply from second part of duodenum to proximal two-thirds of transverse colon.))
((Coeliac trunk::Supplies foregut — stomach, liver, spleen, proximal duodenum.))
((Inferior mesenteric artery::Hindgut — distal transverse colon to upper rectum.))
((Internal iliac artery::Pelvic viscera; rectum below the dentate line.))

Which vessel is compressed in nutcracker syndrome?

((Left renal vein::☑️ Trapped between SMA anteriorly and aorta posteriorly — haematuria, flank pain, left varicocele.))
((Left renal artery::Lies posterior; not involved.))
((Inferior vena cava::Compressed in SMA syndrome's vascular cousin but not nutcracker.))
((Third part of duodenum::This is SMA syndrome, not nutcracker.))

👩‍⚕️ Nutcracker = vein squashed. SMA syndrome = duodenum squashed. Same anatomical pincer.

What is the most common location of a Meckel's diverticulum?

((Antimesenteric border of the ileum, ~60 cm proximal to the ileocaecal valve::☑️ Vitellointestinal duct remnant — the "2 feet" of the rule of 2s.))
((Mesenteric border of the jejunum::Meckel's is antimesenteric, never mesenteric.))
((Caecum::Not the embryological site of the vitelline duct.))
((Duodenojejunal junction::This is the ligament of Treitz — unrelated.))

A 2-year-old passes large-volume painless dark red PR bleeding. Most likely diagnosis?

((Meckel's diverticulum::☑️ Ectopic gastric mucosa secretes acid, ulcerating adjacent ileum — painless brisk bleed.))
((Intussusception::Causes "currant-jelly" stool with colicky pain and a sausage mass, not painless bleeding.))
((Juvenile polyp::Causes small painless bleeds, but typically streaks on stool, not large bleed.))
((Inflammatory bowel disease::Rare under 5; bloody diarrhoea with systemic features.))

👩‍⚕️ Diagnose with a technetium-99m pertechnetate scan — taken up by gastric mucosa.

Which vessel supplies a Meckel's diverticulum?

((Persistent vitelline (omphalomesenteric) artery off the ileocolic::☑️ A true embryological remnant — ileocolic branch of the SMA.))
((Right colic artery::Supplies ascending colon, not the ileum.))
((Middle colic artery::Transverse colon.))
((Inferior mesenteric artery::Hindgut — wrong embryological territory.))

A 30-year-old with months of diarrhoea has a barium follow-through showing contrast flowing from the ileum directly into the sigmoid colon. Diagnosis?

((Crohn's disease::☑️ Transmural inflammation forms fistulae — here an ileosigmoid fistula.))
((Ulcerative colitis::Confined to mucosa; does not form fistulae.))
((Diverticulitis::Can form colovesical fistulae but not ileosigmoid.))
((Coeliac disease::Causes villous atrophy in the proximal small bowel, not fistulae.))

Which of the following is NOT an extraintestinal manifestation of Crohn's disease?

((Polyarteritis nodosa::☑️ Systemic medium-vessel vasculitis — unrelated to IBD.))
((Pyoderma gangrenosum::Painful violaceous ulcers, classically peristomal.))
((Erythema nodosum::Tender shin nodules — both UC and Crohn's.))
((Episcleritis::Eye involvement is well-recognised in IBD.))

👩‍⚕️ Primary sclerosing cholangitis goes with UC; gallstones and oxalate renal stones go with Crohn's.

A Crohn's patient with prior terminal ileal resection has low calcium, low phosphate and raised ALP. Diagnosis?

((Osteomalacia::☑️ Bile-salt loss → fat malabsorption → vitamin D deficiency → osteomalacia.))
((Paget's disease::ALP is markedly raised but calcium and phosphate are normal.))
((Metastatic bone disease::Usually high calcium, not low.))
((Primary hyperparathyroidism::High calcium, low phosphate.))

A Crohn's patient develops cholesterol gallstones. Mechanism?

((Terminal ileal disease reduces bile-salt reabsorption::☑️ Less bile salts in bile → cholesterol supersaturates → stones.))
((Haemolysis::Causes pigment stones, not cholesterol stones.))
((Biliary stasis::Not the dominant mechanism in Crohn's.))
((High oestrogen state::Relevant in pregnancy/OCP, not Crohn's.))

A duodenal biopsy shows villous atrophy, crypt hyperplasia and intraepithelial lymphocytosis. Diagnosis?

((Coeliac disease::☑️ Classic triad — confirm with anti-tTG (after IgA check) and gluten-free diet response.))
((Crohn's disease::Transmural inflammation with non-caseating granulomas; villi typically preserved.))
((Tropical sprue::Similar histology but distinct travel history and responds to antibiotics + folate.))
((Whipple's disease::PAS-positive macrophages, not lymphocytic infiltrate.))

The most appropriate first-line serological test for coeliac disease is?

((IgA anti-tissue transglutaminase (tTG) with total IgA::☑️ First line per NICE — must exclude IgA deficiency to avoid false negatives.))
((Anti-endomysial antibody::Highly specific but more expensive — used as confirmatory.))
((HLA-DQ2/DQ8 typing::High negative predictive value but poor specificity — used to rule out.))
((Duodenal biopsy::Gold standard for diagnosis but not a serological test.))

Which complication of long-standing coeliac disease is most feared?

((Enteropathy-associated T-cell lymphoma (EATL)::☑️ Aggressive jejunal lymphoma; risk reduced by strict gluten avoidance.))
((Colorectal adenocarcinoma::Not associated with coeliac.))
((Hepatocellular carcinoma::Not a recognised complication.))
((Carcinoid tumour::Arises from enteroendocrine cells, unrelated to coeliac.))

A 6-month-old has episodic crying with drawing-up of legs, vomiting and a sausage-shaped RUQ mass. Best initial management?

((Air enema reduction after USS confirmation::☑️ Ileocolic intussusception — USS shows target sign; pneumatic reduction succeeds in ~80%.))
((Immediate laparotomy::Reserved for peritonitis, perforation or failed enema reduction.))
((Barium follow-through::Slow and inferior to USS in this setting.))
((Oral rehydration and observation::Inadequate — risk of bowel ischaemia.))

👩‍⚕️ Adult intussusception almost always has a pathological lead point (often tumour) — resect, don't reduce.

A 70-year-old woman presents with SBO. Plain film shows pneumobilia, a dilated small bowel and an opacity in the right iliac fossa. Diagnosis?

((Gallstone ileus::☑️ Rigler's triad — stone eroded from gallbladder into duodenum and impacted at the ileocaecal valve.))
((Adhesional obstruction::Common but does not cause pneumobilia.))
((Caecal volvulus::Coffee-bean sign in LUQ, no pneumobilia.))
((Incarcerated femoral hernia::Common in this demographic but no biliary tree gas.))

A 60-year-old presents with episodic flushing, diarrhoea and wheeze. CT shows liver metastases. Best initial investigation?

((24-hour urinary 5-HIAA::☑️ Serotonin metabolite — diagnostic of carcinoid syndrome.))
((Serum gastrin::Used for Zollinger–Ellison syndrome.))
((Serum chromogranin A::Useful as a tumour marker but less specific than 5-HIAA.))
((Faecal calprotectin::Marker of bowel inflammation, not neuroendocrine tumours.))

👩‍⚕️ Carcinoid syndrome implies liver metastases — otherwise hepatic first-pass inactivates serotonin.

Which feature on abdominal radiograph favours small-bowel rather than large-bowel obstruction?

((Valvulae conniventes crossing the full lumen, central loops::☑️ Plicae circulares are complete; small bowel sits centrally; dilated >3 cm is abnormal.))
((Haustra not crossing the lumen::This is a colonic feature.))
((Peripheral distribution of loops::Colon sits peripherally.))
((Loops >6 cm in diameter::Threshold for large bowel (6 cm) or caecum (9 cm).))

Which cell type lies over Peyer's patches and samples luminal antigen?

((M cell::☑️ Flat epithelial cell delivering antigen to underlying lymphoid tissue — exploited by Salmonella.))
((Paneth cell::Antimicrobial — at the base of crypts, not over Peyer's patches.))
((Goblet cell::Mucus secretion.))
((Enteroendocrine cell::Hormone secretion.))

Lacteals drain into which structure first?

((Cisterna chyli::☑️ Dilated lymphatic sac anterior to L1–L2, draining the intestinal trunks before the thoracic duct.))
((Thoracic duct directly::Reaches the thoracic duct only after the cisterna chyli.))
((Portal vein::Receives venous, not lymphatic drainage.))
((Azygos vein::Drains thoracic wall, not gut lymphatics.))

Revision summary

➡ Jejunum = proximal 2/5, thick red wall, long vasa recta, few arcades, tall plicae. Ileum = distal 3/5, thin pale, short vasa recta, many arcades, Peyer's patches.

➡ Terminal ileum absorbs B12 and bile salts — resect it and you get megaloblastic anaemia, cholesterol gallstones and fat-soluble vitamin deficiency.

➡ Blood supply = SMA (midgut). Lymph = lacteals → cisterna chyli → thoracic duct.

➡ Cells: enterocytes (absorb), goblet (mucus), Paneth at crypt base (antimicrobial), enteroendocrine (hormones), M cells over Peyer's (antigen sampling).

➡ SBO: adhesions > hernia > malignancy. Colicky pain, early vomiting, distension, late absolute constipation. Loops >3 cm with valvulae conniventes. Treat with drip and suck; operate for strangulation or failure to settle.

➡ Crohn's: skip lesions, transmural, granulomas, terminal ileum, fistulae, cobblestone, smoking worsens, ileocaecal resection most common operation.

➡ Coeliac: gliadin → villous atrophy + crypt hyperplasia; anti-tTG (with IgA); duodenal biopsy is gold standard; EATL is the feared complication.

➡ Meckel's: rule of 2s; antimesenteric ileum; ectopic gastric mucosa → painless PR bleed in toddler; diagnose with technetium pertechnetate scan.

➡ Intussusception: child = ileocolic, currant-jelly stool, target sign on USS, air enema reduction. Adult = pathological lead point — resect.

➡ Carcinoid: ileal neuroendocrine tumour; syndrome (flushing, diarrhoea, wheeze, right-heart valves) only with liver mets; diagnose with urinary 5-HIAA.

➡ Gallstone ileus = Rigler's triad (SBO + pneumobilia + ectopic stone) in an elderly woman.