Jejunum versus Ileum
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Direct answer
Hold a loop of small bowel up to the light and the jejunum names itself: wider calibre, thicker wall, tall crowded plicae circulares giving the mucosa a velvety look, long sparse vasa recta hanging from one or two arterial arcades, and mesentery so transparent that windows exist between the vessels. The ileum is narrower and thinner-walled, its plicae short, sparse and finally absent in the terminal segment, its arcades three to five (or more) tiers deep with short crowded vasa recta, mesenteric fat creeping up to the bowel wall, and aggregated lymphoid follicles — Peyer's patches — along its antimesenteric border. The jejunum is the proximal two-fifths of the small intestine beyond the duodenojejunal flexure, the ileum the distal three-fifths, with no sharp junction, only a gradual change every surgeon reads at laparotomy.
What you must remember
- Calibre and wall: jejunum wider (about 3.5-4 cm) and thicker, ileum narrower (about 2.5 cm) and thinner — thickness is felt, not measured, at laparotomy.
- Mucosal folds: plicae circulares are tall, frequent and spiralled in the jejunum (the main surface-area multiplier), low and widely spaced in the upper ileum and virtually absent in the terminal ileum.
- Vascular pattern: jejunum — one or two arcades with long vasa recta and translucent mesenteric windows; ileum — multilayered (commonly three to five) arcades with short vasa recta and fat encroaching on the wall.
- Lymphoid tissue: Peyer's patches, the aggregated follicles lying along the antimesenteric border of the ileum, hypertrophy in typhoid (the site of ulceration and perforation) and are the prion and viral entry route studied in microbiology.
- Functional correlate: the terminal 60-100 cm of ileum absorbs vitamin B12-intrinsic factor and reabsorbs bile salts — resection produces megaloblastic anaemia, choleretic diarrhoea and gallstone propensity.
- Viva-favourite histology: both have villi and crypts, but the jejunum's myenteric plexus and muscle are conventionally described as richer and thicker, matching its transport work.
- Clinical landmarks: the duodenojejunal flexure (suspended by the ligament of Treitz) starts the jejunum; the ileocaecal junction ends the ileum, guarded by the ileocaecal valve.
Two clinical vignettes
First, the laparotomy. A surgeon handed an unplanned loop of mid-small bowel identifies it by transillumination: long vasa recta and few arcades mean jejunum, best used for a high jejunal stoma; many arcades and fat-wrapped vessels mean ileum. The same reading guides resection margins in mesenteric ischaemia — the transition from pulsatile to thrombosed arcades maps the dead bowel better than colour alone.
Second, the terminal ileum in disease. Crohn's disease favours this segment, its transmural inflammation thickening the wall, narrowing the lumen and producing the cobblestone mucosa that colonoscopy sees; the fat of the mesentery creeps around the wall ("creeping fat"). Because the same segment handles B12 and bile salt recycling, extensive Crohn's or resection leaves the patient B12-deficient (lifelong injections once stores deplete) and prone to both oxalate renal stones and a watery, bile-salt-led diarrhoea that cholestyramine controls. One embryological footnote belongs here too: a Meckel's diverticulum lies on the antimesenteric border within about 100 cm of the ileocaecal valve — its gastric mucosa can ulcerate and bleed, mimicking appendicitis when inflamed, a reminder that the ileum hosts the gut's commonest congenital anomaly.
Where candidates slip
The recurring flip is Peyer's patches assigned to the jejunum because "more lymphoid tissue proximally" sounds plausible; the aggregation is ileal and antimesenteric, a two-word qualifier worth full marks. The second slip is stating the ileum has "no plicae" — they are reduced and lost only progressively, vanishing in the terminal ileum, not at an arbitrary point. The third is the arcade numbers quoted without the vasa recta comparison; the length of the vasa recta (long in jejunum, short in ileum) is the distinction an examiner listens for because it is the one the surgeon actually uses. And remember there is no anatomical junction — a boundary drawn as "proximal two-fifths versus distal three-fifths of the mobile small intestine" is a convention, not a structure.
Frequently asked questions
How do vasa recta differ between jejunum and ileum?
Jejunal vasa recta are long, few and widely spaced with one or two arcades; ileal vasa recta are short, numerous and crowded on multilayered arcades.
Where are Peyer's patches found and what is their clinical importance?
Along the antimesenteric border of the ileum; they hypertrophy and ulcerate in typhoid fever, risking haemorrhage and free perforation.
What happens after resection of the terminal ileum?
Loss of B12-intrinsic factor absorption causes megaloblastic anaemia, and lost bile salt recycling causes diarrhoea, steatorrhoea and gallstone formation.
Which segment shows the tallest plicae circulares and why?
The jejunum, because its tall, spiralled circular folds multiply absorptive surface where most nutrient uptake occurs.
What is the rule for a Meckel diverticulum's location?
It arises from the antimesenteric border of the ileum, classically within 100 cm (about 2 feet) of the ileocaecal valve, from persistence of the vitelline duct.