Jejunum is thicker walled with long vasa recta and few arcades; ileum is thinner with short vasa recta and many arcades
Mnemonic
“Jejunum is thick, red and empty; ileum is thin, pale and full of Peyer’s patches”:
| Jejunum | Ileum | |
|---|---|---|
| Position | Upper left | Lower right |
| Wall | Thicker, more vascular | Thinner, paler |
| Plicae circulares | Tall and numerous | Few, absent distally |
| Vasa recta | Long, few arcades | Short, many arcades |
| Mesenteric fat | Less, “windows” visible | More, encroaching |
| Peyer’s patches | Few | Many |
The name itself is the aid: jejunum means empty, since it is usually found empty at post mortem because of its rapid transit.
The vasa recta and arcade pattern is the reliable operative distinction: long straight vessels with one or two arcades means jejunum, short vessels with four or five arcades means ileum.
Only the terminal ileum absorbs vitamin B12 and bile salts, which is why its resection causes B12 deficiency and bile salt diarrhoea.
Expansion
| Feature | Jejunum | Ileum |
|---|---|---|
| Position | Upper left | Lower right |
| Proportion | Proximal 2/5 | Distal 3/5 |
| Wall | Thicker, more vascular | Thinner, paler |
| Lumen | Wider | Narrower |
| Plicae circulares | Tall and numerous | Sparse or absent distally |
| Arterial arcades | Few (1 to 2) | Many (4 to 5) |
| Vasa recta | Long | Short |
| Mesenteric fat | Less; windows visible | More; fat encroaches to the wall |
| Peyer’s patches | Few | Many |
The vascular pattern is the most reliable intraoperative sign: fewer, larger arcades with long straight vessels means jejunum.
The distal ileum is functionally distinct; it is the only site absorbing vitamin B12 (with intrinsic factor) and bile salts, so ileal resection or Crohn’s disease there causes B12 deficiency and bile salt diarrhoea.