Jejunum
The jejunum and the ilium are the site of major enzymatic processing and absorption of food. Together they are about 6 - 7 m in length and provide a large surface area for the digestive process.
There is no clear boundary between them. However they can be identified from their external appearance and vascular supply.
They are suspended loosely within the peritoneal cavity by means of the mesentery, which is a band like reflection of the peritoneum from the posterior abdominal wall.
The mesentery extends from the duodenojejunal junction (L2) on the left , diagonally across (to the right along the posterior abdominal wall) up to the ileocaecal junction. The root of the mesentery passes across the duodenum, aorta, IVC, right ureter, right psoas major muscle and the right testicular / ovarian vessels.
Blood Supply:
The superior mesenteric artery is the artery of the midgut. It supplies an area of the intestine extending from the distal half of second part of duodenum up to the left colic flexure. The artery originates from the abdominal aorta and passes anterior to the 3rd part of the duodenum to enter the root of the mesentery. It then has a lateral course towards the ileocolic junction. It gives several branches to the ileum and the jejunum that unite to form arterial arcades from which arise the vasa recta that supply the intestine. The jejunum has long vasa recta while the ileum has short vasa recta - a distinguishing feature between them.
Superior mesenteric vein follows the artery along the root of the mesentery. It joins the splenic vein posterior to the neck of pancreas to form the portal vein.
Lymphatic supply:
Most of the lymphatics drain to the Superior Mesenteric nodes. Those from the terminal ileum drains to the ileocolic lymph nodes.
Innervation:
Intestines are supplied by the autonomic nervous system.
Presynaptic sympathetic fibres from T8-T10 segment reach the intestines via the greater, lesser and the least splanchanic nerves to the celiac and the superior mesenteric ganglion. The postsynaptic fibres follow the terminal branches of the superior mesenteric artery to supply the gut.
Parasympathetic fibres from the posterior vagal trunk. Presynaptic fibres from the vagus synapse with post synaptic fibres in the myenteric and the submucosal plexus of the intestines.
Sympathetic stimulation leads to reduction of intestinal secretions and gut motility. Parasympathetic stimulation causes an incresase in secretions and intestinal motility. The intestines are insensitive to pain. However distension is perceived as visceral pain through ascending visceral afferent innervation via the autonomic fibres.
| Characteristic | Jejunum | Ileum |
|---|---|---|
| Colour | Deeper Red | Pale Pink |
| Caliber | 2 - 4 cm | 2-3 cm |
| Wall | Thick | Thin |
| Vascularity | More | Less |
| Vasa recta | Long | Short |
| Arcades | Large loop | Smaller loop |
| Fat in mesentery | Less | More |
| Circular folds | Large and tightly packed | Small and sparse |
