Editing Small intestine (section)

== '''Jejunum/ileum''' ==

* Endoscopic anatomy
** Jejunum is characterised by thinner but more frequent circular folds than the duodenum, and the mucosa is smoother
** Ileal mucosa has a fine, granular appearance, with visible villi underwater. Lymphoid follicles are also seen.
* Arterial supply
** Branches of SMA - see separate topic under 'great vessels'
** Abundant collateral provided by vascular arcades
** In the mesentery, each branch divides into superior and inferior branches, which anastomoses with its fellow above and below to form arcades. Usually only one or two arcades in proximal small bowel, but progressively increases as you move distally to mid-small bowel up to 5 or 6, then comes down again around TI to 1-2.
** Vasa rectae (arteriae rectae) arise from the arcade farthest from the SMA and pass straight to bowel wall - essentially end arteries
** Not much intramural plexus when compared to stomach or oesophagus; only able to sustain 2-4cm of devascularised small bowel without a colour change being observed.
**
* Venous drainage
** Into SMV, which joins the splenic vein behind the neck of the pancreas to form the portal vein
** Artery and vein do not always lie close together
** See full description under 'colon'
* Innervation
** PNS
*** Derived from vagus nerve, traversing the coeliac ganglion
*** Influences secretion, motility and other bowel activity
*** Vagal afferent fibres are present but do not carry pain
** SNS
*** Three sets of splanchnic nerves - ganglions usually located in a plexus around the base of the SMA
*** Motor impulses affect blood vessel motility and probably gut secretion and motility
*** Pain is transmitted through general visceral afferent fibres of the sympathetic system
* Lymphatics
** Seen as Peyer patches in the distal small bowel
** Drainage proceeds from the mucosa through the wall of the bowel to a set of nodes adjacent to the bowel in the mesentery
** Then to a group of regional nodes adjacent to the mesenteric arterial arcades
** Then to a group at the base of the superior mesenteric vessels
** Then into the cisterna chyli and up the thoracic duct