Editing Colon and rectum (section)

=== '''IMA''' ===

*** Arises at anterior/left aorta, at level of L2-L3, behind inferior border of D3, about 3cm above the aortic bifurcation
*** Can be identified about 2cm caudal to the ligament of Treitz; the origin is surrounded by the mesenteric and hypogastric nervous plexus
*** Descends inferiorly and to the left
*** Branches:
**** '''Left colic'''
***** Originates about 3cm distally to the origin of the IMA, which is 3cm below the inferior border of duodenum
***** Ascends obliquely and steeply to the left, then divides 3cm from colon
****** Ascending branch (to splenic flexure)
****** Descending branch (to descending colon)
***** Accessory left colic can come from SMA in some cases, which causes the entire descending colon to be tethered quite high
**** '''Sigmoid branches'''
***** Usually 2-4 in variable fashion
***** First branch usually the largest, called colosigmoid artery by some authors
**** '''Superior rectal (haemorrhoidal) artery'''
***** End branch of IMA - continues on down to pelvis after giving off left colic
***** It comes to lie in the sigmoid mesocolon, which shortens as the rectum is approached, so at the proximal rectum it lies immediately posterior
***** At around S3 (usually around proximal rectum), it divides into left and right branches, running posteriorly and downward. These give off branches which run close to rectal wall and supply the arteries which lie in the anorectal columns.
***** Excellent anastomosis between the superior rectal, lower sigmoid, and probably middle rectal arteries - so superior rectal can be interrupted at any level without risk of ischaemia to the rectal stump
***
***
** Internal iliac artery - blood supply to rectum
*** ''Strong anastomotic network, good collaterals''
*** '''Middle rectal artery'''
**** Paired vessels derived from internal iliac artery, most often from internal pudendal branch
**** Runs superior to S3 nerve root
**** Supply the lower rectum's muscle coat, but anastomose with superior rectal artery
**** Found inconstantly and not considered a major blood supply to the rectum
**** Can be inadvertently injured when dissecting the 'lateral ligaments' during TME
*** '''Inferior rectal artery'''
**** Branches of the internal pudendal arteries
**** Cross through fat of the ischiorectal fossa to reach the anal region, where they supply the sphincter muscles
**** Doubtful whether there is significant anastomoses between these and the higher rectal arteries
**** Misnomer - generally supply the anus distal to the dentate line, '''not''' the rectum
** Two vascular anastomoses between SMA and IMA
*** '''Marginal artery of Drummond'''
**** Runs along the mesenteric margin of the colon from the caecocolic junction to the rectosigmoid junction
***** Sometimes deficient near splenic flexure and upper ascending colon
**** Vasa recta branch off at short intervals and supply the bowel wall directly
**** Important clinically when one of the larger arteries is obstructed, and the colon can receive collateral supply through this artery
*** '''Arc of Riolan''' ('meandering mesenteric artery')
**** Uncommon finding - Jamieson's says 10% have it
**** A thick tortuous vessel that runs a meandering course in the colonic mesentery between the main trunk of the middle colic artery, and the ascending branch of the left colic artery (so kind of parallel to the marginal artery but lower in the mesentery)
**** Collateral in case of SMA or IMA occlusion
**** Flow can be forward (IMA stenosis) or retrograde (SMA stenosis)
**** The presence of a large arc of Riolan suggests stenosis or occlusion of one of the major vessels
** '''Watershed areas''' are found at the edge of the region supplied by the SMA and IMA, which are frequently dependent on collaterals. They are vulnerable during systemic hypoperfusion because they are supplied by the most distal branches.
*** Splenic flexure (Griffiths point). Up to 50% of patients don't have a marginal artery here, so don't have very good supply. Avoid this area for anastomoses.
*** Rectosigmoid junction (Sudeck's point). Gets supply from distal sigmoid and superior haemorrhoidal branches, which are both far along the IMA, and hence can have poor supply.
*** Right colon can be susceptible too, as the ileocolic artery here is an end artery.