What is the blood supply to the rectum in an adult patient with rectal cancer?

Blood Supply of the Rectum

The rectum receives its primary arterial supply from the superior rectal artery (continuation of the inferior mesenteric artery), with variable contributions from the middle rectal arteries (branches of the internal iliac artery) and inferior rectal arteries (branches of the internal pudendal artery). 1, 2, 3

Primary Arterial Supply

Superior Rectal Artery (Chief Blood Supply)

• The superior rectal artery is the principal vessel supplying the rectum, providing blood to the upper two-thirds of the rectal wall 2, 3
• This artery is the continuation of the inferior mesenteric artery and descends within a fibrous sheath connected to the posterior rectal surface by an anterior mesorectum 2
• The superior rectal artery gives rise to four main branches: transverse rectal, descending rectal, rectosigmoid, and terminal branches 2
• The terminal branches (2 branches in 66% of cases, 3 branches in 34%) divide into right and left branches that extend downward and forward around the lower two-thirds of the rectum to the level of the levator ani muscle 2, 3

Vascular Distribution Patterns

• Two distinct arterial patterns exist in the rectum: an annular pattern in the upper half (provided by transverse rectal arteries) and a plexiform pattern in the lower half (supplied by terminal branches of the superior rectal artery) 2
• The transverse rectal arteries arise from the superior rectal artery in 75% of cases and from the descending rectal artery in 25%, distributing to the upper half of the rectum 2

Secondary Arterial Supply

Middle Rectal Arteries (Variable and Inconsistent)

• The middle rectal arteries are present in only 50-57% of individuals, making them an unreliable source of collateral blood supply 4, 3
• When present, they may be bilateral (37%) or unilateral (20%) 4
• These arteries most commonly originate from the internal pudendal artery (40%), inferior gluteal artery (27%), or internal iliac artery (17%) 4
• The average external diameter is 1.7 mm, with an average length of 7 cm, penetrating the rectal wall approximately 6 cm superior to the anus 4
• The middle rectal arteries most frequently pierce the anterior (50%) and posterior (45%) quadrants of the rectum 4
• When present, the middle rectal artery supplies only a limited part of the rectum in 12% of specimens 3

Inferior Rectal Arteries

• The inferior rectal arteries (branches of the internal pudendal artery) consistently supply the anal canal below the levator ani muscle 5, 3
• These vessels provide ample blood supply to the anal canal, explaining the good healing tendency of coloanal anastomoses 5

Critical Vascular Anatomy for Rectal Cancer Surgery

Vessel-Deficient Zone

• A consistently vessel-deficient area exists in the dorso-caudal sector of the rectal ampulla cranial to the levator muscle, which cannot be compensated by other rectal vessels 5
• This anatomical feature explains the relatively high rate of anastomotic leaks after low anterior resection of the rectum, as leaks are constantly observed in the dorso-caudal ampulla 5
• The levator ani muscle insertion serves as a vascular watershed: caudal to the levator, the inferior rectal artery is the main supply; cranial to it, the superior rectal artery predominates 5

Anastomotic Considerations

• Preservation of the superior rectal artery and its branches during anterior resection significantly reduces anastomotic leak rates compared to high ligation of the inferior mesenteric artery at its origin 1
• Postoperative angiography demonstrates that vascularization of the preserved rectal stump is supplied essentially by branches of the superior rectal artery when the inferior mesenteric artery is preserved 1
• In a series of 84 anterior resections, anastomotic leakage rarely occurred when the inferior mesenteric artery was preserved (56 patients) but was frequent when ligated at its origin (28 patients) 1

Vascular Anastomoses

Extramural and Intramural Connections

• Extramural anastomoses between the main rectal arteries are not typically present, limiting collateral circulation 3
• Intramural anastomoses exist primarily between the superior and inferior rectal arteries 3
• Rectal twigs from other pelvic arteries (such as branches from the internal iliac system) terminate at the pararectal tissues and occasionally reach the rectal wall but do not provide significant collateral supply 3

Clinical Implications for Rectal Cancer

Relevance to Imaging and Staging

• Understanding the vascular anatomy is essential when interpreting extramural vascular invasion (EMVI) on MRI, which occurs when rectal cancer invades through the muscularis propria into surrounding blood vessels 6
• EMVI is diagnosed on MRI by tracking blood vessels around the rectum and identifying irregular vascular morphology with partial or complete replacement of vascular flow signals by tumor signals 6
• The superior rectal arteries along the presacral region up to S1-2 level and internal iliac vessels are key vascular structures that must be evaluated during staging 6

Surgical Planning Considerations

• The variable presence and small caliber of middle rectal arteries means they cannot be relied upon for collateral blood supply during rectal resection 4, 3
• Preservation of the superior rectal artery branches when technically feasible may improve anastomotic healing 1
• The consistent vessel-deficient zone in the dorso-caudal rectum should be considered when planning anastomotic technique and level 5