The rectosigmoid peritoneal reflections are marked by the dashes. The mesentery crosses the left iliac vessels at the level of the iliac bifurcation. The left ureter crosses the vessels in the base of the mesentery. The terminal branch of the inferior mesenteric artery (see lower GI bleed), the superior rectal (superior hemorrhoidal) artery travels in the retroperitoneum and crosses the left common iliac vessels to reach the mesorectum.
The rectal blood supply comes from the superior rectal branch of the inferior mesenteric artery above, and from the middle rectal branches of the internal iliac arteries just above the levator ani. Distally, inferior rectal branches from the internal pudendal arteries reach the sphincters and anal canal.
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