The superior, middle, and inferior rectal or haemorrhoidal veins accompany their arteries and drain corresponding parts of the rectum and anal canal. The superior vein bcomes the inferior mesenteric vien and, therefore, belongs to the portal system. The middle and inferior veins are paired and belong to the caval system. The superior vein begins in the anal columna. It has extensive mucous and submucous plexuses, and it receives branches from the perirectal tissues. The middle rectal vein is a much more important vessel than the corresponding artery. It drains the rectum above the Internal Sphincter and communicates both submucously and perimuscularly with the inferior rectal vein, and it makes free anastomoses submucously with the superior rectal vein. Its branches communicate with the prostatic (vaginal and uterine) plexus. It is the chief link between the portal and caval systems, and it ends in the internal iliac vein. The inferior vein drains the anus and Sphincter Ani Externus.
December 2003. Arch Surg. 2003;138(12):1333. doi:10.1001/archsurg.138.12.1333
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