They run lateralward on the pelvic surface of the levator ani to end in the internal iliac vein. Veins superior to the middle rectal vein in the colon and rectum drain via the portal system to the liver. Veins inferior, and including, the middle rectal vein drain into systemic circulation and are returned to the heart, bypassing the liver.[1]
↑van Hoogdalem, Edward; de Boer, Albertus G.; Breimer, Douwe D. (July 1991). "Pharmacokinetics of rectal drug administration, Part I. General considerations and clinical applications of centrally acting drugs". Clinical Pharmacokinetics. 21 (1): 14. doi:10.2165/00003088-199121010-00002. ISSN0312-5963. PMID1717195. The superior rectal vein, perfusing the upper part of the rectum, drains into the portal vein and subsequently into the liver On the other hand, the middle and inferior rectal veins drain the lower part of the rectum and venous blood is returned to the inferior vena cava.
