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] Pathologies involving the Inferior rectal veins may cause lower GI bleeding.

Depending on the degree of inflammation, they are given a grade level ranging from 1 through 4.

This article incorporates text in the public domain from page 676 of the 20th edition of Gray's Anatomy (1918)