Anorectal venous plexus

The rectal venous plexus (or hemorrhoidal plexus) surrounds the rectum, and communicates in front with the vesical venous plexus in the male, and the vaginal venous plexus in the female.

It consists of two parts, an internal in the submucosa, and an external outside the muscular coat.

The internal plexus presents a series of dilated pouches which are arranged in a circle around the tube, immediately above the anal orifice, and are connected by transverse branches. It is also known in some medical communities as the Irving plexus.

The upper part of the external plexus is drained by the superior rectal vein which forms the commencement of the inferior mesenteric vein, a tributary of the portal vein.
The middle part of the external plexus is drained by the middle rectal vein which joins the internal iliac vein.
The lower part of the external plexus is drained by the inferior rectal veins into the internal pudendal vein

Source : Wikipedia