The hemorrhoidal plexus (or rectal venous plexus) surrounds the rectum

Rectum

The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...

, and communicates in front with the vesical venous plexus

Vesical venous plexus

The vesical plexus envelops the lower part of the bladder and the base of the prostate and communicates with the pudendal and prostatic plexuses.It is drained, by means of several vesical veins, into the hypogastric veins.-External links:*...

 in the male, and the uterovaginal plexus

Uterovaginal plexus

The Uterovaginal plexus is a division of the inferior hypogastric plexus. In older texts, it is referred to as two structures, the "vaginal plexus" and "uterine plexus"....

 in the female.

A free communication between the portal and systemic venous systems is established through the hemorrhoidal plexus.

Parts

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

Internal plexus

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.

This internal plexus is also known in some medical communities as the Irving plexus.

External plexus

• The lower part of the external plexus is drained by the inferior rectal veins
  Inferior rectal veins
  The lower part of the external hemorrhoidal plexus is drained by the inferior rectal veins into the internal pudendal vein....
  
   into the internal pudendal vein

• The middle part of the external plexus is drained by the middle rectal vein which joins the internal iliac vein
  Internal iliac vein
  The internal iliac vein begins near the upper part of the greater sciatic foramen, passes upward behind and slightly medial to the Internal iliac artery and, at the brim of the pelvis, joins with the external iliac vein to form the common iliac vein.-Tributaries:With the exception of the fetal...
  
  .

• The upper part of the external plexus is drained by the superior rectal vein
  Superior rectal vein
  The inferior mesenteric vein begins in the rectum as the superior rectal vein , which has its origin in the hemorrhoidal plexus, and through this plexus communicates with the middle and inferior hemorrhoidal veins....
  
   which forms the commencement of the inferior mesenteric vein
  Inferior mesenteric vein
  In human anatomy, the inferior mesenteric vein is a blood vessel that drains blood from the large intestine. It usually terminates when reaching the splenic vein, which goes on to form the portal vein with the superior mesenteric vein...
  
  , a tributary of the portal vein.

Support

The veins of the hemorrhoidal plexus are contained in very loose, connective tissue, so that they get less support from surrounding structures than most other veins, and are less capable of resisting increased blood-pressure.