The middle rectal veins (or middle hemorrhoidal vein) take origin in the hemorrhoidal plexus

Hemorrhoidal plexus

The hemorrhoidal plexus surrounds the rectum, and communicates in front with the vesical venous plexus in the male, and the uterovaginal plexus in the female....

 and receive tributaries from the bladder

Urinary bladder

The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...

, prostate

Prostate

The prostate is a compound tubuloalveolar exocrine gland of the male reproductive system in most mammals....

, and seminal vesicle

Seminal vesicle

The seminal vesicles or vesicular glands are a pair of simple tubular glands posteroinferior to the urinary bladder of male mammals...

.

They run lateralward on the pelvic surface of the levator ani

Levator ani

The levator ani is a broad, thin muscle, situated on the side of the pelvis.It is attached to the inner surface of the side of the lesser pelvis, and unites with its fellow of the opposite side to form the greater part of the floor of the pelvic cavity....

 to end in 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...

. 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.