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

hemorrhoidal 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. A free communication between the portal and systemic venous ...

and receive tributaries from the

bladder The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...

, prostate, and seminal vesicle. 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 In the circulatory system of animals, a portal venous system occurs when a capillary bed pools into another capillary bed through veins, without first going through the heart. Both capillary beds and the blood vessels that connect them are co ...

to the liver. Veins inferior, and including, the middle rectal vein drain into systemic circulation and are returned to the heart, bypassing the liver.

References

Veins of the torso {{circulatory-stub