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...

 begins in 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...

 as the superior rectal vein (superior hemorrhoidal vein), which has its 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 through this plexus communicates with the middle and inferior hemorrhoidal veins.

The superior hemorrhoidal vein leaves the lesser pelvis

Lesser pelvis

The lesser pelvis is that part of the space enclosed by the skeleton of the pelvis which is situated between the pelvic inlet and the pelvic floor. Some authors call this cavity the pelvic cavity...

 and crosses the left common iliac vessels with the superior hemorrhoidal artery, and is continued upward as 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...

.