The rectum is the end portion of the large intestine, and transports waste to the anus. The hemorrhoidal plexus — the network of veins surrounding the rectum — contains the beginning of the middle rectal veins. The veins also receive contributions from the prostate and bladder.
The middle rectal veins are considered inferior veins and are part of the systemic circulation system, which is the system that delivers oxygenated blood from the heart to the rest of the body and oxygen-depleted blood back to the heart.
The veins move laterally (from one side to the other) across the pelvis, ending at the internal iliac vein. The internal iliac vein contributes to the area around the base of the spine and the pelvis.
The veins located in the rectum and colon that are above the rectal veins drain into the hepatic portal, which sends blood to the liver, where it is processed. However, blood drained by the middle rectal veins bypasses the liver on its way up to the heart. The heart oxygenates the blood and sends it back into the body.