Superior rectal vein
The superior rectal vein, or superior hemorrhoidal vein, connects veins surrounding the rectum to the inferior mesenteric vein.
The inferior mesenteric vein brings blood from the large intestines to the splenic vein. Blood in the spleen travels to the liver. From the liver, blood is filtered and eventually makes its way to the heart where more oxygen is added before it continues to cycle through the body. The rectal vein is the main link between blood exiting all veins around the rectum.
If the vein's tributaries swell, they will push into the rectum, becoming internal hemorrhoids. Internal hemorrhoids may cause blood to appear on the surface of the stool. Internal hemorrhoids require medical attention as they may indicate a significant medical issue.
Hemorrhoids occur when pressure on rectal veins cuts blood flow. During pregnancy, the weight of the fetus affects blood flow through the vein. Cancerous growths in the rectum may block the veins. Contractions of the rectum also affect blood flow in the superior rectal vein.