<prompt> Superior rectal vein opens into-Inferior mesenteric veinSuperior mesenteric veinInternal iliac veinCircumflex iliac vein  <response>: Ans. is a i.e., Inferior mesenteric vein Superior rectal vein drains into the inferior mesenteric vein and hence into the portal circulationBlood supply of rectum Major arterial supply of rectum is through superior rectal artery inferior continuation of inferior mesenteric artery. Other arterial supply of rectum includes middle rectal arteries branch of anterior division of internal iliac artery and median sacral artery branch of the aorta.s Venous drainage is through superior rectal vein drains into inferior mesenteric vein, middle rectal vein drains into internal iliac vein and median sacral vein joins left common iliac vein. 