M01.06.027 The Inferior Mesenteric Artery

Written by

in

 

Learning Objectives

  • Identify the anatomical origin of the IMA at the L3 level and its retroperitoneal course.
  • Map the three principal branches supplying the hindgut (distal 1/3 transverse colon to upper rectum).
  • Understand the critical anastomoses (Marginal Artery of Drummond and Arc of Riolan) between the SMA and IMA.
  • Correlate IMA anatomy with clinical conditions like watershed ischemia and Horseshoe Kidney.

Anatomical Position

The Inferior Mesenteric Artery (IMA) is the final major unpaired visceral branch of the abdominal aorta. It arises anteriorly at the L3 vertebral level, approximately 3–4 cm superior to the aortic bifurcation. It is a retroperitoneal structure throughout its course.

Major Branches and Territories

The IMA provides the primary blood supply to the Hindgut. It has three principal branches:

Branch Anatomical Course & Supply
Left Colic Artery First branch. Divides into Ascending (supplies the distal 1/3 transverse colon) and Descending (supplies the descending colon) branches.
Sigmoid Arteries Usually 2–4 branches. Supply the lower descending and sigmoid colon. The uppermost is the superior sigmoid artery.
Superior Rectal Artery The terminal continuation of the IMA. Crosses the left common iliac vessels to supply the rectum down to the S3 level.

Clinical Anatomy: Anastomoses & Watersheds

The transition between the midgut (SMA) and hindgut (IMA) is protected by two key vascular communications:

  • Marginal Artery (of Drummond): A continuous arterial circle along the inner border of the colon formed by SMA and IMA branches.
  • Arc of Riolan: A less common, direct mesenteric anastomosis between the middle colic (SMA) and left colic (IMA) arteries.

High-Yield: The Splenic Flexure (Griffith’s Point)
This is a watershed area. Because it lies at the terminal limits of both the SMA and IMA circulations, it has the poorest perfusion in the colon. It is the most common site for ischemic colitis during states of hypotension.

Clinical Relevance

  • Left Hemicolectomy: Surgical removal of the descending colon. Surgeons must ligate the IMA branches while preserving the marginal artery to ensure the remaining bowel stays viable.
  • Horseshoe Kidney: During embryological ascent, the fused lower poles of the kidneys are blocked by the IMA. Consequently, a horseshoe kidney remains lower in the abdomen than normal kidneys.

 

Activity

Visit mymedschool.org for our Hindgut Surgical Case Studies. Can you identify the Superior Rectal Artery on a pelvic CT scan? Practice tracing it from the L3 level down to where it crosses the left common iliac artery!