The inferior mesenteric artery (IMA) is a major artery that arises from the abdominal aorta. It typically originates around the level of the third lumbar vertebra (L3), slightly below the origin of the renal arteries. The IMA supplies blood to the lower part of the large intestine, specifically the left part of the transverse colon, descending colon, sigmoid colon, and rectum. It descends in the retroperitoneum and branches off into several arteries that provide vascular supply to these regions of the gastrointestinal tract. Its location and branching pattern are critical for surgeries involving the lower abdomen.
Anatomy
Origin
The inferior mesenteric artery (IMA) originates from the anterior surface of the abdominal aorta, typically at the level of the third lumbar vertebra (L3), though in some cases, this level may vary slightly. It arises below the origin of the renal arteries and slightly above the aortic bifurcation into the common iliac arteries.
Course
After its origin, the IMA descends retroperitoneally on the left side of the aorta. It follows a relatively straight downward path in the posterior abdominal wall, traveling inferiorly and slightly to the left as it proceeds towards the lower portion of the large intestine.
Branches
The IMA gives off several key branches that supply blood to various parts of the large intestine:
- Left Colic Artery: This artery is typically the first branch of the IMA. It arises shortly after the IMA itself and travels toward the left side of the abdomen to supply the descending colon. It divides into ascending and descending branches:
- The ascending branch moves upward to anastomose with branches from the middle colic artery.
- The descending branch moves downward to supply the upper part of the descending colon.
- Sigmoid Arteries: These are multiple arteries (usually two to four in number) that branch off from the IMA and supply the sigmoid colon. The sigmoid arteries form an anastomotic network with each other and with branches of the left colic artery and superior rectal artery.
- Superior Rectal Artery: The superior rectal artery is the terminal branch of the IMA. It descends into the pelvis to supply the rectum. As it enters the pelvic cavity, it bifurcates into two branches, which further divide to supply the rectum and upper part of the anal canal.
Relations
- Anteriorly: The inferior mesenteric artery is covered by the peritoneum of the posterior abdominal wall. Overlying it, in the abdominal cavity, are coils of small intestine and the mesentery of the small intestine.
- Posteriorly: It lies in front of the abdominal aorta as it descends, and posteriorly to it are the left psoas major muscle and the sympathetic trunk.
- Medially: The abdominal aorta and the inferior vena cava lie to the right of the artery.
- Laterally: The ureter and gonadal vessels are positioned laterally.
Length and Diameter
The length of the IMA varies depending on the individual’s anatomy, but it is generally about 3-4 cm long. Its diameter also varies but tends to be smaller compared to other major mesenteric arteries like the superior mesenteric artery.
Anastomoses
The IMA forms important anastomoses with branches of the superior mesenteric artery and internal iliac arteries. These anastomoses help maintain the blood supply to the colon in case of vascular occlusion or damage to one of the mesenteric arteries.
Termination
The inferior mesenteric artery terminates as the superior rectal artery, which continues into the pelvis to supply the rectum and upper anal canal.
Function
The primary function of the inferior mesenteric artery (IMA) is to provide arterial blood supply to the distal parts of the large intestine. Its branches ensure that oxygenated blood reaches various sections of the colon and rectum. This vascular supply is essential for maintaining the health of the colonic tissue, aiding in digestion, and facilitating the movement and processing of waste material. Below are the specific functions of the IMA based on its branches.
Left Colic Artery Function
The left colic artery, the first major branch of the IMA, is responsible for supplying blood to the descending colon. It divides into two further branches:
- Ascending Branch: This branch moves upward to supply the upper portion of the descending colon and anastomoses with the middle colic artery, ensuring that the blood supply to the distal transverse colon is preserved. It also supports the formation of the marginal artery, an important anastomotic vessel that ensures continuous blood flow in the case of obstruction.
- Descending Branch: This branch travels downward to provide blood to the lower part of the descending colon. This ensures that the descending colon receives an adequate blood supply for its digestive functions, particularly in the absorption of water and electrolytes.
Sigmoid Arteries Function
The sigmoid arteries are responsible for supplying the sigmoid colon. There are typically two to four of these arteries, which form an anastomotic network. Their function is to maintain blood flow to the sigmoid colon, ensuring the effective continuation of peristaltic movements, nutrient absorption, and waste material transport. The anastomotic connections formed by the sigmoid arteries with other branches (like the left colic and superior rectal arteries) also ensure redundancy in blood flow, maintaining tissue health even in the presence of partial vascular blockage.
Superior Rectal Artery Function
The superior rectal artery is the terminal branch of the IMA and supplies blood to the rectum and upper part of the anal canal. This arterial branch plays a critical role in maintaining the health of the rectal tissues, which are involved in the final stages of waste processing and defecation. The superior rectal artery also forms anastomoses with the middle and inferior rectal arteries (from the internal iliac artery), ensuring a robust blood supply to the rectum and creating collateral circulation to prevent ischemia.
Collateral Circulation and Redundancy
One of the key functions of the IMA is its role in collateral circulation, which is achieved through its anastomotic connections with branches of the superior mesenteric artery (SMA) and the internal iliac artery. These connections allow for alternative pathways for blood to flow if one route becomes obstructed or damaged. The marginal artery, which runs along the length of the colon, is a particularly important vessel that provides a continuous supply of blood from both the SMA and IMA, ensuring the colon remains functional even in cases of vascular compromise.
Nutrient and Oxygen Delivery
The IMA plays a vital role in delivering oxygenated blood and essential nutrients to the tissues of the large intestine. This is necessary for maintaining the metabolic activities of the intestinal cells, including cellular respiration, tissue repair, and the generation of energy needed for peristalsis and the absorption of water and electrolytes.
Clinical Significance
The inferior mesenteric artery (IMA) plays a crucial role in the blood supply to the distal portions of the large intestine, making it significant in various clinical contexts. One of its primary clinical considerations is during abdominal surgeries, particularly colorectal procedures, where the IMA may need to be ligated or manipulated. The presence of collateral circulation through the marginal artery often allows safe ligation, but in cases where these anastomoses are insufficient, ischemia of the colon can occur.
In conditions like atherosclerosis, the IMA can become stenotic or occluded, leading to intestinal ischemia, particularly affecting the descending colon and rectum. This can result in ischemic colitis, which may present with abdominal pain, bloody diarrhea, and in severe cases, bowel infarction.
