The inferior mesenteric vein is the blood vessel responsible for draining the left side of the colon, the sigmoid colon, and the upper rectum, carrying oxygen-depleted blood back toward the liver through the portal venous system. It is smaller and less discussed than its counterpart on the right side, the superior mesenteric vein, but it plays an outsized role in several surgical procedures, in complications of liver disease, and in rare but serious clotting events. Where exactly it empties varies from person to person, and that variability has real consequences on the operating table.
Where It Runs and What It Collects
The inferior mesenteric vein begins as a continuation of the superior rectal vein deep in the pelvis. It climbs upward along the left side of the abdomen, running behind the peritoneum (the membrane lining the abdominal cavity) and passing just to the left of a key abdominal landmark called the ligament of Treitz, where the small intestine transitions from the duodenum to the jejunum. Along the way, it picks up smaller tributaries: the left colic vein from the descending colon and the sigmoid veins from the sigmoid colon. All of these carry blood loaded with absorbed nutrients and waste products from the left colon and rectum.
Its job is to funnel that blood into the portal venous system so it can pass through the liver for filtering before returning to the heart. In most people, the inferior mesenteric vein empties into the splenic vein behind the pancreas, but this is not universal, and the variation matters.
Drainage Patterns Vary More Than You Might Expect
Textbooks typically show the inferior mesenteric vein joining the splenic vein, but imaging studies consistently find that a large minority of people have a different arrangement. In one CT venography study of 51 patients, the vein drained into the splenic vein in about 56% of cases, into the superior mesenteric vein in roughly 26%, and into the junction where the splenic and superior mesenteric veins merge (the splenomesenteric confluence) in about 18%.1PubMed. Anatomic variants of mesenteric veins: depiction with helical CT venography A larger study using 3D CT portography found a somewhat different distribution: roughly 69% into the splenic vein, about 19% into the superior mesenteric vein, and around 8% into the confluence.2PubMed. Analysis of anatomic variants of mesenteric veins by 3-dimensional portography using multidetector-row computed tomography
A Turkish population study of nearly 900 patients identified five drainage types rather than the traditional three, with additional rare patterns making up about 3% of cases combined.3Journal of Surgery and Medicine. Evaluation of inferior mesenteric vein drainage patterns in the Turkish population: A multidetector computed tomography study The bottom line is that while most people follow the textbook pattern, somewhere between a quarter and a third do not. This is not a quirk of academic interest. Surgeons who assume the textbook layout without checking preoperative imaging can accidentally cut or clip a vessel they didn’t expect to find where it is.
Why Surgeons Pay Such Close Attention to It
The inferior mesenteric vein sits right in the surgical field for a number of common and complex operations, particularly left-sided colectomies (removal of part of the left colon) and anterior resections for rectal cancer. During these procedures, surgeons often need to decide how high up to cut the vessel. A “high tie” means dividing the vein near the lower border of the pancreas, while a “low tie” takes it closer to its origin. The choice affects how much extra length the remaining colon gains, which determines whether a tension-free reconnection (anastomosis) is possible without pulling too hard on the bowel.
One study comparing the two approaches found that dividing the inferior mesenteric vein high, near the pancreas, gave an average additional colon length of about 8 cm, compared with roughly 2.5 cm with a low division. The cumulative gain from a high-tie strategy averaged about 19 cm versus roughly 9 cm with a low tie.4Diseases of the Colon & Rectum. High Tie Versus Low Tie Vascular Ligation of the Inferior Mesenteric Artery in Colorectal Cancer Surgery: Impact on the Gain in Colon Length and Implications on the Feasibility of Anastomoses That extra length can be the difference between a successful reconnection and a patient needing a temporary colostomy bag.
The Arc of Riolan Problem
High ligation comes with a risk that is easy to overlook. A collateral artery called the arc of Riolan, which connects the blood supply of the superior and inferior mesenteric arteries, frequently crosses directly in front of the inferior mesenteric vein just below the pancreas. If a surgeon ligating the vein high inadvertently clips this artery, the left colon can lose a critical backup blood supply. The result can be ischemia (starved tissue), colitis, or breakdown of the surgical connection.
A 3D angiography mapping study confirmed that the arc of Riolan is the vessel most at risk during high ligation and recommended that surgeons specifically identify and preserve it when performing splenic flexure takedown.5PubMed Central. 3D mesenteric angiogram-based assessment of Arc of Riolan crossing the inferior mesenteric vein: important considerations in high ligation during splenic flexure takedown in anterior resection Another study proposed classifying the relationship between the collateral vessel and the vein into three anatomical types, finding that in over half of patients the collateral either crossed the vein or ran close enough to be endangered during ligation.6PubMed Central. The relation between inferior mesenteric vein ligation and collateral vessels to splenic flexure: anatomical landmarks, technical precautions and clinical significance A separate surgical team echoed that recognizing and preserving the arc of Riolan during high vein ligation and splenic flexure takedown may be an important step in reducing acute postoperative ischemia.7Diseases of the Colon & Rectum. Arc of Riolan-Preserving Splenic Flexure Takedown During Anterior Resection: Potentially Critical to Prevent Acute Anastomotic Ischemia
A Landmark in Laparoscopic Surgery
With the widespread adoption of minimally invasive surgery, the inferior mesenteric vein has taken on an additional role as a navigational landmark. In laparoscopic left-sided colectomies performed using a medial-to-lateral approach (where the surgeon works from the middle of the abdomen outward), the vein serves as a visible starting point for dissection. A cadaveric feasibility study found that using the inferior mesenteric vein as the initial landmark was workable in about two-thirds of cases.8PubMed. Anatomical basis of laparoscopic medial-to-lateral mobilization of the descending colon It has also been described in a video demonstration as a reliable first structure to identify when beginning the medial dissection of the descending colon.9PubMed. Inferior mesenteric vein as initial landmark for laparoscopic medial to lateral dissection of descending colon – a video vignette
Preoperative 3D CT angiography has proven useful for mapping the vein’s course and its relationship to the inferior mesenteric artery before surgery, allowing the team to plan around anatomical variations rather than discovering them intraoperatively.10PubMed. Anatomical study of the inferior mesenteric vein using three-dimensional computed tomography angiography in laparoscopy-assisted surgery for left-sided colorectal cancer
Its Role in Pancreatic Surgery
The inferior mesenteric vein also matters in operations on the pancreas, particularly pancreaticoduodenectomy (the Whipple procedure) when tumor involvement requires removing part of the portal or superior mesenteric vein as well. Because the inferior mesenteric vein runs along the left side of the pancreas and enters the splenic vein behind the gland, it can serve as a guide for where to cut the pancreatic body. One study concluded that when the vein drains into the splenic vein, it can serve as an alternative guide for transecting the pancreas during complex Whipple procedures involving vascular resection.11PubMed Central. Inferior mesenteric vein serves as an alternative guide for transection of the pancreatic body during pancreaticoduodenectomy with concomitant vascular resection: a comparative study evaluating perioperative outcomes
In some locally advanced pancreatic cancers, the tumor encases the superior mesenteric vein so thoroughly that removing a segment of it becomes necessary. In these cases, surgeons have rerouted venous flow from the superior mesenteric vein through the inferior mesenteric vein, effectively using it as an outflow tract for the intestinal blood that normally takes a different path. This has been described as a safe procedure that may offer a chance at curative resection in patients who would otherwise be considered inoperable.12PubMed Central. Redirecting venous flow from the superior mesenteric vein to the inferior mesenteric vein in resections for locally advanced pancreatic cancer
The vein also matters when the splenic vein is ligated during pancreatic resection. If the inferior mesenteric vein enters the confluence and gets taken along with the splenic vein, or if it is too small to carry the splenic vein’s drainage, the patient can develop sinistral (left-sided) portal hypertension, which engorges the gastric veins and can cause dangerous bleeding from gastric varices.13PubMed. Optimal management of the splenic vein at the time of venous resection for pancreatic cancer: importance of the inferior mesenteric vein
Portal Hypertension and Rectal Varices
In liver cirrhosis, the portal venous system backs up because the scarred liver resists blood flow. This elevated pressure, called portal hypertension, forces blood to find alternate escape routes into the systemic venous circulation. The inferior mesenteric vein is one of the conduits through which this rerouting happens. Normally, blood in the superior rectal veins drains upward through the inferior mesenteric vein toward the liver. When portal pressure rises, that flow reverses: blood pushes backward through the inferior mesenteric vein, down into the superior rectal vein, and engorges the rectal venous plexus. These swollen vessels then connect with the middle and inferior rectal veins, which drain into the internal iliac veins and thus into the systemic circulation, bypassing the liver entirely.14PubMed Central. Portosystemic collateral pathways on portal hypertension: a comprehensive review on MDCT
The result is rectal varices, which are dilated submucosal veins in the rectum distinct from ordinary hemorrhoids (though they are sometimes confused with them). A study of cirrhosis patients found that those whose inferior mesenteric vein carried blood away from the liver (hepatofugal flow, meaning reversed) had significantly higher rates of rectal varices compared with those whose flow remained directed toward the liver: roughly 56% versus 13%.15PubMed. Effects of inferior mesenteric vein flow in patients with cirrhosis The same group also had more ascites and more advanced liver disease overall. In patients with reversed flow, the inferior mesenteric vein can dilate considerably, with one study reporting mean diameters of about 7.7 mm and flow volumes that sometimes exceeded 4,000 mL per minute in extreme cases.16Clinical Gastroenterology and Hepatology. Clinical Significance of Inferior Mesenteric Vein in Portal Hypertension Associated With Cirrhosis
Thrombosis of the Inferior Mesenteric Vein
Mesenteric venous thrombosis, where a clot blocks one of the mesenteric veins, is an uncommon but dangerous cause of intestinal ischemia. The vast majority of cases involve the superior mesenteric vein. Isolated thrombosis of the inferior mesenteric vein is exceptionally rare, accounting for fewer than 5% of all mesenteric venous thrombosis cases.17ScienceDirect / Medicina Clínica (English Edition). Review Mesenteric venous thrombosis When it does occur, it typically presents with left-sided abdominal pain, bloody stools, and signs of ischemic colitis affecting the descending or sigmoid colon.
A classic case report documented a 32-year-old man who developed ischemic colitis from inferior mesenteric vein thrombosis with no identifiable predisposing factors. The clot was confirmed by angiography, and the course of the disease was tracked endoscopically.18PubMed. Ischemic colitis in a young adult due to inferior mesenteric vein thrombosis Part of the reason isolated inferior mesenteric vein thrombosis is rare is that the left colon has more robust collateral pathways than the small intestine, so a clot in this vessel is more likely to be compensated by alternate routes before the tissue becomes critically oxygen-starved. That said, delayed diagnosis still carries the risk of bowel infarction, so clinicians consider it in patients with unexplained left-sided abdominal pain and rectal bleeding, particularly those with clotting disorders, recent surgery, or inflammatory bowel disease.
Endovascular Interventions Through the Vein
Interventional radiologists sometimes access the inferior mesenteric vein directly to treat complications of portal hypertension. Rectal varices that bleed despite medical management can be embolized by threading a catheter into the vein and deploying coils or sclerosing agents to shut down the offending vessels. One reported approach involved reaching the rectal varices through a recanalized umbilical vein on the abdominal wall, then embolizing the varices continuing from the inferior mesenteric vein using coils and a sclerosant agent.19Surgical Laparoscopy, Endoscopy & Percutaneous Techniques. Treatment of Bleeding Rectal Varices With Transumbilical Venous Obliteration of the Inferior Mesenteric Vein
Stomal varices, which form around surgical stomas in patients with portal hypertension, represent another scenario where the inferior mesenteric vein serves as the feeding vessel. Angiographic imaging in these cases has shown the varix and inferior mesenteric vein acting as the afferent (inflow) vessel, guiding embolization.20Radiology Case Reports. Transcatheter embolization for stomal varices: A report of three patients Even in pediatric patients, direct percutaneous catheterization of the inferior mesenteric vein under ultrasound guidance has been performed to embolize perirectal varices, with a microcatheter advanced selectively into the rectal varicose veins and coils deployed to reduce flow.21Radiology Case Reports. Direct percutaneous embolization of superior rectal veins via an inferior mesenteric vein in a pediatric patient: A case report
Congenital Anomalies Involving the Vein
Rarely, the inferior mesenteric vein is involved in congenital portosystemic shunts, where an abnormal vessel connects the portal venous system directly to the systemic circulation, bypassing the liver. One documented case described a Y-shaped shunt vessel linking the inferior mesenteric vein to both the inferior vena cava and the left ovarian vein.22PubMed. A rare congenital extrahepatic portosystemic shunt affecting the inferior mesenteric vein, inferior vena cava, and left ovarian vein These shunts can divert nutrient-rich portal blood away from the liver, which in some cases leads to elevated blood ammonia levels, hepatic encephalopathy, or liver nodules. Many are discovered incidentally on imaging performed for unrelated reasons. Whether they require treatment depends on the volume of shunted blood and whether the patient develops symptoms.
Congenital shunts involving the inferior mesenteric vein are far less common than those involving the larger portal or splenic veins, likely because the inferior mesenteric vein carries a comparatively small fraction of total portal flow. When they do occur, however, they can complicate abdominal surgeries by presenting unexpected vascular connections that the surgical team needs to recognize and manage.
How It Differs From the Superior Mesenteric Vein
The inferior mesenteric vein is often overshadowed by the superior mesenteric vein, which drains the entire small intestine and right colon and carries a much larger volume of blood. The practical differences between the two go beyond size. The superior mesenteric vein runs on the right side of the abdomen, travels within the mesentery alongside the superior mesenteric artery, and joins the splenic vein behind the pancreas to form the portal vein. The inferior mesenteric vein, by contrast, runs retroperitoneally on the left side and typically empties into the splenic vein rather than forming the portal vein directly.
This anatomical separation has clinical consequences. Thrombosis of the superior mesenteric vein threatens the small bowel, where ischemia progresses rapidly and can become life-threatening within hours. Thrombosis of the inferior mesenteric vein threatens the left colon, which tolerates reduced blood flow somewhat better thanks to its collateral supply. Similarly, in portal hypertension, the superior mesenteric vein is associated with esophageal and gastric varices (the classic and most dangerous type), while the inferior mesenteric vein route produces rectal and colonic varices. Both can bleed, but esophageal variceal bleeding carries a higher mortality rate, which partly explains why the inferior mesenteric vein receives less attention in the portal hypertension literature despite being a meaningful contributor to the collateral circulation.