The Anatomy of the Superior Mesenteric Vein

The superior mesenteric vein (SMV) is a large blood vessel in the abdomen. Its function is to drain blood from the small intestine as well as the first sections of the large intestine and other digestive organs. This large vein receives blood from several other veins (tributaries) in the digestive tract.

It lies to the right of the superior mesenteric artery. Nutrients absorbed from the small intestine are carried in the blood and through the SMV to the hepatic portal vein and then into the liver. Once in the liver, the nutrients can be dispersed throughout the body.

Anatomy

Veins are the blood vessels that carry blood from various parts of the body and toward the heart. This is different from arteries, which carry blood away from the heart and back toward the organs and structures of the body.

Location

The SMV is located on the right anatomical side of the abdomen and is part of the hepatic portal venous system. The superior mesenteric artery, which carries blood into the digestive system, is on the left side of the abdomen.

The SMV joins with the splenic vein and becomes the hepatic portal vein. It is the hepatic portal vein that carries the blood to the liver. The SMV receives blood coming from several parts of the digestive tract. This includes veins coming from the small intestine, which is organized into three sections (duodenum, jejunum, and ileum).

Other veins that feed into the SMV come from sections of the large intestine which include the cecum, ascending colon, and transverse colon. Still other contributing veins come from the stomach and the pancreas.

These veins include:

• The jejunal vein from the second part of the small intestine
• The ileal vein from the last part of the small intestine
• The ileocolic vein from the ileum, colon, and cecum
• The right colic vein from the ascending colon

The gastrocolic trunk also joins, which includes:

• The right gastroepiploic vein from the stomach area
• The middle colic vein from the transverse colon
• The anterior and inferior pancreaticoduodenal veins from the pancreas and duodenum

Anatomical Variations

The SMV is normally located on the right side of the abdomen. If it is found to be on the left, such as with an imaging test or during surgery, that could be caused by a congenital condition (such as midgut malrotation), by the vein being shifted by a growing tumor, or by a short-term rotation of the gut.

In some people, the SMV may have one main trunk. In others, it may be split into two.

A midgut malrotation is a congenital condition (one that a person is born with). It often does not cause any symptoms, and it’s unknown how often it occurs. However, when there are symptoms, the condition is usually found in infants and children under the age of 5. Being diagnosed as an adult is rare.

In some cases, other congenital conditions occur along with a midgut malrotation, such as heart disease and abdominal wall defects. Surgery might be done to correct any complications such as a bowel obstruction or a twisted bowel.

There are some differences in how the tributary veins connect to the SMV. These variations can be diverse and complex.

For instance, about 50% of people don’t have a right colic vein, and a little less than half of those who do, drain into a different branch. The right gastroepiploic vein and colonic drainage veins may also drain into either the main trunk or into a different branch.

Because of these variations, it is important for surgeons to know the patient’s abdominal vein structure before surgery is done on the digestive tract. A CT angiography scan or another test may be done to look for these differences. Understanding any variations may help in avoiding certain surgical complications.

Function

The purpose of the SMV is to transport blood from the digestive tract to the liver. The blood comes from the small intestine, large intestine, appendix, stomach, and pancreas. The many tributary veins bring blood from all of these parts of the digestive tract to the SMV, which feeds into the hepatic portal vein, bringing the blood to the liver.

Blood coming from the midgut contains vitamins and nutrients from the food that is digested there. In the liver, the nutrients are processed into a form that the body can use. The liver stores these nutrients and parses them out when the body has a need.

Clinical Significance

Some conditions can affect the SMV. In most cases, they are uncommon or rare.

Pylephlebitis

Pylephlebitis is an uncommon condition that can follow significant infection in the abdomen, leading to clots and infection of the vein. Before the era of antibiotics, the condition was uniformly lethal. Now, it carries a mortality of up to 30%. When the SMV is affected, it is called a portomesenteric venous thrombosis.

Conditions that might lead to the development of pylephlebitis include diverticulitis, acute appendicitis, inflammatory bowel disease, pancreatitis, and gastroenteritis. Symptoms can include fever, abdominal pain, liver dysfunction, and a bacterial infection in the blood. Pylephlebitis is often treated with antibiotics.

In some cases, the lack of blood flow to the small intestine leads to complications such as the death of cells there (necrosis) or an inflammation of the abdominal wall (peritonitis). In these instances, a bowel resection, where a part of the bowel is removed, may be needed.

Isolated Superior Mesenteric Venous Thrombosis

Isolated superior mesenteric venous thrombosis (SMVT) is when a blood clot forms in the SMV. It is a rare condition.

SMVT can occur as a result of cancer, peritonitis, increased blood clotting (hypercoagulable state), protein C deficiency, polycythemia vera, recent abdominal surgery, high blood pressure in the portal vein (portal hypertension), or sepsis.

Symptoms can include fever, nausea, blood in the stool, abdominal distention or pain, and vomiting blood. This condition is most often diagnosed with a CT scan. Treatment could include antibiotics, anticoagulants, surgery to remove the clot or to place drugs to dissolve the clot, or a small intestine resection.

Injury

It’s quite rare for an injury to occur to the SMV, but when it does, the fatality rate can be as much as approximately 50%. Injuries might be caused by a car accident or when there is piercing damage to the abdomen.

Patients might present with severe blood loss causing hemorrhagic shock, hyperthermia, increased acidity in the blood (acidosis), and lack of blood clotting (coagulopathy). A CT scan may be done to assess the injury, but often, exploratory surgery is done to repair the damage.

In other cases, when a patient isn’t stable enough to undergo a lengthy repair surgery, the SMV may be clamped (ligation).