Superior Mesenteric Artery
The gastrointestinal tract is comprised of a number of hollow organs that extend from the mouth all the way down to the rectum. The GI tract enables us to process, digest and gain nutrients from the food we eat. Its various parts are specialized to absorb certain nutrients and vitamins such as vitamin B12 which is absorbed in the terminal ileum. The bowel, which part of the GI tract, develops in three separate sections; namely the foregut, midgut and hindgut. Separate nerves, arteries, and veins supply these various sections. This article will describe the superior mesenteric artery (which supplies the midgut), including its branches, and clinical relevance.
There are three major parts that make up the gastrointestinal tract. These are the foregut, midgut and hindgut. The foregut extends from the mouth to the major duodenal papilla (where the ampulla of Vater empties into the duodenum). The midgut extends from this point to the proximal two thirds of the way along the transverse colon. The hindgut runs from this point to the dentate line of the anal canal. The blood supply to the rectum arises from the superior, middle and inferior rectal arteries.
The superior mesenteric artery arises from the abdominal aorta at the level of the first lumbar vertebral body L1, approximately a centimeter below the coeliac trunk. It arises above the renal arteries (that arise at vertebral level L1-L2). The superior mesenteric artery supplies the midgut, while the inferior mesenteric artery supplies the hindgut. Each of these arteries give rise to further major branches that supply regions of the gastrointestinal tract. The superior mesenteric arterys initial course is forwards and downwards, and it travels posterior to the neck of the pancreas and the splenic vein. The superior mesenteric artery is usually found running to the left side of the superior mesenteric vein (which drains the same region as the artery supplies).
Similarly, the sympathetic inputs to the foregut, midgut and hindgut arise from the thoracic and lumbar splanchnic nerves, i.e. T5-T9, T9-T12 and L1-L2 respectively. The parasympathetic innervation to the foregut and midgut is supplied by the vagus nerve (cranial nerve 10) while the pelvic splanchnic nerves provide parasympathetic innervation to the hindgut.
Inferior pancreaticoduodenal artery- This is the first branch of the superior mesenteric artery, arising from its right side, and supplies the head of the pancreas as well as the inferior and ascending regions of the duodenum. This artery gives off two further branches i.e. anterior and posterior branches. Both branches run between the c shaped internal curvature of the duodenum, and the head of the pancreas. They anastomose with the superior pancreaticoduodenal artery that is the terminal branch of the gastroduodenal artery.
Middle colic artery- This is the second branch arising from the right side of the superior mesenteric artery and supplies the proximal two thirds of the transverse colon. It branches off just below the pancreas, and travels upwards within the mesentery of the transverse colon. Its left and right branches anastomose with various arteries i.e. the left one anastomoses with the left colic artery (a branch of the inferior mesenteric artery), and the right anastomoses with the right colic artery.
Right colic artery- This artery runs directly to the right, and supplies the ascending colon. In order to reach this part of the bowel, it passes anteriorly to the gonadal vessels and the psoas major (as well as the ureter, which lies retroperitoneally, these vessels run within the greater omentum, which is a quadruple layer of peritoneum). It divides to form an ascending and descending branch. The former anastomoses with the middle colic artery and the latter anastomoses with the ileocolic artery.
Ileocolic artery- This artery supplies the caecum, appendix and ileum. In order to reach the ascending colon, it passes downward and to the right. It has further branches including the anterior and posterior cecal arteries (which supply their respective parts of the caecum), the ileal branch of the ileocolic, which supplies the junction between the ileum and the caecum, as well as the appendicular artery which supplies the appendix. The appendicular artery is an end artery, and hence the appendix is vulnerable to ischaemic necrosis if this artery is blocked. The colic branch runs superiorly and supplies the ascending colon.
Jejunal and Ileal branches- These arteries branch off from the left side of the superior mesenteric artery to supply the jejunum and ileum. They form arterial arcades (a network of arteries that lies near the bowel and supplies it). The arcades for the jejunum are longer and less connected (i.e. fewer anastomoses). The arcades of the ileum are shorter and are more connected (i.e. more anastomoses). This reflects the function of the various parts of the bowel.
Marginal artery of Drummond- The terminal branches of the superior mesenteric artery (middle colic, right colic and ileocolic arteries) and inferior mesenteric artery (left colic and sigmoid arteries) anastomose to form the marginal artery, that runs along the internal border of the colon. Sir David Drummond described the artery first. In surgical terms, during bowel surgery when the major arteries may be clamped off to reduce blood loss, the marginal artery is sufficient to perfuse the colon.
Nutcracker syndrome- This results from an elevated left renal vein pressure due to the compression of the renal vein between the superior mesenteric artery and the abdominal aorta. Symptoms include blood in the urine, pain in the abdomen (usually in the flank). Varicoceles may also result, as the left gonadal vein drains into the left renal vein, and hence becomes backlogged, resulting in venous engorgement.
Mesenteric ischaemia- Chronic mesenteric ischaemia can result from atherosclerosis of the mesenteric arteries. Acute mesenteric ischaemia commonly results from an embolus that becomes lodged in any of the branches of the mesenteric arteries. Risk factors include atrial fibrillation, chronic renal failure and heart failure. Treatment includes surgical revascularization, but radiological interventions are being developed.
Superior mesenteric artery syndrome- This is a rare disease that occurs when the duodenum is trapped between the abdominal aorta and the superior mesenteric artery, causing compression or complete obstruction. The patient will present with bloating after meals, nausea and vomiting and a feeling of abdominal fullness. Treatment includes diet modification to treat the underlying cause (which is often rapid weight loss, or previous abdominal surgery), and surgery.