Inferior Mesenteric Artery
The gastrointestinal tract extends from the mouth to the rectum. It is comprised of a number of hollow organs that enable us to process, digest and gain nutrients from the food we eat. Its various parts are specialized to absorb certain nutrients and vitamins, for example, essential micronutrients like vitamin B12 are absorbed in the terminal ileum. The bowel 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 inferior mesenteric artery (which supplies the hindgut), including its branches, and clinical relevance.
There are three major parts in the gastrointestinal tract. These are the foregut, midgut and hindgut. The foregut extends from the mouth to the major duodenal papillae (where the ampulla of Vater empties into the duodenum). The midgut extends from this point to two thirds of the way along the transverse colon. The hindgut runs from this point to the dentate line of the anal canal. The lower anal canal, below the dentate line, is perfused by the middle rectal artery (a branch of the internal iliac artery) and the inferior rectal artery (a branch of the internal pudendal artery).
Course and supply
The coeliac trunk arises from the abdominal aorta at the level of the vertebral body of T12/L1 to supply the foregut. The superior mesenteric artery supplies the midgut and the inferior mesenteric artery supplies the hindgut. Each of these arteries give off major branches that supply regions of the gastrointestinal tract. The inferior mesenteric artery arises from the abdominal aorta at the level of the third lumbar vertebra, making it the third major unpaired branch of the abdominal aorta. It supplies the distal third of the transverse colon, the splenic flexure, the descending colon, the sigmoid colon and the rectum, as well as the superior part of the anal canal down to the dentate line. The area of the colon supplied by the left colic artery overlaps with the middle colic artery (a branch of the superior mesenteric artery), forming the marginal artery of Drummond. The superior rectal artery anastomoses with the middle and inferior rectal arteries.
Similarly, the sympathetic input to the foregut, midgut and hindgut are supplied by the thoracic and lumbar splanchnic nerves, i.e. T5-T9, T9-T12 and L1-L2 respectively. The parasympathetic input 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.
Left Colic Artery- This is the first branch of the inferior mesenteric artery, and its initial section runs retroperitoneally and crosses in front of the left kidney and the psoas major muscle. It has both an ascending and a descending branch. The ascending branch anastomoses with branches of the middle colic artery (a branch of the superior mesenteric artery), and the descending branch anastomoses with the first sigmoid artery.
Sigmoid arteries- The sigmoid colon is around 40 cm in length, and is S-shaped. The position of this part of the bowel is variable, as it can be intra-abdominal or lie within the pelvis. The sigmoid colon (as well as the transverse colon) lies within the peritoneum, and begins at the superior aperture of the lesser pelvis. There are usually around 3 branches of this artery, which pass inferiorly and obliquely posterior to the peritoneum, but anterior to psoas major, the kidneys and ureters. These branches supply the inferior part of the descending colon and the S-shaped sigmoid colon.
Superior Rectal Artery- At the midpoint of the sacrum, the S-shaped sigmoid colon will lose its mesentery (double layer of peritoneum) and will become the rectum. The rectum is approximately 14-15 cm long, and ends in the anus. This artery supplies the rectum above the pectinate line. It divides into two branches (anterior and posterior). They pierce the muscular wall of the rectum, and descend as straight arteries to the level of the internal anal sphincter. Here they form a net-like mesh of arteries that anastomose with the middle rectal and inferior rectal arteries, which supply the anal canal below the pectinate line.
Marginal artery of Drummond- This is a continuous arterial arcade that runs along the internal border of the colon, and is formed by the anastomosis of the terminal branches of the superior and inferior mesenteric arteries. The left colic (a branch of the inferior mesenteric artery) and the middle colic (a branch of the superior mesenteric artery) have a focally small anastomosis, which makes the splenic flexure a watershed area that is vulnerable to ischaemia.
Left Hemicolectomy- The presence of a tumour in the descending colon will require surgery. This procedure involves resection of the descending colon, and can be performed via a laparoscopic technique or an open approach. The blood vessel, nerves and lymph nodes that supply the region of bowel will also need to be removed, and the remaining colon will be reattached to the rectum. A stoma (ileostomy) is not usually required for this procedure.
Ischaemic Colitis- This is an inflammatory condition of the colon (or large bowel) that results from inadequate blood supply. It is most common in those over 60. Atherosclerosis (vascular disease) causes a chronic form of the disease, and can be resolved with mild treatment e.g. antibiotics and liquid diet. Diagnosis can be made using a CT scan, or an x-ray that will show distension. A clot will cause an acute presentation of the disease, and the bowel can become ischaemic, necrotic and gangrenous in a matter of hours. Treatment includes intravenous fluids and painkillers and keeping the patient nil by mouth to rest the bowel. Death can result from sepsis if the bowel perforates, so emergency surgery is needed.
Volvulus- This is an acute presentation resulting from malrotation of the bowel, where a loop of bowel twists about a focal point in the mesentery, and may result in bowel obstruction. Symptoms of this illness include abdominal distension and bilious vomiting, as well as severe abdominal pain. On x-ray, a sigmoid volvulus will cause the coffee-bean sign, and gastrografin will show the location of the obstruction. The caecum is a vulnerable area for volvulus, as well as the splenic flexure, transverse colon, and sigmoid colon.