Inferior mesenteric artery

The inferior mesenteric artery supplies the left half of the transverse part of the colon, the whole of the descending and the sigmoid colon, and the greater part of the rectum.

The inferior mesenteric artery is smaller than the superior mesenteric artery, and arises from the aorta, about 3 or 4 cm above its division into the common iliacs and close to the lower border of the inferior part of the duodenum.

The inferior mesenteric artery passes downward posterior to the peritoneum, lying at first anterior to and then on the left side of the aorta. It crosses the left common iliac artery and is continued into the lesser pelvis under the name of the superior anorectal artery, which descends between the two layers of the sigmoid mesocolon and ends on the upper part of the rectum.   

The branches of the inferior mesenteric artery are:

• The left colic artery runs to the left behind the peritoneum and in front of the psoas major, and after a short, but variable, course divides into an ascending and a descending branch; the stem of the artery or its branches cross the left ureter and left internal spermatic vessels. The ascending branch crosses in front of the left kidney and ends, between the two layers of the transverse mesocolon, by anastomosing with the middle colic artery; the descending branch anastomoses with the highest sigmoid artery. From the arches formed by these anastomoses branches are distributed to the descending colon and the left part of the transverse colon.

• The sigmoid arteries, two or three in number, run obliquely downward and to the left behind the peritoneum and in front of the Psoas major, ureter, and internal spermatic vessels. Their branches supply the lower part of the descending colon, and the sigmoid colon; anastomosing above with the left colic artery, and below with the superior anorectal artery.

• The superior anorectal artery (superior hemorrhoidal artery) is the continuation of the inferior mesenteric. It descends into the pelvis between the layers of the mesentery of the sigmoid colon, crossing, in its course, the left common iliac vessels. It divides, opposite the third sacral vertebra, into two branches, which descend one on either side of the rectum, and about 10 or 12 cm. from the anus break up into several small branches. These pierce the muscular coat of the bowel and run downward, as straight vessels, placed at regular intervals from each other in the wall of the gut between its muscular and mucous coats, to the level of the sphincter ani internus; here they form a series of loops around the lower end of the rectum, and communicate with the middle anorectal branches of the internal iliac artery, and with the inferior anorectal branches of the internal pudendal artery.