Blood supply of the midgut
Superior mesenteric artery This is the artery of the midgut and supplies the gut from the entrance of the bile duct to a level just short of the splenic flexure of the colon. The artery arises from the front of the aorta a centimetre below the coeliac trunk, at the level of the lower border of L1 vertebra. It is directed steeply downwards behind the splenic vein and the body of the pancreas, with the superior mesenteric vein on its right side. It lies anterior to the left renal vein, the uncinate process of the pancreas and the third part of the duodenum, in that order from above downwards (Fig. 5.26). With its vein it enters the upper end of the mesentery of the small intestine and passes down to the right along the root of the mesentery (Fig. 5.23). Pressure of the superior mesenteric artery on the left renal vein may produce left-sided varicocele, and pressure on the duodenum may give symptoms of chronic duodenal ileus, particularly when the angle between the artery and the aorta is smaller than usual. The inferior pancreaticoduodenal artery is its first
branch, arising from the posterior surface; it may come off the first jejunal branch. It divides into anterior and posterior branches which run in the curve between the duodenum and the head of the pancreas, supply both, and anastomose with the terminal branches of the superior pancreaticoduodenal artery. The jejunal and ileal branches arise from the left of the
main trunk and pass down between the two layers of the mesentery. The pattern of anastomosing arcades from which vessels enter the gut wall is described with the jejunum and ileum (see p. 254). The ileocolic artery (Fig. 5.23) arises from the right side
of the superior mesenteric trunk low down in the base of the mesentery. It descends to the right iliac fossa and divides into superior and inferior branches. The superior branch runs up along the left side of the ascending colon to anastomose with the right colic artery. The inferior branch runs to the ileocolic junction, and gives off anterior and posterior caecal arteries, an appendicular artery, and an ileal branch which ascends to the left on the ileum to anastomose with the terminal branch of the superior mesenteric artery. The right colic artery (Fig. 5.23) arises from the right
side of the superior mesenteric artery, or in common with the ileocolic artery. It runs to the right across the right psoas muscle, gonadal vessels, ureter and genitofemoral
nerve, and quadratus lumborum, just behind the peritoneal floor of the right infracolic compartment. it divides near the left side of the ascending colon into two branches. the descending branch runs down to anastomose with the superior branch of the ileocolic artery. the ascending branch runs up across the inferior pole of the right kidney to the hepatic flexure where it anastomoses with a branch of the middle colic artery. the middle colic artery (fig. 5.23) arises from the right
side of the superior mesenteric artery, as the artery emerges at the lower border of the neck of the pancreas, and descends between the two leaves of the transverse mesocolon. it lies to the right of the midline and at the intestinal border of the transverse mesocolon it divides into right and left branches which run along the margin of the transverse colon. the right branch anastomoses with the ascending branch of the right colic artery. the left branch supplies the transverse colon almost to the splenic flexure (the distal part of the midgut) where it anastomoses with a branch of the left colic artery. as the middl
colic lies to the right of the midline it leaves a large avascular window to its left in the transverse mesocolon (fig. 5.23). this window is the site of election for surgical access to the lesser sac and the posterior wall of the stomach