The midgut consists of the duodenum (third and fourth parts), jejunum, ileum, cecum, ascending colon, and the proximal half of the transverse colon. The superior mesenteric arteries and veins provide the primary vascular supply for the midgut. Lymph from the midgut drains into the superior mesenteric nodes surrounding the superior mesenteric artery.
DUODENUM (THIRD AND FOURTH PARTS)
The duodenum chemically digests food because of the secretion of pancreatic enzymes. The parts of the duodenum associated with the midgut are as follows:
Third Part (horizontal). Crossed anteriorly by the superior mesenteric artery and vein.
Fourth Part (ascending). The distal portion of the fourth part is transitional, from retroperitoneal to intraperitoneal in the region of the duodenojejunal junction.
The jejunum and ileum are intraperitoneal organs and are tethered to the posterior abdominal wall via the mesentery. This double-layered peritoneum serves as the conduit for the mesenteric neurovascular structures.
Jejunum. The jejunum is the second part of the small intestine and has the most highly developed circular folds lining the lumen, thereby increasing the surface area of the mucosal lining for absorption.
Ileum. The ileum is the third part of the small intestine and contains large lymphatic aggregates known as Peyer's patches. In some conditions, such as lymphomas, these Peyer's patches may enlarge and cause intestinal obstruction.
In contrast to the jejunum, the ileum has fewer circular folds lining the lumen and more vascular arcades.
The terminal end of the ileum has a thickened smooth muscle layer known as the ileocecal valve (sphincter), which prevents feces from the cecum to move backward from the large intestine into the small intestine.
Meckel's diverticulum. Approaching the distal end of the ileum is the site of the embryonic vitellointestinal duct attachment. If a remnant of this duct persists it presents as Meckel's diverticulum. This diverticulum is important clinically in that it may contain ectopic gastric mucosa, which secretes acid. This may result in inflammation, bleeding from intestinal mucosa, bowl obstruction, intussusception, or perforation. A helpful way of remembering this condition is the rule of “2s”: occurs in 2% of the population, is 2 inches in length, may contain 2 types of ectopic tissue (gastric and pancreatic), found within 2 feet of the ileocecal valve, and most become symptomatic before 2 years of age.▼