Chapter 10. Midgut and Hindgut

Big Picture

The midgut consists of the distal half of the duodenum, jejunum, ileum, cecum, ascending colon, and the proximal half of the transverse colon (Figure 10-1A). Branches of the superior mesenteric arteries and veins provide the primary (but not exclusive) vascular supply for the midgut (Figure 10-1B).

Distal Half of the Duodenum

The duodenum is the first part of the small intestine. The chemical digestion of food (i.e., carbohydrates to simple sugars; fats to fatty acids and glycerol; proteins to amino acids) primarily occurs in the duodenum because of the secretion of pancreatic enzymes. The remainder of the small intestine (i.e., jejunum and ileum) primarily functions in absorption of these nutrients into the blood stream.

The duodenum is part of the foregut (supplied by branches of the celiac artery) and the midgut (supplied by branches of the superior mesenteric artery), as noted by its dual vascular supply (Figure 10-1B). The junction between the duodenum and the jejunum is marked by the suspensory ligament of the duodenum (ligament of Treitz). The suspensory ligament consists of connective tissue and smooth muscle and courses from the left crus of the diaphragm to the fourth part of the duodenum. Contraction of the smooth muscle within the ligament helps to open the duodenojejunal flexure, enabling the flow of chyme.

The submucosal layer of the duodenum contains Brunner's glands, which protect the duodenum against the acidic chyme from the stomach. Despite this protection, the duodenum is a relatively common site of ulcer formation.

Jejunum and Ileum

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. In contrast to the ileum, the jejunum also has a greater number of vasa recti. A histologic section of the jejunum is usually identified negatively: it lacks Brunner's glands (like the duodenum) or Peyer's patches (like the ileum).

The ileum is the third part of the small intestine and contains large lymphatic aggregates known as Peyer's patches. 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.

The jejunum and ileum receive their blood supply primarily via ...