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).
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.
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 jejunal and ileal branches of the superior mesenteric artery.
The cecum is the blind-ended sac at the beginning of the large intestine (Figure 10-1A and B). The cecum is inferior to the ileocecal valve and is located in the right lower quadrant of the abdomen, within the iliac fossa. Attached to the cecum is the vermiform appendix (unknown function in humans). The taenia coli (longitudinal smooth muscle bands) of the ascending colon lead directly to the base of the appendix.
By identifying the taenia coli during surgery, surgeons locate the origin of the appendix on the cecum. The position of the remainder of the appendix varies because it is intraperitoneal (mobile). The surface projection of the appendix (McBurney’s point) is most often located one-third of the distance between the right anterior superior iliac spine and the umbilicus.
The ileocolic artery, a branch of the superior mesenteric artery, supplies the cecum. In addition, a small branch of the ileocolic artery, the appendicular artery, supplies the appendix (Figure 10-1B).
The appendix may become inflamed, resulting in appendicitis. Sensory neurons from the visceral peritoneum of the appendix signal the central nervous system that the appendix is inflamed. These signals are transmitted via visceral sensory neurons in the lesser splanchnic nerve, which enters the T10 vertebral level of the spinal cord. However, somatic sensory neurons from the skin around the umbilicus also enter at the T10 vertebral level of the spinal cord. Because both visceral and somatic neurons enter the spinal cord at the same level and synapse in the same region, the brain interprets the inflammation from the appendix as if the pain originated in the region of the umbilicus. This phenomenon is known as referred pain.
The ascending colon arises from the cecum and courses vertically to the liver, where the colon bends at the right colic (hepatic) flexure (Figure 10-1B). The parietal peritoneum covers its anterior surface, and thus the ascending colon is considered a retroperitoneal organ. A depression between the lateral surface of the ascending colon and the abdominal wall is known as the right paracolic gutter. Branches of the right colic artery that supply the ascending colon enter the bowel on its medial surface. It is possible during surgery to mobilize the ascending colon by cutting the peritoneum along the right paracolic gutter without injuring its major vessels or lymphatics.
The colon continues horizontally as the transverse colon to the spleen on the opposite side of the abdomen, inferior to the liver, gallbladder, and the greater curvature of the stomach. The transverse colon is connected to the greater curvature of the stomach via the gastrocolic ligament, which is part of the greater omentum. The duodenum, pancreas, duodenojejunal flexure, and parts of the small intestine are all located deep to the transverse colon.