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.

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 jejunal and ileal branches of the superior mesenteric artery.

Cecum

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.

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).

Ascending and Transverse Colon

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.

The transverse colon is surrounded by visceral peritoneum and thus is considered an intraperitoneal structure.

The right colic artery supplies the ascending colon, whereas the middle colic artery supplies the transverse colon (both are branches of the superior mesenteric artery). The right and left colic arteries contribute to the blood supply of the transverse colon.

Big Picture

The hindgut consists of the distal half of the transverse colon, descending colon, sigmoid colon, and the proximal third of the rectum. Branches of the inferior mesenteric artery and vein provide vascular supply to the hindgut.

Transverse, Descending, and Sigmoid Colon

The transverse colon ends at the spleen, where the colon bends as the left colic (splenic) flexure (Figure 10-2A). The colon continues vertically down the left wall of the abdomen as the descending colon. The parietal peritoneum covers only the anterior surface of the descending colon, and thus the descending colon is considered a retroperitoneal structure.

A depression between the lateral surface of the descending colon and the abdominal wall is known as the left paracolic gutter. Branches of the left colic artery supplying the descending colon enter the bowel on its medial surface. As such, it is possible to surgically mobilize the descending colon by cutting the peritoneum along the left paracolic gutter without injuring major vessels or lymphatics.

The transverse colon is part of both the midgut (supplied by left and middle colic arteries, branches of the superior mesenteric artery) and the hindgut (supplied by left colic arteries, branches of the inferior mesenteric artery). The descending colon also receives its vascular supply mainly from the left colic artery (branch of the inferior mesenteric artery) (Figure 10-2B).

Near the left iliac fossa, the descending colon continues as the sigmoid colon, which is an intraperitoneal structure. The vascular supply of the sigmoid colon is via sigmoid arteries from the inferior mesenteric artery.

Rectum and Anus

In the pelvic cavity, the rectum is the terminal, straight portion of the colon and ultimately terminates as the anus. The transition from sigmoid colon to rectum occurs at approximately the S3 vertebral level, where the rectum is covered anteriorly by parietal peritoneum and thus is a retroperitoneal structure.

Big Picture

The midgut and hindgut receive sympathetic innervation from the midthoracic to the upper lumbar spinal nerves, parasympathetic innervation from the vagus nerves and pelvic splanchnic nerves (S2–S4) (Figure 10-3).

Sympathetic Motor Innervation

Sympathetic motor innervation to the gastrointestinal tract decreases motility, peristalsis, sphincter muscle contraction, absorption, and glandular secretions, in addition to causing vasoconstriction. Sympathetic innervation to the gastrointestinal tract is accomplished via the following nerves:

• Greater splanchnic nerve. Carries preganglionic sympathetics originating from the T5–T9 level of the spinal cord that most likely synapse in the prevertebral plexus (celiac and superior mesenteric ganglia or plexuses).
• Lesser splanchnic nerve. Carries preganglionic sympathetics originating from the T10–T11 level of the spinal cord that most likely synapse in the prevertebral plexus (celiac and superior mesenteric ganglia or plexuses).
• Least splanchnic nerve. Carries preganglionic sympathetics from the T12 level of the spinal cord that most likely synapse in the prevertebral plexus (aorticorenal and inferior mesenteric ganglia or plexuses).
• Lumbar splanchnic nerve. Carries preganglionic sympathetics from the L1–L2 level of the spinal cord that most likely synapse in the prevertebral plexus (inferior mesenteric and inferior hypogastric ganglia or plexuses).

Parasympathetic Motor Innervation

Parasympathetic motor innervation increases motility, absorption, smooth muscle contraction, and glandular secretions. In addition, parasympathetic motor innervation relaxes the sphincter muscles. Parasympathetic innervation to the gastrointestinal tract is accomplished as follows:

• Midgut. Preganglionic parasympathetic fibers originating in the brainstem course in the vagus nerve (CN X) to the prevertebral plexus and accompany sympathetic fibers to regions of the midgut.
• Hindgut. Preganglionic parasympathetic fibers originating at the S2–S4 levels of the spinal cord are transported via the sacral splanchnic nerves to the prevertebral plexus (inferior hypogastric plexus). Here, they accompany sympathetic fibers to regions of the hindgut and urogenital systems.

Big Picture

The portal system is responsible for transporting blood from most of the gastrointestinal tract to the liver for metabolic processing before the blood returns to the heart. The portal system drains venous blood from the distal end of the esophagus, stomach, small and large intestines, proximal portion of the rectum, pancreas, and spleen. The portal system is the venous counterpart to areas supplied by the celiac trunk and the superior and inferior mesenteric arteries.

Blood Flow of the Portal System

The liver is unique in that it receives both nutrient-rich deoxygenated blood (portal vein) and oxygenated blood (hepatic arteries). The portal vein branches as it enters the liver, where its blood percolates around hepatocytes in tiny vascular channels known as sinusoids. Hepatocytes detoxify the blood, metabolize fats, carbohydrates, and drugs, and produce bile. The sinusoids receive deoxygenated blood from the portal veins (provide blood for metabolism and detoxification) and oxygenated blood from the hepatic arteries (provide oxygen for hepatocytes). Blood exits the sinusoids into a central vein, which empties into the hepatic veins and ultimately into the inferior vena cava, which passes through the diaphragm before entering the right atrium of the heart.

Veins of the Portal System

Veins of the portal system generally mirror the arterial branches of the celiac trunk and the superior and inferior mesenteric arteries (Figure 10-4A). The major veins of the portal system are as follows:

• Splenic vein. Drains blood from the foregut, including the spleen, pancreas, and part of the stomach. The splenic vein courses deep to the pancreas.
• Superior mesenteric vein. Drains blood from the midgut and part of the foregut. The superior mesenteric vein is located to the right of the superior mesenteric artery as it courses over the third part of the duodenum.
  • Gastro-omental veins. Drain blood from the greater curvature of the stomach into the superior mesenteric vein.
• Inferior mesenteric vein. Drains blood from the hindgut, including the proximal third of the rectum. The inferior mesenteric vein usually drains into the superior mesenteric vein, inferior to its union with the portal vein.
• Portal vein. Collects blood from the foregut, midgut, and hindgut. The portal vein is located deep to the hepatic artery and cystic duct and is formed by the union of the superior mesenteric vein and splenic vein, deep to the neck of the pancreas.
  • Gastric veins. Drain blood from the lesser curvature of the stomach into the portal vein.

Portal–Caval Anastomoses

To better understand the portal–caval anastomoses, recall that veins in the abdomen return blood to the heart via two routes (Figure 10-4B):

• Portal system. Veins from the foregut, midgut, and hindgut drain blood to the liver before the blood enters the inferior vena cava and ultimately returns to the heart.
• Caval system. Veins from the lower limbs, pelvis, and posterior abdominal wall transport blood directly to the inferior vena cava before the blood returns to the heart.

Portal–caval anastomoses occur at regions of the gastrointestinal tract that are drained by both the portal and systemic (-caval) systems. The principal portal–caval anastomoses are as follows:

1. Distal portion of the esophagus. The left gastric vein of the hepatic portal system drains blood from the distal portion of the esophagus. However, most of the blood drained from the esophagus is through the esophageal veins, which drain into the azygos (caval) vein.

2. Anterior abdominal wall. The paraumbilical veins drain the tissue surrounding the umbilicus: Embryologically, these veins communicated with the umbilical veins. These connections may reopen during chronic portal hypertension. Normally in the adult, most of the venous drainage is from the inferior epigastric veins.

3. Rectum. The proximal portion of the rectum is drained via the superior rectal vein, which drains into the inferior mesenteric vein of the hepatic portal system. However, the remainder of the rectum is drained by the middle rectal vein (branch of the internal iliac vein) and inferior rectal vein (branch of the internal pudendal vein).