Chapter 9. Foregut

The foregut consists of the distal end of the esophagus, the stomach, and a portion of the duodenum. In addition, the pancreas, liver, and gallbladder form embryologically from the foregut and thus also are included in this discussion. The celiac trunk is the principal (but not exclusive) artery supplying the foregut. The celiac trunk arises from the abdominal aorta.

Esophagus

The distal end of the esophagus enters the abdominal cavity in the upper left quadrant by traversing the diaphragm at the T10 vertebral level. The esophagus immediately transitions into the stomach, with the cardiac sphincter serving as the transition boundary. Coursing parallel to the esophagus are the anterior and posterior vagal trunks. The vagal trunks consist of visceral motor and sensory parasympathetic fibers from the left and right vagus nerves, respectively.

Stomach

The stomach is a dilated, J-shaped portion of the foregut, juxtaposed between the esophagus and the duodenum (Figure 9-1A and B). The stomach is located in the upper left quadrant of the abdomen, with the spleen, pancreas, and aorta located deep to the stomach body.

Gastric secretions are churned in the stomach, with food, into a semifluid mixture (chyme) that is eventually transported from the stomach into the duodenum.

The stomach is partitioned into the following four regions:

• Cardia. Surrounds the gastroesophageal opening.
• Fundus. Dome-shaped region superior to the cardia.
• Body. Largest region of the stomach that consists of a lesser curvature and a greater curvature, where the lesser omentum and greater omentum attach, respectively.
• Pylorus. Distal end of the stomach containing the pyloric sphincter, which is located in the transpyloric plane at the L1 vertebral level.

When food enters the stomach, it begins to expand and stretch, resulting in a vagovagal reflex. Visceral sensory neurons from the vagus nerve [cranial nerve (CN) X] relay the stretching of the stomach to the brainstem. In response, the brainstem relays impulses via the vagus nerve, inhibiting the tone of the muscularis externa (smooth muscle of the stomach). In this way, the wall of the stomach progressively expands to accommodate greater quantities of food. The stomach can hold up to 1.5 L of food before pressure within the stomach lumen increases.

Duodenum

The duodenum is approximately 25-cm long and curves around the pancreatic head. It is divided into the following four parts (Figure 9-1A and B):

• Part one (superior). The stomach is an intraperitoneal organ as is the first part of the duodenum. The first part of the duodenum contains the duodenal cap, a dilation of the proximal duodenum that is easily identifiable on radiographs. In contrast, the second part of the duodenum courses deep to the parietal peritoneum and thus is retroperitoneal, as are the third and fourth parts of the duodenum. However, the distal portion of the fourth part is transitional, from retroperitoneal to intraperitoneal in the region of the duodenojejunal junction.
• Part two (descending). Courses deep to the transverse colon and anterior to the right kidney. The common bile duct enters its posterior wall. Within the duodenal wall, the common bile duct receives the main pancreatic duct (of Wirsung). Immediately after the junction, there is an enlargement called the major duodenal papilla (ampulla of Vater). The papilla is surrounded by smooth muscle called the sphincter of Oddi. An accessory pancreatic duct (of Santorini) may enter the duodenum proximal to the main pancreatic duct.
• Part three (horizontal). Turns left and courses horizontally across the inferior vena cava, the aorta, and the vertebral column. In addition, the superior mesenteric artery and vein course anteriorly to the third part of the duodenum.
• Part four (ascending). Ascends anterior to the aorta at the L2 vertebral level.

The duodenum receives its blood supply from branches of both the celiac trunk (superior pancreaticoduodenal arteries) and the superior mesenteric trunk (inferior pancreaticoduodenal arteries).

Big Picture

In addition to its numerous metabolic activities, the liver secretes bile. Bile is transported to the gallbladder, where it is stored. When food reaches the duodenum, the gallbladder releases bile, which emulsifies fat in the duodenum.

Liver

The liver produces bile, which emulsifies fat. The liver is also involved in cholesterol metabolism, the urea cycle, protein production, clotting factor production, detoxification, phagocytosis via the Kupffer cells lining the sinusoids, and receiving blood from the portal vein and hepatic artery.

The liver is attached to the inferior surface of the right dome of the diaphragm via the coronary ligaments. The bare area of the liver is a region devoid of peritoneum between the coronary ligaments and, therefore, lies in direct contact with the diaphragm. The falciform ligament is a peritoneal structure that courses between the left and right lobes of the liver and the anterior abdominal wall. The ligamentum teres is within the falciform ligament and is the embryonic remnant of the ductus venosus of the umbilical cord. The four lobes of the liver are as follows (Figure 9-2A and C):

• Right lobe. Positioned to the right of the inferior vena cava and gallbladder.
• Left lobe. Positioned to the left ligamentum teres.
• Quadrate lobe. Positioned posterior to the portal triad.
• Caudate lobe. Positioned anterior to the portal triad.

Functionally, the quadrate and caudate lobes are part of the left lobe because they are supplied by the left hepatic artery, drained by the left branch of the portal vein, and deliver bile via the left bile duct.

Portal Triad

The portal triad lies between the caudate and quadrate lobes and is the structural unit of the liver (Figure 9-2B and C). The portal triad consists of the portal vein, proper hepatic artery, and the common hepatic duct. The portal vein is deep to the hepatic artery and the common hepatic duct.

• Proper hepatic artery. Branches from the celiac trunk via the common hepatic artery. The hepatic artery supplies oxygenated blood to the liver. The cystic artery arises from the hepatic artery to supply the gallbladder.
• Portal vein. Formed through the union of the splenic and superior mesenteric veins, deep to the pancreas. The portal vein collects nutrient-rich venous blood from the small and large intestines, where it is transported to the hepatic sinusoids of the liver for filtration and detoxification. The hepatic sinusoids empty into the common central vein, which empties into the hepatic veins and ultimately drains into the inferior vena cava. The flow of blood from one capillary bed (intestinal capillaries) through a second capillary bed (liver sinusoids) before its return by systemic veins to the heart is defined as the hepatic portal system.
• Common hepatic duct. The union of the left and right hepatic ducts forms the common hepatic duct. The common hepatic duct transmits bile produced in the liver to the gallbladder for storage.

Gallbladder

The gallbladder lies on the visceral surface of the liver, to the right of the quadrate lobe, and stores and concentrates bile secreted by the liver (Figure 9-2A–C). Bile is released into the duodenum when the gallbladder is stimulated after eating a fatty meal. Bile enters the cystic duct, which joins the common hepatic duct, becoming the common bile duct. The common bile duct courses within the hepatoduodenal ligament of the lesser omentum, deep to the first part of the duodenum, where it joins the main pancreatic duct. Together, the common bile duct and the main pancreatic duct enter the second part of the duodenum at the hepatopancreatic ampulla (of Vater). The sphincter of Oddi surrounds the ampulla and controls the flow of bile and pancreatic digestive enzyme secretions into the duodenum.

Big Picture

The pancreas is an organ of dual function: exocrine secretion for digestion and endocrine function for the regulation of glucose metabolism. By comparison, the spleen contributes to the formation of blood cells during fetal and early postnatal life and is involved in the development of immune cells (lymphocytes) and the clearance of red blood cells from the blood (tissue macrophages).

Pancreas

The pancreas is a retroperitoneal organ located at the L2 vertebral level and contains the following structures (Figure 9-3A and B):

• Head and neck. Both parts are nestled within the concavity of the duodenum. The head and neck receive their blood supply from the superior pancreaticoduodenal arteries (celiac trunk) and the inferior pancreaticoduodenal arteries (superior mesenteric artery).
• Body and tail. Both parts are located anterior to the left kidney, with the tail touching the spleen. The body and tail receive their blood supply from the splenic artery (celiac trunk) and pancreatic branches from the superior mesenteric artery. The end of the pancreatic tale, where it touches the spleen, is the only portion of the pancreas that is not retroperitoneal.

The pancreas is a gland that consists of both exocrine tissue (a gland that produces and secretes products locally) and endocrine tissue (a gland that produces and secretes hormones into the blood).

• Pancreas as an exocrine organ. The pancreas produces enzymes that chemically digest carbohydrates, proteins, and fats. These enzymes are produced in the pancreatic exocrine glands and are secreted into the main pancreatic duct, which joins the common bile duct within the wall of the duodenum. An accessory pancreatic duct may open separately into the duodenum, proximal to the common bile duct.
• Pancreas as an endocrine organ. Endocrine tissue islands, called pancreatic islets (of Langerhans), are found within the pancreas and produce the hormones insulin and glucagon. These hormones are secreted into the blood stream via the pancreatic vein, where they are transported by the blood to distant cellular targets.

Innervation most likely plays a limited role in the digestive process. Parasympathetic stimulation of the vagus nerve may increase exocrine digestive secretions. Sympathetic input (T5–T9 spinal cord levels via the greater splanchnic nerve) increases the tone of smooth muscle cells on the neck of the secretory units, thus inhibiting the release of digestive enzymes.

Spleen

The spleen is located in the left upper quadrant of the abdomen, between the stomach and the diaphragm (Figure 9-3B). The spleen is the size of a fist and stores blood, phagocytizes foreign blood particles, and produces mononuclear leukocytes. It also maintains “quality control” over erythrocytes by the removal of senescent and defective red blood cells. The spleen receives its blood supply from the splenic artery (via the celiac trunk) and venous drainage through the portal vein.

Big Picture

Knowledge of the three unpaired arterial trunks that arise from the anterior surface of the abdominal aorta will facilitate in the understanding of the three subdivisions of the gastrointestinal tract. Each subdivision receives a primary, but not sole, arterial supply: foregut–celiac trunk, midgut–superior mesenteric artery, and hindgut–inferior mesenteric artery. The foregut organs are the stomach, the first half of the duodenum, and the liver, gallbladder, pancreas, and spleen.

Arterial Supply

The principal blood supply to the organs of the foregut is the celiac trunk. The celiac trunk is an unpaired artery arising from the abdominal aorta, immediately below the aortic hiatus of the diaphragm at the T12 vertebral level. The celiac trunk divides into the left gastric, splenic, and common hepatic arteries (Figure 9-4A).

• Left gastric artery. The smallest branch of the celiac trunk. The left gastric artery provides blood supply to the lesser curvature of the stomach and the inferior portion of the esophagus.
• Splenic artery. The largest branch of the celiac trunk. The splenic artery provides blood to the spleen, the greater curvature of the stomach (left gastroomental branch), the fundus (short gastric branches), and the pancreas (pancreatic branches). The splenic artery follows a highly tortuous course along the superior border of the pancreas.
• Common hepatic artery. Divides into the proper hepatic artery, the gastroduodenal artery, and the right gastric artery.
  • Proper hepatic artery. Ascends in the free edge of the lesser omentum and divides near the portal triad into the left and right hepatic arteries. The right hepatic artery gives rise to the cystic artery, supplying the gallbladder.
  • Right gastric artery. Supplies the lesser curvature of the stomach and usually forms an anastomosis with the left gastric artery.
  • Gastroduodenal artery. Descends deep to the first part of the duodenum, giving rise to the following two principal branches:
    • Right gastroomental artery. Supplies the right half of the greater curvature of the stomach.
    • Superior pancreaticoduodenal artery. Courses between the duodenum and the head of the pancreas to further split into anterior and posterior divisions.

Venous Drainage

The principal venous drainage of the foregut is to the portal venous system (Figure 9-4B). The portal venous system drains nutrient-rich venous blood from the gastrointestinal tract and the spleen to the liver. Three collecting veins converge to form the portal vein: the splenic vein and the superior mesenteric and inferior mesenteric veins. Portal venous blood flows to the liver, where nutrients are metabolized. The metabolic products are collected in central veins, which are tributaries of the hepatic veins. The hepatic veins emerge from the liver to drain into the inferior vena cava. The portal and hepatic veins are connected within the liver, at the liver lobules; thus, the combined venous system is called the hepatic portal system of veins.