19 The Gastrointestinal System – Foregut and Midgut

GASTROINTESTINAL SYSTEM

 Foregut and  Midgut 

Learning Objectives

By the end of the course students will be able to:

1. Identify the parts of the stomach and describe its spatial relationships to surrounding organs and mesenteries
2. Describe the blood supply of the GI tract via branches of the celiac, superior mesenteric and inferior mesenteric arteries, and the basic pattern of lymphatic drainage in this region.
3. Describe the anatomy of the foregut peritoneal ligaments, omenta and omental bursa, and their development from the embryological ventral and dorsal mesogastria
4. Describe the organization of the GI tract and its parts
5. Describe the organization of the autonomic innervation of the GI tract
6. Describe the basic anatomy of the large and small intestines, including blood supply and internal structure

 

Reference: Moore, Clinically Oriented Anatomy, chapter 2.

Particularly relevant Blue Boxes in Moore:

●Pylorospasm, Congenital Hypertrophic Pyloric Stenosis, p. 254

●Gastric Ulcers and Vagotomy, p. 256

●Visceral Referred Pain, 257

●Duodenal Ulcers, p. 257

●Appendicitis, p. 259

●Colonoscopy, p.260

●Diverticulosis, p. 261

 

 

 

To access the Netter Presenter Database click here

Grant’s Dissector, 15th Edition, pp 107 – 116

To access Gray’s Photographic Dissector section on the Gastrointestinal tract click here

 To access the Primal Pictures software click here 

Check out the Primal Pictures model of the Abdomen and Intestines

 

 

 

CONTENTS OF THE ABDOMINAL CAVITY

General Organizing Principal:

Contents within the abdominal cavity are divided into three segments based on embryological development. These segments include the Foregut, Midgut, and Hindgut.

1-Foregut

Contents: abdominal esophagus, stomach, duodenum (1^st and 2^nd parts), liver, pancreas, and spleen

Arterial supply: Celiac trunk

Venous drainage: Splenic vein and left gastric veins

Innervation: Thoracic Splanchnic Nerves (Sympathetic), Celiac Plexus via CNX (Parasympathetic)

2-Midgut

Contents: remaining duodenum (parts 3-4), jejunum, ileum, ascending colon, right 2/3 of transverse colon

Arterial supply: Superior Mesenteric Artery

Venous drainage: Superior mesenteric vein

Innervation: Thoracic splanchnic nerves (Sympathetic), Superior mesenteric plexus via CNX (Parasympathetic)

3-Hindgut

Contents: left 1/3 of transverse colon, descending colon, sigmoid colon, rectum

Arterial supply: Inferior Mesenteric Artery

Venous drainage: Inferior mesenteric vein

Innervation: Lumbar splanchnic nerves (Sympathetic), Pelvic Splanchnic nerves (Parasympathetic)

 

THE FOREGUT

The Esophagus (Netter 229, 230, 231, 233, 234, 235, 236, Moore 229-230)

Overall Function: Peristaltic passage of food to the stomach. Reverse peristalsis if necessary.

Structure: Beginning at the inferior border of the laryngopharynx and terminating with its entry to the stomach via the cardiac orifice, the esophagus is comprised of voluntary and involuntary portions. The first 2/3 are under voluntary control, and consists of two layers of striated muscle, inner circular and outer longitudinal, which surround the mucosa and submucosa. The last 1/3 is solely under involuntary control. The muscle fiber direction is the same, but they are smooth in nature.

Blood Supply: Esophageal arterial branches emerge from the thoracic aorta. Furthermore, the left gastric artery also provides some lower esophageal arterial supply. The esophageal venous plexus drains to the azygos/hemiazygos veins (see hepatic portocaval anastomoses below for significance).

Innervation: The Esophageal Nervous Plexus is formed from portions of the Vagus nerves (parasympathetic) as well as having contribution from the Sympatheic trunk

Lymphatic Drainage: Juxto-esophageal lymphatics drain directly to the thoracic duct.

Clinical Correlation: Esophagus and Pyrosis (GERD), Moore pg 254 Women may be at greater risk for hiatal hernia as they age because of increased kyphosis of the spine due to osteoporosis. However, this is not always problematic. Up to 60% of patients will have some degree of hiatal hernia by the age of 60. Many of them will be asymptomatic. A patient’s complaint of dysphagia (difficulty swallowing) must always be taken seriously by the physician. Often, this is the only symptom of esophageal cancer, an uncommon disease with a very low 5-year survival rate. Typically, the disease has already spread to other portions of the body by the time dysphagia is reported. Esophagitis refers to pain in the upper chest (precordium) and the epigastric area. Occasionally gastric contents reflux into the esophagus. Theh acidic peptic chyme burns and inflames the unprotected stratified squamous epithelium of the esophageal mucosa, producing regurgitative eophagitis – the uncomfortable sensation of “heartburn“. A hiatal hernia may produce discomfort and pain referred to the precordium and epigastrium. Herniation of the stomach through the esophageal hiatus produces a saclike dilation above the diaphragm. In ~90% of cases, the esophagus ends above the diaphragm (sliding haital hernia). In the remainig 10%, the fundus region of the stomach dissects along the esophagus through a defect in the esophageal hiatus to produce an intrathoracic sac (a paraesophageal hernia)

Stomach (Netter 269, 270) Moore 230 – 238

The stomach is a dilated portion of the gastro-intestinal tract. It lies inferior to the esophagus, which enters the abdominal cavity through an opening in the right crus of the diaphragm, and supero-lateral to the duodenum, which is the first 26 cm. of the small intestine. Contractions of the stomach along with the production of gastric enzymes, pepsin, and gastrin, produce a partially digested substance, chyme. The cardiac orifice is the esophageal opening into the stomach; there is no muscle sphincter at this opening which lies to the left of Tl0.

The stomach is divided into four regions for descriptive purposes. The fundus lies superior to the line connecting the cardiac notch to the greater curvature; it is usually full of gas. The body lies inferior to this line and superior to a line connecting the incisura angularis to the greater curvature (Netter 270). The pyloric antrum is the small area between the body and the pylorus. The pylorus is the muscular area made up of the pyloric sphincter and the pyloric canal. It lies to the right of Ll.

The stomach has the two muscle layers (Netter 270) seen in the rest of the gastro-intestinal tract, the circular and the longitudinal muscle layers, as well as having an additional innermost oblique muscle layer. The innermost muscle layer of the stomach has oblique fibers which run parallel to the lesser curvature and loop over the fundus.

The middle layer of muscle is composed of circular fibers. There is a thin circular muscle layer in the fundus and a thick circular muscle layer in the region of the pylorus which becomes the pyloric valve.

The outer muscular coat is the longitudinal layer.

Functionally, the stomach is divided into two regions: the superior region produces acid and pepsin, the inferior region produces alkali and gastrin.

The stomach is covered with peritoneum. Nerves and vessels serving the stomach course between the two layers of the lesser omentum and the gastro-colic ligament.

The stomach is innervated by parasympathetic branches of the vagus nerve; the left vagus nerve innervates the anterior stomach and the right vagus nerve innervates the posterior stomach (Netter 298, 299, 300). Sympathetic innervation is via the celiac plexus and the greater splanchnic nerves.

Circulation to the stomach arises from the CELIAC TRUNK (Netter 283). The celiac trunk (or artery) is about 1 cm. long and lies at the base of the diaphragm between the right and left crus. It gives off three branches: the left gastric artery, the splenic artery, and the common hepatic artery. The left gastric artery supplies the lower esophagus and the stomach along the lesser curvature.

The splenic artery gives off short gastric branches which supply the fundus, and it gives off the left gastro-epiploic artery which supplies the left stomach along the greater curvature.

The common hepatic artery usually gives off the right gastric artery and the gastroduodenal artery, and it becomes the hepatic artery proper.

 

Clinical Correlations: Gastritis. Excessive vagal activity may produce gastritis by excessively stimulating secretin production that activates acid-secreting glands. Substances that irritate the gastric mucosa such as aspirin and steroids, also produce gastritis Peptic ulcers are erosions in the lining of the stomach or duodenum. They are most commonly caused by an infection with a bacterium called Helicobacter pylori, but are also caused by stress, acid and pepsin. They occur most commonly in the pyloric region of the stomach (gastric ulcer) or the first part of the duodenum (duodenal ulcers).  Ulcers may produce severe bleeding, obstruction from edema or scarring and peritonitis from perforation. Erosion into a blood vessel results in massive hematemesis or intra-abdominal bleeding with complicating peritonitis. Pain from ulceration of the lower esophagus, stomach or superior duodenum is referred to the 5th and 6th dermatomes whichh include the epigastric region. Perofration of the stomach causes leakage of gastric contents, leading to peritonitis. The parietal peritoneum is innervated by branches of the intercostal and lumbar nerves, so intense peritoneal pain is perceived at the location of irritation. The gastrin secreting portion of the stomach has been implicated in the occurance of gastric ulcers (Moore Blue Box p. 256). Surgical removal of this portion of the stomach, a gastrectomy, is sometimes indicated in order to eliminate or cut down the amount of gastrin secreted. The stomach is then reconnected to the duodenum to re-establish the continuity of the digestive tract.

THE MIDGUT

The Small Intestines (Netter 271, 272, 287; Moore 239-246)

The small intestine is divided into three parts: the duodenum, the jejunum, and the ileum. The jejunum, the ileum and the first few centimeters of the duodenum have retained their mesenteries, but the rest of the duodenum lies behind the peritoneum as a result of the gut rotation during development.

The duodenum (Netter 271, 272) is a C (or G) shaped tube surrounding the head of the pancreas and is the shortest but widest part of the small intestine. It is retroperitnoeal except fo rthe beginning of the first part wich is connected to the liver by the hepatoduodenal ligament of the lesser omentum. It receives blood from the celiac (foregut) and superior mesenteric (midgut) arteries. It is divided into four parts:: ●the first part (superior portion) is 5 cm long and has a mobile section called the duodenal cap into which the pyloris invaginates. ●the second part (descending portion) is 8 cm. long and contains the junction of the foregut and midgut where the common bile and main pancreatic ducts open. It contains the major duodenal papilla, on which terminal openings of the bile and main pancreatic ducts are located, and the lesser papilla, which lies about 2 cm above the greater papilla and marks the site of entry of the accessory pancreatic duct (if there is one). ●the third part (horizontal portion) is 8 cm. long and crosses the IVC, aorta and vertebral column to the left ●the fourth part (ascending portion) is 5 cm. long and ascends to the left of the aorta to the level of the second lumbar vertebra and terminates in the duodenojejeunal junction, which is fixed in position by the suspensory ligament of Treitz (Netter 262), a surgical landmark. This fibromuscular band is attached to the right crus of the diaphragm..

 

The lesser omentum is attached to the first few centimeters of the duodenum along its superior border; the greater omentum is attached along its inferior border. The mucous membrane in the first part of the duodenum is smooth. Along the rest of the duodenum, the mucous membrane is thrown into folds, the plicae circulares, which continue throughout most of the small intestine. The small intestine tapers from the duodenum to the ileum so that the duodenum is the widest portion and the terminal ileum is the narrowest.

The duodenum is spatially related to some important structures: the aorta, the inferior vena cava, the renal vessels, the celiac trunk, the superior mesenteric artery, the splenic vessels, the common bile duct, the pancreas, the ascending colon, the transverse colon, the right kidney, and the right ureter.

The duodenum receives its blood supply from the gastro-duodenal artery which gives off duodenal branches in addition to its contribution to the anterior and posterior pancreatico-duodenal arcade which also supplies the duodenum.

The lymphatic drainage is along the route of the blood supply to the celiac nodes.

The jejunum (Netter 287) is the upper 2/5 of the small intestine excluding the duodenum. The ileum is the lower 3/5 of the small intestine. The jejunum has a greater diameter than the ileum and a thicker wall due to the large plicae circulares in the jejunum. The plicae circulares are few in the initial ileum and absent in the terminal ileum.

The mesentery of the small intestine is attached to the posterior wall in an oblique line which is approximately 15-20 cm. long and runs from the upper left quadrant to the lower right quadrant. Consequently, the jejunum typically occupies the upper left quadrant and the ileum the lower right quadrant.

Blood Supply

The blood supply (Netter 287) to the small intestine forms anastomosing arcades before the terminal end arteries of the superior mesenteric artery, the vasa rectae, reach the intestinal wall. The vasa rectae do not anastomose. They are relatively long in the jejunum when compared to the shorter vasa rectae of the ileum. The arcades of the jejunum usually form only a single or a double level of anastomoses, but the arcades of the ileum anastomose for several levels before terminating in the vasa rectae. Venous return is through the superior mesenteric vein, which drains into the portal system.

Mesenteric fat does not encroach upon the jejunum as it does upon the ileum. As a result, there is a “window” of mesentery along the mesenteric border of the jejunum. Along the ileum, this “window” area is obliterated by mesenteric fat.Lymphoid tissue, Peyer’s patches, are present on the mucous membrane along the border of the lower ileum opposite the mesentery. Peyer’s patches are not present in the jejunum or the upper ileum (this was probably discussed in CT-Bio).

The right side of the superior mesenteric artery gives off two branches which supply the large intestine. These branches will be discussed with the large intestine, but the terminal ileum receives some blood from one of these branches, the ileocolic artery.

Lymphatic drainage

Lymphatic drainage  (Netter 295) is to the pre-aortic nodes which surround the superior mesenteric artery (superior mesenteric nodes) as it exits from the aorta.

Innervation

Sympathetic innervation to the midgut  arises (mostly) from the lesser splanchnic nerves (T10 – T11) (Netter 301) and synapses in the superior mesenteric plexuses. Parasympathetic innervation is via the vagus nerve. Remember that the main functions of the autonomic system are “fight or flight” in nature and act in opposition to control blood flow and digestion.

Clinical Correlations: Mobilization of the duodenum. Because the blood supply is from the medial side, incision of the peritoneum along the right edge of the descending (2nd part) mobilizes the duodenum as well as the head of the pancreas. Metastatic routes. Because the venous and lymphatic channels may anastamose with those of the dorsal body wall, carcinoma of the duodenum and pancreas frequently is associated with a poor prognosis. Duodenal ulcers. Peptic ulceration in the duodenal cap is four times more frequent than gastric ulcers. Anomalies are not uncommon in the small intestine. Considering the movement of the small intestine during development, some anomalies might be predicted . A common anomaly is Meckel’s diverticulum (1%). Meckel’s diverticulum (Blue Box p. 258) is the persistence of the yolk stalk in the adult. Meckle’s diverticulum occurs in the ileum about 2-3 feet from its terminal end. It is usually asymptomatic, but it may become inflamed and give rise to appendicitis-like symptoms. Frequently, a Meckle’s diverticulum may contain tissue resembling pancreatic or gastric tissue (thus it may develop an “ulcer”). The embryological origin of a Meckle’s diverticulum will be discused in the GI Embryology lecture. An omphalocele occurs when the small intestine fails to be completely drawn into the abdomen after rotation. Consequently, a hernia filled with small bowel and covered with amniotic membrane protrudes out of the abdomen. A volvulus results from the incomplete rotation of the gut. The mesentery of the midgut does not retract and secure itself to the posterior wall, but instead, it remains extended. Thus, the midgut is loosely suspended by a long mesentery which may become twisted or tangled. Superior mesenteric artery obstruction is caused by a thrombus, an atherosclerosis, an aortic aneurysm, a tumor in the uncinate process of the pancreas, compression by the third part of the duodenum, or surgical scar tissue. The obstruction leads to small and large intestinal ischemia resulting in necrosis of all or part of the involved intestinal segment. Symptoms are abdominal pain, nausea, vomiting, diarrhea and electrolyte imbalance. Small intestine obstruction is caused by postoperative adhesions, tumors, Crohn’s disease, hernias, peritonitis, gallstones, volvulus, congenital malrotation, stricture and intussusception (invagination of one part of the intestine into another). Strangulated obstructions are surgical emergencies and may cause death if intreated because the arterial occlusion leads to bowel ischemia and necrosis. Symptoms include colicky abdominal pain and cramping, nausea and vomiting, abdominal distention and high-pitched bowel sounds. Inflammatory bowel disease involves the large or small intestine or both and includes Crohn’s disease and ulcerative colitis.

Superior mesenteric artery (SMA) syndrome is a very rare, life-threatening gastro-vascular disorder characterized by a compression of the third portion of the duodenum by the  abdominal aorta and the overlyingsuperior mesenteric artery. The syndrome is typically caused by an angle of 6°-25° between the AA and the SMA, in comparison to the normal range of 38°-56°, due to a lack of retroperitoneal and visceral fat.