Updated: 03/13/08 12:17 PM
HOME HEAL EDUCATE RESEARCH DIRECTORY OUTREACH


Authors: C.E. Rubin and D.R. Saunders
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“Thought depends absolutely on the stomach, but in spite of that,
those who have the best stomachs are not the best thinkers”
Voltaire

A. Objectives
  1. Understand the anatomy of the stomach in its relation to the vagus nerve, gastric secretion, surgical resection, and the spread of gastric carcinoma.
  2. Understand the motor functions of the stomach so that abnormalities of gastric emptying can be described.
  3. Understand the autoregulation of gastric acid secretion.
  4. Know the contribution of the stomach to digestion and hematopoiesis.
  5. Have a good understanding of peptic ulcer disease so that logical therapy can be devised.
B. Structure

1. Gross Anatomy

Fixed at esophageal & pyloric ends; mobile in between.







Pyloric opening and closing 		The stomach begins at the distal end of the esophagus which enters the abdomen by piercing the diaphragm opposite the tenth thoracic vertebra (T10), to the left of the midline. The stomach is relatively fixed at its esophageal and its pyloric ends. Between these two positions the stomach is quite mobile; it usually lies entirely within the left upper quadrant of the abdomen, or, uncommonly, descends into the pelvis. It may be J-shaped, or steer-horn-shaped. It is divided into cardia, fundus, body and antrum. These are descriptive subdivisions which are used when discussing X-rays of the barium-filled stomach. The radiologist's fundus is not synonymous with the histologist's fundal gland area. The opening between the stomach and the duodenum is the pylorus which contains prominent bands of connective tissue partially separating gastric from duodenal muscle fibers. Unlike the LES, the pylorus is usually open; it is closed when a peristaltic wave reaches the distal antrum.



Figure 1

Relations of the Stomach:

The anterior and posterior surfaces of the stomach meet along the lesser curvature on the right, and along the greater curvature on the left.
  1. Directly behind the stomach, separated by peritoneal reflections, are:

    1. the crura of the diaphragm
    2. the spleen
    3. the left adrenal gland and kidney
    4. the body and tail of the pancreas

  2. Superior to the gastric fundus is the dome of the left diaphragm.


  3. Inferior to it one finds the coils of small bowel and the transverse colon with its attached greater omentum.


  4. Anteriorly over the most of the stomach is the lower rib cage and over the antrum and distal stomach is the anterior abdominal wall.
2. Blood Supply







Celiac
Common hepatic
Gastroduodenal
R gastroepiploic
Splenic
L gastroepiploic 		The arterial channels along the lesser curvature are supplied by the left gastric artery which arises directly from the celiac artery, and by the right gastric artery which arises from the hepatic artery.

The arterial channel along the greater curvature is formed by branches from the splenic artery and the gastroduodenal artery which courses posterior to the proximal duodenum after originating from the hepatic artery. The stomach is one of the most vascular organs in the body. Branches from arterial arcades on each of its mesenteric borders penetrate into the submucosa to form abundant anastomoses. The intramural vessels are interconnected so that they remain completely filled, even after all but one of the major gastric arteries have been ligated. At rest, about 25% of the cardiac output enters the splanchnic circulation - about 1500 ml per minute in a 70 Kg man; roughly 15% of this amount is distributed to the stomach.

Venous pattern is similar to arterial, then enters portal system. The venous drainage of the stomach corresponds to the arterial supply, but enters the portal system. An alternate pathway is available in the veins of the lower esophagus which drain upward into the azygos system and then to the superior vena cava.

Portal-Caval anastomoses allow alternate venous return routes. There is a rich lymphatic submucosal plexus which drains into a subserosal plexus, a network of channels just beneath the peritoneum. Four primary lymphatic pathways (which follow the gastric arteries) drain into celiac lymph nodes which connect with the para-aortic nodes.

3. Nerve Supply

Nerve supply = Vagus until mid-transverse colon.

90% of vagal fibers are afferent. 		The gastrointestinal tract, from esophagus to mid-transverse colon, is connected to the medulla by the vagus nerves. Left (anterior) and right (posterior) vagal trunks descend along the esophagus. About 90% of vagal fibers are afferent which relay messages from intestinal stretch receptors, osmoreceptors, etc.; it is unlikely that they transmit painful stimuli. Visceral sensation is carried by sympathetic pathways which enter the spinal cord through dorsal nerve roots (see section in Pain from Digestive Organs).

Pain afferents go through sympathetic trunk. The motor fibers in the vagi originate in the dorsal motor nucleus of the vagus and synapse with ganglion cells in the myenteric and submucosal plexuses or in exocrine glands. Acetylcholine is the best characterized neurotransmitter at these nerve endings.

4. Histologic Anatomy

Fundal & Pyloric Gland Areas The stomach has four concentric layers: the mucosa, the submucosa, the muscularis externa, and the serosa (Figure 2).

Mucous neck cells replace surface mucous cells.

Stomach lining replaced every~5 days.

Glandular cell population is very stable. 		For practical purposes, the human gastric mucosa is divided into the fundal gland area and the pyloric gland area. The surface of the entire stomach is covered throughout by mucus-producing cells which dip into pits. The mouths of the underlying pyloric or fundal glands open into the bottoms of these pits. Mucous neck cells are also found at the bottom of gastric pits; they are regenerative cells which divide and will replace the surface mucous cells as they move upward to the surface where they exfoliate. This process is rapid and averages five days; this rate is probably increased when there is an injury to be repaired. The population of glandular cells in the fundal and pyloric gland areas is very stable and their replacement is probably only accelerated when these glandular elements have to be replaced after injury.




Tightly packed fundal glands: upper part = parietal cells & lower part = chief cells.

Parietal cells are eosinophilic, have intracellular canaliculae, and produce HCl & intrinsic factor.

Intrinsic factor + Vit B12 are absorbed in terminal ileum. 		a. Fundal Gland Area:

Most of the thickness of the fundal gland area is occupied normally with tightly packed parallel glands. Parietal cells predominate in the upper portion of the gland. The parietal cell is roughly triangular in shape, has intracellular canaliculae and an eosinophilic cytoplasm which is filled with mitochondria. Parietal cells are the source of gastric HCl; the maximal rate of gastric acid secretion (which can be provoked by stimulation with histamine or pentagastrin) seems to correlate well with the number of parietal cells. In man, intrinsic factor is also produced by the parietal cells; it is combined with dietary vitamin B12 so that this vitamin can be absorbed in the terminal ileum. This vitamin is essential for normal red blood cell formation, and nervous system function.



Figure 2

Chief cells are columnar & basophilic; they secrete pepsinogen Chief cells usually predominate in the lower portion of the fundal glands. They are columnar in shape with a basally-located nucleus; the upper two-thirds of the cells is filled with basophilic zymogen (enzyme precursor) granules. The infranuclear basal portion of the cell has the stacks of rough endoplasmic reticulum by electron microscopy which are typical of a protein manufacturing cell. These cells secrete pepsinogens.

Interspersed among parietal and chief cells are entero-chromaffin-like (ECL) cells and mast cells which can stimulate acid and pepsinogen secretion by releasing histamine and peptides close to the exocrine cells.



Figure 3



Pyloric glands secrete mucus and gastrin 		b. Pyloric Gland Area (Figure 3):

Pyloric glands are present in the pyloric canal, in the antrum, and in a transitional area between the antrum and the body of the stomach. The major secretion from pyloric glands is mucus. The hormone, gastrin, is produced by triangular cells with clear cytoplasm (G cells) which are located in the upper one-third of the pyloric glands.

5. Microvascular Anatomy

The microvascular arrangement defends against luminal acid by carrying HCO3- to the surface mucous cells. Vascular casts of the stomach have revealed that arteries break up into capillaries at the submucosal aspect of the mucosa. The capillaries drain into venules just under the surface mucus cells. The venules, in turn, converge into collecting venules which pass directly to the submucosal venous plexus. Mucosal blood flow, therefore, seems to be unidirectional without arterio-venous shunts. This vascular arrangement may assist gastric mucosal defense against luminal acid. For each H+ secreted by a parietal cell into the canaliculus, an OH- is made which, with CO2, forms HCO3-. The HCO3- is then delivered via capillary blood from the parietal cells to the luminal surface where it diffuses into the overlying, mucous layer.


Figure 4




Next Section (C): Gastric Emptying »



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