148 The Stomach
Gross Anatomy of the Stomach
The stomach is divided into four sections, each of which has different cells and functions.
Learning Objectives
Describe the gross anatomy of the stomach
Key Takeaways
Key Points
- The stomach is divided into four sections: the cardiac region, the fundus, the body, and the pylorus or atrium.
- The stomach is lined by a mucous membrane that contains glands (with chief cells) that secrete gastric juices.
- Two smooth muscle valves, or sphincters, keep the contents of the stomach contained: the cardiac or esophageal sphincter and the pyloric sphincter.
- The arteries supplying the stomach are the left gastric, the right gastric, and the right gastroepiploic branches of the hepatic, and the left gastroepiploic and short gastric branches of the lineal.
- Lymphatics consist of a superficial and a deep set, and pass to the lymph glands found along the two curvatures of the organ.
- The nerves are the terminal branches of the right and left urethra and other parts; the former are distributed upon the back, and the latter upon the front, part of the organ.
Key Terms
- chief cell: A cell located in the stomach and parathyroid gland that secretes precursor enzymes.
- pylorus: The opening in a vertebrate, including humans, at the lower end of the stomach that opens into the duodenum.
- sphincter: A ringlike band of muscle that surrounds a bodily opening and constricts and relaxes as required for normal physiological functioning.
The stomach is a thick, walled organ that lies between the esophagus and the first part of the small intestine (the duodenum). It is on the left side of the abdominal cavity, the fundus of the stomach lying against the diaphragm. Lying beneath the stomach is the pancreas. The greater omentum hangs from the greater curvature.
A mucous membrane lines the stomach that contains the glands (with chief cells) that secrete gastric juices. Up to three quarts of this digestive fluid is produced daily. The gastric glands begin secreting before food enters the stomach due to the parasympathetic impulses of the vagus nerve, that also make the stomach a storage vat for that acid.
The stomach is divided into four sections, each of which has different cells and functions. The sections are:
- The cardiac region, where the contents of the esophagus empty into the stomach.
- The fundus, which is formed by the upper curvature of the organ.
- The body, the main central region.
- The pylorus or atrium, the lower section of the organ that facilitates the emptying of the contents into the small intestine.
Two smooth muscle valves, or sphincters, keep the contents of the stomach contained. They are the:
- Cardiac or esophageal sphincter that divides the tract above.
- Pyloric sphincter or pyloric orifice that divides the stomach from the small intestine.
The arteries that supply the stomach are the left gastric, the right gastric, and right gastroepiploic branches of the hepati;, and the left gastroepiploic and short gastric branches of the lineal. They supply the muscular coat, ramify in the submucous coat, and are finally distributed to the mucous membrane.
The arteries break up at the base of the gastric tubules into a plexus of fine capillaries that run upward between the tubules. They anatomize with each other and end in a plexus of larger capillaries that surround the mouths of the tubes and also form hexagonal meshes around the ducts.
From these the veins arise, and pursue a straight course downward, between the tubules, to the submucous tissue; they end in the lineal and superior mesenteric veins or directly in the portal vein.
The lymphatics are numerous. They consist of a superficial and a deep set, and pass to the lymph glands found along the two curvatures of the organ.
The nerves are the terminal branches of the right and left urethra and other parts, the former being distributed upon the back, and the latter upon the front part of the organ. A great number of branches from the celiac plexus of the sympathetic are also distributed to it.
Nerve plexuses are found in the submucous coat and between the layers of the muscular coat as in the intestine. Fibrils are distributed from these plexuses to the muscular tissue and the mucous membrane.
Microscopic Anatomy of the Stomach
The layers of the stomach produce mucus to protect itself, enyzmes to break down the food for digestion, and muscles to churn the food.
Learning Objectives
Diagram the microscopic anatomy of the stomach
Key Takeaways
Key Points
- The stomach walls are made of the following layers (inside to outside): mucosa, muscularis mucosa, submucosa, and muscularis externa.
- The epithelium of the stomach forms deep pits (fundic or oxyntic glands) where chief cells produce pepsinogen, an inactive precursor of pepsin that degrades proteins. The secretion of pepsinogen prevents self-digestion of the stomach cells.
- Gastric acid kills most of the bacteria in food, stimulates hunger, and activates pepsinogen into pepsin; it also denatures the complex protein molecule as a precursor to protein digestion.
- Goblet cells produce mucus that protects the stomach from self-digestion.
- The muscularis externa has three layers of smooth muscle. The innermost obliquely-oriented layer is responsible for creating the motion that churns and physically breaks down the food, and is unique to the stomach. The other layers are present as in other parts of the GI tract.
Key Terms
- muscularis externa: A region of muscle in many organs in the vertebrate body that is adjacent to the submucosa membrane. It is responsible for gut movements, such as peristalsis.
- muscularis mucosae: Also called the lamina muscularis mucosae, this is the thin layer of smooth muscle found in most parts of the gastrointestinal tract. It is located outside the lamina propria mucosae and separates it from the submucosa.
- submucosa: A layer of connective tissue beneath a mucous membrane.
Examples
If there is low or no gastric acid in the stomach, this could lead to problems as the disinfectant properties of the gastric lumen are decreased. In such conditions, there is a greater risk of infections in the digestive tract (such as infection with Vibrio or Helicobacter bacteria).
Anatomy of the Stomach
Like the other parts of the gastrointestinal tract, the stomach walls are made of a number of layers. From the inside to the outside, the first main layer is the mucosa.
Mucosa
The stomach wall: A micrograph that shows a cross section of the stomach wall, in the body portion of the stomach.
This consists of an epithelium, the lamina propria underneath, and a thin bit of smooth muscle called the muscularis mucosae. The submucosa lies under this and consists of fibrous connective tissue that separate the mucosa from the next layer, the muscularis externa.
The muscularis in the stomach differs from that of other GI organs in that it has three layers of muscle instead of two. Under these muscle layers is the adventitia, layers of connective tissue continuous with the omenta.
The epithelium of the stomach forms deep pits, called fundic or oxyntic glands. Different types of cells are at different locations down the pits. The cells at the base of these pits are chief cells that are responsible for the production of pepsinogen, an inactive precursor of pepsin, which degrades proteins. The secretion of pepsinogen prevents self-digestion of the stomach cells.
Further up the pits, parietal cells produce gastric acid and a vital substance, intrinsic factor. The function of gastric acid is two fold:
- It kills most of the bacteria in food, stimulates hunger, and activates pepsinogen into pepsin.
- It denatures the complex protein molecule as a precursor to protein digestion through enzyme action in the stomach and small intestines.
Near the top of the pits, closest to the contents of the stomach, there are mucus-producing cells called goblet cells that help protect the stomach from self-digestion.
The Muscularis Externa
The muscularis externa is made up of three layers of smooth muscle.
- The innermost layer is obliquely-oriented: this is not seen in other parts of the digestive system. This layer is responsible for creating the motion that churns and physically breaks down the food.
- The next layers are the square and then the longitudinal, which are present as in other parts of the GI tract.
- The pyloric antrum has thicker skin cells in its walls and performs more forceful contractions than the fundus. The pylorus is surrounded by a thick circular muscular wall that is normally tonically constricted, forming a functional (if not anatomically discrete) pyloric sphincter that controls the movement of chyme.
Digestive Properties of the Stomach
The movement and flow of chemicals into the stomach is controlled by the autonomic nervous system and various digestive system hormones.
Learning Objectives
List the various digestive properties of the stomach
Key Takeaways
Key Points
- Digestive hormones include: gastrin, somatostatin, cholecystokinin, secretin, gastric inhibitory peptide, and enteroglucagon.
- Gastrin causes an increase in the secretion of HCl from the parietal cells, an increase in pepsinogen from the chief cells in the stomach, and an increase in stomach motility.
- Somatostatin inhibits the activity of gastrin. Cholecystokinin (CCK) causes gall bladder contractions; it also decreases gastric emptying and increases the release of pancreatic juice that is alkaline and neutralizes the chyme. Secretin diminishes acid secretion in the stomach.
- Gastric inhibitory peptide (GIP) and enteroglucagon decrease gastric acid release and motility. They also turn off stomach action when the gall bladder and liver have not yet finished absorption.
- Epidermal growth factor (EGF), produced by the salivary glands, heals oral and gastroesophageal ulcers, inhibits gastric acid secretion, and stimulates DNA synthesis as well as mucosal protection.
- The stomach can sense, independently to the tongue and oral taste receptors, glucose, carbohydrates, proteins, fats, and sodium glutamate via the vagus nerve, which allows the brain to link the nutritional value of foods to their tastes.
Key Terms
- gastrin: A hormone that stimulates the production of gastric acid in the stomach.
- antrum: A bodily cavity, especially one having bony walls, especially in the sinuses.
- enteroglucagon: A peptide hormone derived from preproglucagon that is secreted from the mucosal cells, primarily of the colon and terminal ileum, that follows ingestion of a mixed meal to delay gastric emptying.
- somatostatin: A polypeptide hormone, secreted by the pancreas, that inhibits the production of certain other hormones.
Examples
Gastric ulcers can be healed by supplemental daily doses of epidermal growth factor (EGF). EGF inhibits bacterial colonization and speeds healing just as well as antibiotics.
Stomach Hormones
The movement and the flow of chemicals into the stomach are controlled by both the autonomic nervous system and the various digestive system hormones.
The hormone gastrin causes an increase in the secretion of HCl from the parietal cells and pepsinogen from the chief cells in the stomach. It also causes increased motility (movement, peristalsis) in the stomach.
Gastrin is released by G-cells in the stomach, via the base cells of the pyloric, cardiac, and fundic glands, in response to distension of the antrum, and digestive products (especially large quantities of incompletely digested proteins). It is inhibited by a pH normally less than four (high acid), as well as the hormone somatostatin.
Cholecystokinin (CCK) primarily effects the gall bladder and causes it to contract, but it also decreases gastric emptying and increases release of pancreatic juice, which is alkaline and neutralizes the chyme.
In a different and rare manner, secretin that is produced in the small intestine primarily effects the pancreas, but it will also diminish acid secretion in the stomach. Gastric inhibitory peptide (GIP) decreases both gastric acid release and motility. Enteroglucagon decreases both gastric acid and motility.
Other than gastrin, these hormones all act to turn off the stomach’s action. This is in response to food products in the liver and gall bladder that have not yet been absorbed. The stomach needs to push food into the small intestine only when it is not busy. While the intestine is full and still digesting food, the stomach will act as storage for food.
Epidermal Growth Factor
Epidermal growth factor (EGF) results in cellular proliferation, differentiation, and survival. EGF is a low-molecular-weight polypeptide that was first purified from a mouse’s submandibular gland, but has since been found in many human tissues including the submandibular and parotid glands.
Salivary EGF, which seems to be regulated by dietary inorganic iodine, plays an important physiological role in the maintenance of oroesophageal and gastric tissue integrity. The biological effects of salivary EGF include the healing of oral and gastroesophageal ulcers, the inhibition of gastric acid secretion, and the stimulation of DNA synthesis, as well as providing mucosal protection from intraluminal injurious factors—such as gastric acid, bile acids, pepsin, and trypsin, and to physical, chemical, and bacterial agents.
The stomach can taste sodium glutamate using glutamate receptors. This information is passed to the lateral hypothalamus and limbic system in the brain as a palatability signal through the vagus nerve. The stomach can also sense, independently from the tongue and oral taste receptors, glucose, carbohydrates, proteins, and fats. This allows the brain to link the nutritional value of foods to their tastes.
Although absorption is the primary function of the small intestine, some absorption of certain small molecules does nevertheless occur in the stomach through its lining. This includes water if the body is too dehydrated; medication, like aspirin; and amino acids (e.g., whey protein shake).
There are many different gastric glands that secrete many different chemicals. Parietal cells secrete hydrochloric acid and intrinsic factor
- Chief cells secrete pepsinogen.
- Goblet cells secrete mucus.
- Argentaffin cells secrete serotonin and histamine.
- G cells secrete the hormone gastrin.
