The stomach is a muscular, hollow, dilated part of the digestion system which functions
as an important organ of the digestive tract in some animals, including vertebrates, echinoderms,
insects, and molluscs.
It is involved in the second phase of digestion, following mastication.
The stomach is located between the esophagus and the small intestine.
It secretes protein-digesting enzymes called proteases and strong acids to aid in food
digestion, through smooth muscular contortions before sending partially digested food to
the small intestines.
Structure
The stomach lies between the esophagus and the duodenum.
It is on the left upper part of the abdominal cavity.
The top of the stomach lies against the diaphragm.
Lying behind the stomach is the pancreas.
The greater omentum hangs down from the greater curvature.
Two sphincters keep the contents of the stomach contained.
They are the esophageal sphincter dividing the tract above, and the pyloric sphincter
dividing the stomach from the small intestine.
The stomach is surrounded by parasympathetic and orthosympathetic plexuses, which regulate
both the secretions activity and the motor activity of its muscles.
In adult humans, the stomach has a relaxed, near empty volume of about 45 to 75 milliliters.
Because it is a distensible organ, it normally expands to hold about one litre of food.
The stomach of a newborn human baby will only be able to retain about 30 milliliters.
Sections In classical anatomy, the stomach is divided
into four sections, beginning at the cardia, each of which has different cells and functions.
The cardia is where the contents of the esophagus empty into the stomach.
The cardia is defined as the region following the "z-line" of the gastroesophageal junction,
the point at which the epithelium changes from stratified squamous to columnar.
Near the cardia is the lower esophageal sphincter.
The fundus is formed by the upper curvature of the organ.
The body is the main, central region.
The Pylorus is the lower section of the organ that facilitates emptying the contents into
the small intestine.
Blood supply
The lesser curvature of the stomach is supplied by the right gastric artery inferiorly, and
the left gastric artery superiorly, which also supplies the cardiac region.
The greater curvature is supplied by the right gastro-omental artery inferiorly and the left
gastro-omental artery superiorly.
The fundus of the stomach, and also the upper portion of the greater curvature, is supplied
by the short gastric artery which arises from the splenic artery.
Histology
Like the other parts of the gastrointestinal tract, the stomach walls consist of an outer
mucosa, and inner submucosa, muscularis externa, and serosa.
The mucosa of the stomach consists of the epithelium and the lamina propria, with a
thin layer of smooth muscle called the muscularis mucosae separating it from the submucosa beneath.
The submucosa lies under the mucosa and consists of fibrous connective tissue, separating the
mucosa from the next layer.
Meissner's plexus is in this layer.
The muscularis externa lies beneath the submucosa, and is unique from other organs of the gastrointestinal
tract, consisting of three layers: The inner oblique layer: This layer is responsible
for creating the motion that churns and physically breaks down the food.
It is the only layer of the three which is not seen in other parts of the digestive system.
The antrum has thicker skin cells in its walls and performs more forceful contractions than
the fundus.
The middle circular layer: At this layer, the pylorus is surrounded by a thick circular
muscular wall which is normally tonically constricted forming a functional pyloric sphincter,
which controls the movement of chyme into the duodenum.
This layer is concentric to the longitudinal axis of the stomach.
Auerbach's plexus is found between the outer longitundinal and the middle circular layer
and is responsible for the innervation of both
The outer longitudinal layer The stomach also possesses a serosa, consisting
of layers of connective tissue continuous with the peritoneum.
Glands Different types of cells are found at the
different layers of these glands:
Function Digestion
Bolus enters the stomach through the esophagus via the esophageal sphincter.
The stomach releases proteases and hydrochloric acid, which kills or inhibits bacteria and
provides the acidic pH of two for the proteases to work.
Food is churned by the stomach through muscular contractions of the wall called peristalsis
– reducing the volume of the fundus, before looping around the fundus and the body of
stomach as the boluses are converted into chyme.
Chyme slowly passes through the pyloric sphincter and into the duodenum of the small intestine,
where the extraction of nutrients begins.
Depending on the quantity and contents of the meal, the stomach will digest the food
into chyme anywhere between forty minutes and a few hours.
The average human stomach can comfortably hold about a liter of food.
Gastric juice in the stomach also contains pepsinogen and prorennin.
Hydrochloric acid activates these inactive forms of enzymes into active forms which are
pepsin and rennin.
Rennin digests the milk protein caseinogen into casein thus curdling the milk.
Pepsin breaks down proteins into polypeptides.
Control of secretion and motility The movement and the flow of chemicals into
the stomach are controlled by both the autonomic nervous system and by the various digestive
system hormones: Other than gastrin, these hormones all act
to turn off the stomach action.
This is in response to food products in the liver and gall bladder, which have not yet
been absorbed.
The stomach needs to push food into the small intestine only when the intestine is not busy.
While the intestine is full and still digesting food, the stomach acts as storage for food.
EGF in gastric defense Epidermal growth factor results in cellular
proliferation, differentiation, and survival.
EGF is a low-molecular-weight polypeptide first purified from the mouse submandibular
gland, but since then found in many human tissues including submandibular gland, parotid
gland.
Salivary EGF, which seems also regulated by dietary inorganic iodine, plays also an important
physiological role in the maintenance of oro-oesophageal and gastric tissue integrity.
The biological effects of salivary EGF include healing of oral and gastroesophageal ulcers,
inhibition of gastric acid secretion, stimulation of DNA synthesis as well as mucosal protection
from intraluminal injurious factors such as gastric acid, bile acids, pepsin, and trypsin
and to physical, chemical and bacterial agents.
Stomach as nutrition sensor The stomach can "taste" sodium glutamate using
glutamate receptors and 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 to tongue and oral taste receptors, glucose,
carbohydrates proteins, and fats.
This allows the brain to link nutritional value of foods to their tastes.
Absorption Although the absorption is mainly a function
of the small intestine, some absorption of certain small molecules nevertheless does
occur in the stomach through its lining.
This includes: Water, if the body is dehydrated
Medication, like aspirin Amino acids
10–20% of ingested ethanol Caffeine
Vitamin B12 absorption The parietal cells of the stomach are responsible
for producing Intrinsic Factor, which is necessary for the absorption of Vitamin B12.
Vitamin B12 is used in cellular metabolism and is necessary for the production of red
blood cells, and the functioning of the nervous system.
Clinical significance Diseases
A large number of studies have indicated that most cases of peptic ulcers, gastritis, and
stomach cancer are caused by Helicobacter pylori infection.
The stomach has to regenerate a new layer of mucus every two weeks, or else damage to
the epithelium may result.
Surgery Many bariatric surgery procedures involve
the stomach, in order to lose weight.
A gastric band may be placed around the cardia area, which can adjust to limit intake.
The anatomy of the stomach may be modified, or the stomach may be bypassed entirely.
Surgical removal of the stomach is called a Gastrectomy, and removal of the cardia area
is a called a "cardiactomy".
"Cardiectomy" is a term that is also used to describe removal of the heart.
This may be carried out because of gastric cancer or severe perforation of the stomach
wall.
History There were previously conflicting statements
in the academic anatomy community over whether the cardia is part of the stomach, part of
the esophagus or a distinct entity.
Modern surgical and medical textbooks have agreed that "The gastric cardia is now clearly
considered to be part of the stomach."
Etymology The word stomach is derived from the Latin
stomachus which is derived from the Greek word stomachos, ultimately from stoma, "mouth".
The words gastro- and gastric are both derived from the Greek word gaster.
Other animals
Although the precise shape and size of the stomach varies widely among different vertebrates,
the relative positions of the esophageal and duodenal openings remain relatively constant.
As a result, the organ always curves somewhat to the left before curving back to meet the
pyloric sphincter.
However, lampreys, hagfishes, chimaeras, lungfishes, and some teleost fish have no stomach at all,
with the esophagus opening directly into the intestine.
These animals all consume diets that either require little storage of food, or no pre-digestion
with gastric juices, or both.
The gastric lining is usually divided into two regions, an anterior portion lined by
fundic glands, and a posterior with pyloric glands.
Cardiac glands are unique to mammals, and even then are absent in a number of species.
The distributions of these glands vary between species, and do not always correspond with
the same regions as in man.
Furthermore, in many non-human mammals, a portion of the stomach anterior to the cardiac
glands is lined with epithelium essentially identical to that of the esophagus.
Ruminants, in particular, have a complex stomach, the first three chambers of which are all
lined with esophageal mucosa.
In birds and crocodilians, the stomach is divided into two regions.
Anteriorly is a narrow tubular region, the proventriculus, lined by fundic glands, and
connecting the true stomach to the crop.
Beyond lies the powerful muscular gizzard, lined by pyloric glands, and, in some species,
containing stones that the animal swallows to help grind up food.
Additional Images
See also Bariatric surgery, weight loss surgery
Foveolar cells, mucous producing cells of the stomach
Peristalsis, muscular movement that occurs in the gastrointestinal system, including
the stomach Borborygmus, growling of stomach
Digestion Gut flora
Gastroesophageal reflux disease Proton pump inhibitors
References
External links "Stomach" article from the Encyclopedia of
Nursing & Allied Health, from enotes.com Digestion of proteins in the stomach or tiyan
Site with details of how ruminants process food
Medical diagram of gastric blood supply