Gastric secretion

For other uses, see Stomach (disambiguation).
"Gastric" redirects here. For the sauce flavoring, see Gastrique.
Stomach
The stomach is located in the centre of the human body.
Diagram from Work of the United States Government
Latin Ventriculus (Greek: Gaster)
Gray's subject #247 1161
System Digestive system
Artery Right gastric artery, left gastric artery, right gastro-omental artery, left gastro-omental artery, short gastric arteries
Vein Right gastric vein, left gastric vein, right gastro-omental vein, left gastro-omental vein, short gastric veins
Nerve Celiac ganglia, vagus[1]
Lymph Celiac lymph nodes[2]
Precursor Foregut
MeSH Stomach
Dorlands/Elsevier Stomach

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 (mid-gut), and molluscs. It is involved in the second phase of digestion, following mastication (chewing).

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, (sent to it via esophageal peristalsis) through smooth muscular contortions (called segmentation) before sending partially digested food (chyme) to the small intestines.

Etymology

The word stomach is derived from the Latin stomachus which is derived from the Greek word stomachos (στόμαχος), ultimately from stoma (στόμα), "mouth". The words gastro- and gastric (meaning related to the stomach) are both derived from the Greek word gaster (γαστήρ).

Role in digestion

Bolus (masticated food) enters the stomach through the esophagus via the esophageal sphincter. The stomach releases proteases (protein-digesting enzymes such as pepsin) 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[3] and the body of stomach as the boluses are converted into chyme (partially digested food). 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 (proteases). Rennin digests the milk protein caesinogen (soluble) into caesin (insoluble) thus curdling the milk. Pepsin breaks down proteins into polypeptides.

Anatomy of the stomach

The stomach lies between the esophagus and the duodenum (the first part of the small intestine). 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 (found in the cardiac region, not an anatomical sphincter) dividing the tract above, and the pyloric sphincter dividing the stomach from the small intestine.

The stomach is surrounded by parasympathetic (stimulant) and orthosympathetic (inhibitor) plexuses (networks of blood vessels and nerves in the anterior gastric, posterior, superior and inferior, celiac and myenteric), which regulate both the secretions activity and the motor (motion) activity of its muscles.

In adult humans, the stomach has a relaxed, near empty volume of about 45 to 75 ml.[4] Because it is a distensible organ, it normally expands to hold about one litre of food,[5] but can hold as much as two to three litres. The stomach of a newborn human baby will only be able to retain about 30 ml.

Sections

The stomach is divided into four sections, each of which has different cells and functions. The sections are:

Cardia Where the contents of the esophagus empty into the stomach.
Fundus Formed by the upper curvature of the organ.
Body or Corpus The main, central region.
Pylorus 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 gastroepiploic artery inferiorly and the left gastroepiploic 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 splenic artery.

Like the other parts of the gastrointestinal tract, the stomach walls are made of the following layers, from inside to outside:

mucosa The first main layer. This consists of the epithelium and the lamina propria (composed of loose connective tissue), with a thin layer of smooth muscle called the muscularis mucosae separating it from the submucosa beneath.
submucosa This layer lies under the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer. The Meissner's plexus is in this layer (AKA submucosal plexus).
muscularis externa

Over the submucosa, the muscularis externa in the stomach differs from that of other GI organs in that it has three layers of smooth muscle instead of two.

  • 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.
  • middle circular layer: At this layer, the pylorus is surrounded by a thick circular muscular wall which is normally tonically constricted forming a functional (if not anatomically discrete) 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 (AKA myenteric plexus) is found between the outer longitundinal and the middle circular layer and is responsible for the innervation of both (causing peristalsis and mixing)
  • outer longitudinal layer
serosa This layer is over the muscularis externa, consisting of layers of connective tissue continuous with the peritoneum.


Glands

Cardiac glands
(at cardia)
Pyloric glands
(at pylorus)
Fundic glands
(at fundus)

Different types of cells are found at the different layers of these glands:

Layer of stomach Name Secretion Region of stomach Staining
Isthmus of gland Mucous neck cells mucus gel layer Fundic, cardiac, pyloric Clear
Body of gland parietal (oxyntic) cells gastric acid and intrinsic factor Fundic only Acidophilic
Base of gland chief (zymogenic) cells pepsinogen Fundic only Basophilic
Base of gland enteroendocrine (APUD) cells hormones gastrin, histamine, endorphins, serotonin, cholecystokinin and somatostatin Fundic, cardiac, pyloric

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:

Gastrin The hormone gastrin causes an increase in the secretion of HCl from the parietal cells, and pepsinogen from chief cells in the stomach. It also causes increased motility in the stomach. Gastrin is released by G-cells in the stomach 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 4 (high acid), as well as the hormone somatostatin.
Cholecystokinin Cholecystokinin (CCK) has most effect on the gall bladder, causing gall bladder contractions, but it also decreases gastric emptying and increases release of pancreatic juice which is alkaline and neutralizes the chyme. CCK is synthesized by I-cells in the mucosal epithelium of the small intestine.
Secretin In a different and rare manner, secretin, produced in the small intestine, has most effects on the pancreas, but will also diminish acid secretion in the stomach.
Gastric inhibitory peptide Gastric inhibitory peptide (GIP) decreases both gastric acid release and motility.
Enteroglucagon enteroglucagon decreases both gastric acid and motility.

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 (EGF) results in cellular proliferation, differentiation, and survival.[7] 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.[8]

Stomach as nutrition sensor

The stomach can "taste" sodium glutamate using glutamate receptors[9] and this information is passed to the lateral hypothalamus and limbic system in the brain as a palatability signal through the vagus nerve.[10] The stomach can also sense independently to tongue and oral taste receptors glucose,[11] carbohydrates[12] proteins,[12] and fats.[13] This allows the brain to link nutritional value of foods to their tastes.[11]

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:

  • 10–20% of ingested ethanol (e.g. from alcoholic beverages)[14]
  • Caffeine

Diseases of the stomach

Main article: Stomach disease

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.

In 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.[15]

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.[15]

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.[15]

Additional Images

See also

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