In some
animals, including
vertebrates,
echinoderms,
insects (mid-gut) and
molluscs, the
stomach is a
muscular, hollow, dilated part of the
alimentary canal which functions as an important
organ of the digestive tract. It is involved in the second phase of
digestion, following
mastication (chewing). The stomach is located between the
oesophagus and the
small intestine. It secretes protein-digesting
enzymes and strong
acids to aid in food digestion, (sent to it via
oesophageal peristalsis) through
smooth muscular contortions (called segmentation) before sending partially digested food (
chyme) to the small intestines.
The word
stomach is derived from the
Latin stomachus which is derived from the
Greek word
stomachos, ultimately from
stoma (
στόμα), "mouth". The words
gastro- and
gastric (meaning related to the stomach) are both derived from the Greek word
gaster (
γαστήρ).
[edit] Role in digestion
Bolus (masticated food) enters the stomach through the
oesophagus via the
oesophageal 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 2 for the proteases to work. Food is churned by the stomach through muscular contractions of the wall - 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, 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 40 minutes and a few hours.
[edit] Anatomy of the stomach
The stomach lies between the
oesophagus 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 ml. Because it is a distensible organ, it normally expands to hold about 1 litre of food,
[4] but can hold as much as 2-3 litres. The stomach of a newborn human baby will only be able to retain about 30ml.
[edit] Sections
The stomach is divided into 4 sections, each of which has different cells and functions. The sections are:
| Cardia | Where the contents of the oesophagus 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. |
[edit] Blood supply
A more realistic image, showing the celiac artery and its branches; the liver has been raised, and the lesser omentum and anterior layer of the greater omentum removed.
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, are supplied by the
short gastric 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 over the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer. The Meissner's plexus is in this layer. |
| 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.
- outer longitudinal layer: Auerbach's plexus is found between this layer and the middle circular layer.
|
| serosa | This layer is over the muscularis externa, consisting of layers of connective tissue continuous with the peritoneum. |
Micrograph showing a cross section of the stomach wall, in the body portion of the stomach.
H&E stain.
Microscopic cross section of the pyloric part of the stomach wall.
[edit] Glands
The
epithelium of the stomach forms deep pits. The glands at these locations are named for the corresponding part of the stomach:
Different types of cells are found at the different layers of these glands:
[edit] 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 distenstion 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. |
| 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 only to push food into the small intestine when the intestine is not busy. While the intestine is full and still digesting food, the stomach acts as storage for food.
[edit] EGF in gastric defense
Epidermal growth factor or
EGF results in cellular proliferation, differentiation, and survival.
[6] 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.
[7]
[edit] Stomach as nutrition sensor
The stomach can "taste"
sodium glutamate using glutamate receptors
[8] and this information is passed to the
lateral hypothalamus and
limbic system in the
brain as a
palatability signal through the
vagus nerve.
[9] The stomach can also sense independently to tongue and oral taste receptors
glucose,
[10] carbohydrates[11] proteins,
[11] and
fats.
[12] This allows the brain to link
nutritional value of foods to their tastes.
[10]
[edit] Diseases of the stomach
Historically, it was widely believed that the highly acidic environment of the stomach would keep the stomach immune from
infection. However, a large number of studies have indicated that most cases of
peptic ulcers,
gastritis, and
stomach cancer are caused by
Helicobacter pylori infection.
[edit] In other animals
An
endoscopy of a normal stomach of a healthy 65-year-old woman.
Although the precise shape and size of the stomach varies widely among different vertebrates, the relative positions of the oesophageal 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 oesophagus 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.
[13]
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 oesophagus.
Ruminants, in particular, have a complex stomach, the first three chambers of which are all lined with oesophageal mucosa.
[13]
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.
[13]
[edit] See also