The stomach is a muscular, hollow, dilated part of the digestive system which functions as an important organ of the digestive tract in many animals, including vertebrates, echinoderms, insects (mid-gut), and molluscs. It is involved in the second phase of digestion, following mastication (chewing).
In most vertebrates, the stomach is located between the esophagus and the small intestine. It secretes protein-digesting enzymes called proteases and gastric acid to aid in food digestion, (sent to it via esophageal peristalsis) through smooth muscular contractions before sending partially digested food (chyme) to the small intestines.
Structure
The stomach lies between the esophagus and the duodenum (the first part of the small intestine). It is in 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; the lower esophageal sphincter (found in the cardiac region), at the junction of the oesophagus and stomach, and the pyloric sphincter at the junction of the stomach with the duodenum.
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 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 gastric mucosa of the stomach 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. 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 (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)
- The outer longitudinal layer
The stomach also possesses a serosa, consisting of layers of connective tissue continuous with the peritoneum.
Different types of cells are found at the different layers of these glands:
Function
Digestion
Bolus (masticated food) enters the stomach through the esophagus via the lower 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 â€" reducing the volume of the fundus, before looping around the fundus 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 caseinogen (soluble) into casein (insoluble) thus curdling the milk. Pepsin breaks down proteins into polypeptides.
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
- 10â€"20% of ingested ethanol (e.g. from alcoholic beverages)
- Caffeine
The parietal cells of the stomach are responsible for producing intrinsic factor, which is necessary for the absorption of vitamin B12. B12 is used in cellular metabolism and is necessary for the production of red blood cells, and the functioning of the nervous system.
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.
Stomach acid
Epidermal growth factor (EGF) 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.
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. The former procedure 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 (στόμαχος), ultimately from stoma (στόμα), "mouth". The words gastro- and gastric (meaning related to the stomach) are both derived from the Greek word gaster (γαστήÏ, meaning "belly").
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