In
Anatomy , the (in ancient
Greek ''στόμαχος'') is an
Organ in the
Gastrointestinal Tract used to
Digest food. In general, the stomach's primary function is not the absorption of nutrients from digested food; this task is usually performed by the intestine. In most animals, the main job of the stomach is to break down large fat molecules into smaller ones, so that they can be absorbed into the intestines more easily.
Latin names for the stomach include ''Ventriculus'' and ''Gaster''; many medical terms related to the stomach start in "" or "'''gastric'''".
In
Human s, the stomach is a highly
Acid ic environment (maintained at pH 1.5 - 2 by the secretion of
Hydrochloric Acid ) with
Peptidase digestive
Enzyme s, primarily pepsin. Pepsinogen is secreted by chief cells of the stomach and the acidic environment activates pepsinogen to form
Pepsin . In fact, the stomach's interior can secrete 2 to 3 litres of gastric fluid per day.
The stomach 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 , and the
Greater Omentum hangs from the ''greater curvature''.
It is divided into five sections, each of which has different cells and functions. The
Gastric Juice , which is in the stomach, is highly acidic with a
PH of 1-3. Gastric acid may cause or compound damage to the stomach wall or its layer of mucus, causing a
Peptic Ulcer .
In humans, the stomach has a
Volume of about 50 ml when empty. When full, it can expand to hold about 1 litre of food.
- The arteries break up at the base of the Gastric Tubules into a plexus of fine Capillaries , which run upward between the tubules, Anastomosing with each other, and ending in a Plexus of larger capillaries, which surround the mouths of the tubes, and also form hexagonal meshes around the ducts.
- 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. From these plexuses Fibrils are distributed to the muscular tissue and the mucous membrane.
Like the other parts of the gastrointestinal tract, the stomach walls are made of a number of layers.
From inside to outside, the first main layer is the ''
Mucosa ''. This consists of an
Epithelium , the
Lamina Propria underneath, and a thin bit of
Smooth Muscle called the ''muscularis mucosa''.
The submucosa lies under this and consists of fibrous
Connective Tissue , separating 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 Cell s, responsible for 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 Cell s produce
Gastric Acid , which kills most of the bacteria in food, stimulates hunger, and activates pepsinogen into pepsin.
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'' 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 skin layers are the square and then the longituditinal, which are present as in other parts of the GI tract. Theoso antrum which 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 which is normally tonically constricted forming a functional (if not anatomically discrete) pyloric
Sphincter , which controls the movement of
Chyme into the
Duodenum .
The movement and the flow of chemicals into the stomach are controlled by both the
Autonomic Nervous System and by the various digestive system
Hormone s.
The hormone
Gastrin causes an increase in the secretion of HCl, pepsinogen and
Intrinsic Factor from
Parietal Cell s in the stomach. It also causes increased motility in the stomach. Gastrin is released by G-cells in the stomach to distenstion of the antrum, and digestive products. It is inhibited by a
PH normally less than 4 (high acid), as well as the hormone
Somatostatin .
Cholecystokinin (CCK) has most effect on the
Gall Bladder , but it also decreases gastric emptying. 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 (GIP) and
Enteroglucagon decrease 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.
This pattern is also present in the symbiotic control of the stomach.
In
Ruminant s, such as bovines, the stomach is a large multichamber organ in which enzymes reside. It hosts
Symbiotic Bacteria that produce
Enzyme s required for the digestion of
Cellulose from
Plant matters. The partially-digested plant matter pass through each of the intestines' chambers in sequence, being regurgitated or vomited and rechewed at least once in the process.
