|The human stomach|
The stomach (from ancient Greek στόμαχος,
stomachos, stoma means mouth) is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital
digestive organ. In the digestive system the stomach is involved in the second phase .
In humans and many other animals, the stomach is located between the oesophagus and the small intestine. It secretes digestive enzymes and gastric acid to aid in food digestion. The pyloric sphincter controls the passage of partially digested food (chyme ) from the stomach into the duodenum where peristalsis takes over to move this through the rest of the intestines.
Sections of the human stomach
In humans, the stomach lies between the oesophagus 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 . A large double fold of visceral peritoneum called the greater omentum hangs down from the greater curvature of the stomach. Two sphincters keep the contents of the stomach contained; the
lower oesophageal 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 sympathetic (inhibitor) plexuses (networks of blood vessels and nerves in the anterior gastric, posterior ,
superior and inferior , celiac and myenteric), which regulate both the secretory activity of the stomach and the motor (motion) activity of its muscles.
In adult humans, the stomach has a relaxed, near empty volume of about 75 millilitres. 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 millilitres.
In classical anatomy, the human stomach is divided into four sections, beginning at the gastric cardia ,each of which has different cells and functions.
The cardia is where the contents of the oesophagus 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 oesophageal sphincter.
The fundus (from Latin, “bottom”) is formed by the upper curvature of the organ.
The body is the main, central region.
The pylorus (from Greek, “gatekeeper”) is the lower section of the organ that facilitates emptying the contents into the small intestine.
Schematic image of the blood supply to the human stomach: left and right gastric artery , left and right gastroepiploic artery and short gastric artery.
The lesser curvature of the human 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 the splenic artery.
Micrograph showing a cross section of the human stomach wall, in the body portion of the stomach.
Like the other parts of the gastrointestinal tract, the human stomach walls consist of an outer mucosa, 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 longitudinal and the middle circular layer and is responsible for the innervation of both (causing peristalsis and mixing)
The outer longitudinal layer is responsible for moving the bolus towards the pylorus of the stomach through muscular shortening.
The stomach also possesses a serosa, consisting of layers of connective tissue continuous with the peritoneum .