Anatomy - Subtotal gastrectomy, type Billroth II with Roux-en-Y reconstruction

The stomach is, formally speaking, a dilation of the digestive tract located between the esophagus and the intestine, tasked with storing and mixing food. This muscular hollow organ produces acidic gastric juice (mucus and HCl) and enzymes that partially digest some components of food, subsequently transferring the chyme in portions to the small intestine.

The stomach is usually located in the left and middle upper abdomen directly beneath the diaphragm. The position, size, and shape of the stomach vary significantly from person to person and depending on age, state of fullness, and body position. When moderately filled, the stomach is on average 25-30 cm long and has a storage capacity of 1.5 liters, and in extreme cases, up to 2.5 liters.

The stomach is anchored and stabilized in the abdominal cavity by ligaments that extend to the liver and spleen, among others. It forms the greater curvature (greater gastric curvature/ Curvatura major) with its convex side and the lesser curvature (lesser gastric curvature/ Curvatura minor) with its concave side. Its anterior wall is referred to as Paries anterior, and its posterior wall as Paries posterior.

The stomach is intraperitoneal, thus covered by serosa, except for the dorsal cardia, which is free of serosa. The embryonic mesogastria move from their former sagittal position to a frontal one through gastric rotation: The lesser omentum extends from the lesser curvature to the hepatic portal, while the greater omentum spreads from the greater curvature to the transverse colon, spleen, and diaphragm.

The stomach can be divided into different sections:

Cardia / Ostium cardiacum:
The upper stomach entrance is an area of 1-2 cm where the esophagus opens into the stomach. Here, the sharp transition from the esophageal mucosa to the gastric mucosa can usually be well recognized with an endoscope.

Fundus / Fundus gastricus:
Above the stomach entrance, the fundus arches upwards, also known as the "gastric dome" or Fornix gastricus. The fundus is typically filled with air that is involuntarily swallowed while eating. In an upright person, the fundus forms the highest point of the stomach, so in an X-ray, the collected air appears as a "gastric bubble." Opposite the stomach entrance, the fundus is demarcated by a sharp fold (Incisura cardialis).

Body of the Stomach / Corpus gastricum:
The main part of the stomach is formed by the body of the stomach. Here, deep longitudinal mucosal folds (Plicae gastricae) extend from the stomach entrance to the pylorus, also referred to as the "gastric street."

Pylorus / Pars pylorica:
This section begins with the expanded pyloric antrum, followed by the pyloric canal (Canalis pyloricus), and ends with the actual pylorus. Here lies the pyloric sphincter muscle (M. sphincter pylori), formed by a strong circular muscle layer, which closes the lower stomach opening (Ostium pyloricum). The pylorus closes the stomach exit and periodically allows some chyme to pass into the subsequent duodenum.

Stomach Wall

Under the microscope, the stomach wall shows a characteristic layered structure from the inside out:

• Internally, the stomach wall is lined by mucosa (Tunica mucosa). The gastric mucosa is divided into three sublayers: The lamina epithelialis mucosae forms a tough neutral mucus that protects the gastric mucosa from mechanical, thermal, and enzymatic damage. Below it follows the lamina propria mucosae, which contains the gastric glands (Glandulae gastricae). Lastly, there is a narrow lamina muscularis mucosae that can alter the relief of the mucosa.
• The gastric mucosa is followed by a loose connective tissue layer (Tela submucosa), consisting of connective tissue in which a dense network of blood and lymph vessels runs, as well as a nerve plexus, the submucosal plexus (Meissner's plexus), which controls gastric secretion. This plexus operates independently of the central nervous system (CNS) but can be influenced by it through the autonomic nervous system.
• Next is a strong muscular layer (Tunica muscularis), divided into three sublayers with fibers running in different directions: an inner layer of small obliquely running muscle fibers (Fibrae obliquae), then a circular muscle layer (Stratum circulare), and an outer longitudinal muscle layer (Stratum longitudinale). This musculature is responsible for the peristalsis of the stomach, which ensures the constant mixing of the chyme with gastric juice. Between the circular and longitudinal muscle layers runs a nerve plexus, the myenteric plexus (Auerbach's plexus), which controls the function of the musculature. Like the submucosal plexus, this plexus operates largely autonomously but is influenced by the autonomic nervous system.
• This is followed by another connective tissue layer (Tela subserosa).
• The peritoneum as the abdominal lining (Tunica serosa) forms the conclusion.

Gastric Glands

The gastric glands (Glandulae gastricae) are located in the lamina propria mucosae and can be found in the fundus and corpus of the stomach. Up to 100 glands are located on 1 mm² of the mucosal surface. Various cells are located in the wall of the glandular tube:

• Mucous cells: They produce the same neutral mucus as the epithelial cells.
• Neck cells: These cells are located quite superficially in the gland and secrete alkaline mucus, i.e., the pH value is high due to the bicarbonate ions (OH- ions) contained in it. This property is important to control and, if necessary, regulate the pH value of the stomach. The mucus coats the gastric mucosa and thus protects against self-digestion by the aggressive hydrochloric acid (HCl) and enzymes as self-digesting proteins. This cell type is found predominantly in the cardia and fundus of the stomach.
• Chief cells: These cells produce the inactive precursor enzyme pepsinogen, which is converted into the active enzyme pepsin by hydrochloric acid (HCl) after release and is responsible for the digestion of dietary proteins. Since the enzyme only comes into contact with hydrochloric acid at the surface of the gland, self-digestion of the glands by the enzyme is prevented. This cell form is mainly located in the corpus of the stomach.
• Parietal cells: These cells, found predominantly in the gastric corpus, produce abundant hydrogen ions (H+ ions) needed for the formation of hydrochloric acid (HCl). Hydrochloric acid has a very low pH value of 0.9-1.5. Additionally, parietal cells produce the so-called intrinsic factor. This substance forms a complex with vitamin B12 from food in the intestine, which can then pass through the intestinal wall. This vitamin is of particular importance in erythropoiesis (removal of the stomach can lead to anemia).
• G-cells: These cells, preferably located in the antrum of the stomach, produce gastrin to increase HCl production in the parietal cells.

The stomach serves as a reservoir for ingested food. It can store food for hours, allowing us to meet our daily nutritional needs with a few larger meals. Through peristalsis, the chyme is mixed with gastric juice, the food is chemically broken down, partially digested, and then gradually passed into the duodenum.