The stomach is located between the esophagus and the duodenum. It serves as a reservoir for mixing and primary digestion of incoming food. Several zones are distinguished in the stomach. The cardia is located near the esophageal opening, the bottom is located cranially in relation to the esophageal-gastric junction. 

The main part of the stomach is represented by a body, in which a shorter lesser curvature and a longer greater curvature are distinguished. The mucous membrane of the body and the fundus of the stomach forms folds, or ridges. The distal third of the stomach – antrum – smooth, without folds. Proximally, it is indicated by an angular incision, distally by a pylorus. The pyloric sphincter is formed by circular muscle fibers. Sometimes the fundus of the stomach hangs down at an angle to the body, and such a variant of the normal structure on X-ray examination has the characteristic appearance of a “stomach with an inflection”.  
  

The innervation of the stomach is carried out by parasympathetic vagus nerves and sympathetic fibers of the celiac plexus. The vagus nerve consists of the anterior and posterior trunks, the anterior trunk branches into the anterior parasympathetic plexus of the stomach, which innervates its anterior wall, and the hepatic plexus, which innervates the proximal duodenum. The posterior wall of the stomach receives innervation from the branches of the posterior trunk of the vagus nerve. 

In addition to afferent fibers , three types of efferent fibers pass through the vagus nerve: cholinergic, adrenergic, and non-adrenergic type I inhibitory fibers, which are involved in the regulation of secretion and motility. 

The mucous membrane of the fundus and body of the stomach is represented by a cylindrical epithelium of a regular shape, forming gastric fossae in small areas. At the bottom of these pits, the glands of the stomach open. The glands of the gastric body include two types of secretory cells: parietal (or acid-producing), hydrochloric acid and intrinsic Castle factor, and major pepsin-producing cells. These cells are absent in the mucus-secreting antral (pyloric) glands. 

The main amount of G-cells synthesizing and secreting gastrin is contained in the antrum, where they are scattered among the mucous cells. 

From the inside, the stomach is covered with a layer of mucus – a gel, consisting mainly of glycoproteins and mucopolysaccharides, secreted by the surface epithelium. This substance protects the gastric mucosa from physical damage, and also serves as a buffer to maintain normal hydrochloric acid levels, but its role in the regulation of stimulated acid secretion is negligible. In addition, the mucus layer is of particular importance for the colonization of Helicobacter pylori.  

The vagus nerve stimulates parietal cell acid production and gastrin release from G cells, both of which are mediated by acetylcholine. In addition, acetylcholine potentiates the effect of other stimulating agents on parietal cells. At the same time, gastrin is released under the direct influence of proteins and amino acids, as well as when the antrum is stretched. The effect of histamine on the H ₂ receptors of parietal cells leads to an increase in acid production, however, the main way of increasing acid secretion in the stomach is mediated by the K +, Na + -ATPase system. 

However, histamine and gastrin analog pentagastrin can also induce maximum acid production (this is the basis of the histamine-pentagastrin test to assess acid secretion capacity).  

The volume of an empty stomach is no more than 50 ml, but when food and liquid enters it, the body of the stomach relaxes, which is partially due to the action of ghrelin, and leads to a slight increase in intragastric pressure. The pacemaker, located in the fundus of the stomach, triggers peristalsis waves at a frequency of approximately three per minute, which facilitates the gradual advancement of the food bolus towards the antrum.  

Unlike the cardia , the gatekeeper gapes at rest, but closes only during peristalsis. The speed with which the food bolus enters the duodenum depends on its physical properties and chemical composition: solid food, hypertonic solutions and especially fats move more slowly than isotonic solutions. This fact is confirmed by scintigraphy, in which different isotopes are used for the liquid and solid phases. 

With an increase in pressure in the antrum, part of the stomach contents through the open gatekeeper enters the duodenum. The receptors of the duodenum transmit a reverse signal, as a result of which the gatekeeper closes, the pressure in the antrum rises and part of the food in it moves back into the body of the stomach. This antral “pump” or “press” mechanism ensures thorough mixing of gastric contents with digestive juices, and also controls gastric emptying.