Regions of the Stomach

The stomach is an elongated sac-shaped organ located between the esophagus and the duodenum. About 25 cm long, it has a capacity of 1.5-2 liters.

The stomach is anatomically divided into the following regions:

• the fundus, located above and to the left of the junction between the esophagus and the stomach (esophagogastric);
• the cardia, corresponding to the gastroesophageal junction;
• the body, which represents the largest portion of the stomach and is located between the fundus and the antrum;
• the antrum, the final portion of the stomach, which extends from the lesser curvature to the pylorus;
• the pylorus, which represents the border between the stomach and the duodenum.

Functions of the stomach

The stomach performs numerous and important functions. First of all, it acts as a real reservoir for the bolus that arrives from the esophagus, storing it until it is completely attacked by gastric juices. The storage capacity is entrusted to the regions of the fundus and the body, where the nutritious material remains for about 1-3 hours, in relation to the quantity and quality of the food ingested.

In this area of the stomach the bolus, waiting to be pushed into the intestine, is attacked by gastric juice.

Gastric juice

Viscous fluid produced by gastric glands located along the gastric wall and body.

Like all gastrointestinal secretions, gastric juice is mostly made up of water (about 97%). The abundant presence of liquids is necessary to dilute the bolus, which from a semi-solid mass becomes a dense broth, called chyme.

In addition to water, gastric juice also contains digestive enzymes that, since they all perform the same function, take on a singular name (pepsinogen). Other peptides are also included in the composition of gastric secretion, such as mucoproteins, intrinsic factor and an enzyme called lipase.

The amount of gastric juice secreted in 24 hours amounts to approximately three liters.

Hydrochloric Acid, Pepsin and Protein Digestion

Hydrochloric acid activates pepsinogen to pepsin.

Pepsinogen is a set of enzymes secreted into the lumen of the stomach. These are produced in the form of inactive precursors that, in order to fully perform their digestive function, must be activated to pepsin.

pepsinogen = inactive form pepsin = active enzyme.

Proteolytic enzymes must necessarily be secreted in an inactive form because, if this were not the case, they would digest the same cells that produced and stored them. The function of pepsin is in fact to begin the digestion of food proteins.

Hydrochloric acid, in addition to activating pepsin, creates environmental conditions favorable to its action. Remember that each enzyme works at an optimal pH, which in the case of pepsin is particularly low (2-3).

Hydrochloric acid is an excellent defense against germs introduced with food, which are largely inactivated by the strong acidity. Its antiseptic properties had already been tested by Spallanzani in the seventeenth century who, after having immersed fragments of meat in gastric juice, noted a delay in the putrefaction processes.

Thanks to the presence of hydrochloric acid, gastric juice is able to digest particularly resistant cells and tissues, such as connective tissue. This substance, present especially in animal muscles, is particularly difficult to digest, since it is made up of very resistant proteins such as collagen.

Hydrochloric acid denatures proteins, making them easier to digest. Most proteins, including those consumed with food, are in a tertiary structure. In this form, the amino acid chains are coiled up to form a sort of ball. Denaturing means breaking the internal bonds that keep the protein in this globular configuration. In practical terms, proteins introduced with food are “unrolled” thanks to the presence of hydrochloric acid. The activity of the enzymes responsible for digesting proteins, and therefore for detaching individual amino acids (pepsin), is thus significantly facilitated.

Intrinsic factor

Glycoprotein secreted by the gastric mucosa which, by binding vitamin B12 introduced with the diet, allows its absorption.

In the duodenum, a bond is formed between intrinsic factor and vitamin B12. The complex, resisting the digestive action of numerous proteolytic enzymes, continues unchanged along the digestive tract until it reaches the ileum (the last part of the small intestine) where vitamin B12 is absorbed.

In the absence of intrinsic factor, vitamin B12 is almost completely eliminated with the feces. The resulting avitaminosis is responsible for a typical anemia, called pernicious (or megaloblastic).