Gastric juice

Gastric juice is a liquid secretion from the gastric mucosa, containing a heterogeneous mixture of clear liquid and transparent, lumpy mucus. It comes from secretions of various specialized epithelial cells, both superficial and from the gastric glands. Its chemical composition consists of water, hydrochloric acid, traces of potassium chloride, sodium chloride, bicarbonate and enzymes. Through the action of gastric juice, the bolus forms a pasty substance called chyme that passes into the duodenum.

Main chemical composition

The "gastric juice", more correctly the gastric secretion, is a mixture of the secretions of several types of specialized epithelial cells within the stomach.

In the basal state (fasting), gastric juice is basically a NaCl solution with small amounts of H+ and K+. With the ingestion of food, the concentration of H+ increases considerably and that of Na+ decreases in equivalent proportions and up to 2 liters of hydrochloric acid#Gastric acid (HCl) are produced per day, with a pH as low as 1. The concentration H+ is 3 million times lower than in blood, and chloride secretion is done both against the concentration gradient and the electrical gradient. Thus the ability of parietal cells to secrete acid depends on active transport. Gastric secretion is the most relevant phase of digestion because when the food comes into contact with a low pH and lytic enzymes, it dissociates it into collagen fibers and denatures (proteolysis) the proteins present. Constituting the chemical phase of digestion at the same time as the mechanical action is carried out by the contractions of the stomach.

Mucus and HCO3-

Mucus is a viscous, slippery gel that covers the mucosal surfaces of the gastrointestinal tract. Its characteristics are due to glycoproteins that form gels called mucins. Mucins are the main organic product secreted by surface epithelial cells. Mucin monomers are glycoproteins with a molecular mass of approximately 50 kDa; highly glycosated with only about 15%-20% of its mass made up of the protein core. About 100-200 oligosaccharides are linked along the protein core via hydroxyl groups (O-bond glycosidation) to serine and threonine residues.

In addition to the visible cloudy mucus, the superficial cells secrete a fluid rich in NaHCO3, which acts as a means of protection against the low pH and peptic conditions of the gastric lumen.

Pepsins

The main enzyme in gastric juice is pepsin, although there are other enzymes important for specific functions, but in much smaller quantities, such as gastric lipase, which is the most effective against triglycerides with short chain fatty acids and gelatinous, all derived from the principal cells or zymogen cells.

Gastric pepsin is actually a heterogeneous set of proteins responsible for the proteolytic activity of gastric juice. These are secreted in the form of inactive zymogen precursors called pepsigenes, pepsigene I (PGI) and pepsigene II (PGII), both molecular variants that differ in net charge and/or molecular weight (isozymes).

The catalytic activity of pepsin is that of an endoprotease, which preferentially cleaves peptide bonds involving aromatic amino acids (e.g. phe, try, tyr) and an adjacent amino acid, generating fragmented digestion products of very different sizes..

Hydrochloric acid

Hydrochloric acid represents the exclusively chemical and corrosive component contained in gastric juice. The acidity (low pH) and ionic composition of the final product of gastric secretion is not constant, and varies with the rate of secretion. An average adult stomach secretes 1.5 liters of gastric juice per day, but only a fraction of that volume is gastric acid. It usually does not cause damage to the mucosa, as long as an ingestion with large doses of external acids is not added.
In the stomach at rest and fasting, basal acid secretion has a diurnal pattern and its proportion varies widely among normal people. Normal gastric pH is between 1.2-2.0. Basal acid secretion is 1 meq/h in normal subjects and 2-4 mEq/h in ulcer patients.

Intrinsic factor

It is a 55,000 Da glycoprotein, secreted in humans by parietal cells together with HCl. This factor binds to vitamin B12 forming a complex that is resistant to digestion and binds to receptors in the ileum to promote the absorption of vitamin B12.

Physiology of its formation

The components of gastric juice, its origin and its function are briefly listed below.

Inorganic components

1. Water: main component of the solution.
2. Chlorhydric acid (HCl): segregated by the parietal or oxin cells. It performs the denaturalization function of proteins, acts as a sterilizing medium of bacteria. Activate the pepsinogen to return the pepsinogen and fulfill your lytic function.
3. Ion bicarbonate (HCO)₃^-) and mucus: they are formed in epithelial cells, they perform a function of protection against the decrease of pH and pepsin respectively.
4. Sodium chloride and potassium chloride: proton pump regulators.

Organic components

1. Intrinsic factor: it forms as well as HCl in the parietal cells, forms a complex with vitamin B12 that prevents it from digesting, transporting it to the ileum where it is absorbed.
2. Pepsinogen: it is formed in the main cells or zimogenic cells. It's the primordial enzyme in proteolysis.
3. Trefoil factors: they form in epithelial cells, they perform a protective function.
4. Histamine: It is generated in enterochromaphin regulation cells in acid secretion
5. Gastrine: forms in G cells. Regulates acid secretion.
6. Somatostatin: it forms in the D cells and also regulates acid secretion.
7. Rennina or Quimosine: it is an enzyme that is only found in the babies, until it begins the generation of gastric juice.

Control of gastric secretion and motility

The movement and flow of chemicals in the stomach are controlled by the autonomic nervous system and several hormones in the digestive system:

The regulation of gastric juice secretion in the human body goes through three stages:

• The cephalic phase, in which when you see, smell or taste a food is generated 40% of the maximum volume of gastric juice.
• The gastric phase, which happens when food has reached the stomach and causes the largest generation of acid secretion of the three phases.
• The last phase is the intestinal phase, where the cymo reaches the duodenum, which performs two more sequences of stimulation of the gastric acid and a second in which the same is inhibited. In duodenum, gastric acid is neutralized by sodium bicarbonate. This also blocks the gastric enzymes (pepsins) that have their optimal action in a low pH range.

The secretion of sodium bicarbonate from the pancreas is stimulated by secretin. This polypeptide hormone is activated and secreted from the so-called S cells in the mucosa of the duodenum and jejunum when the pH in the duodenum falls between 4.5 and 5.0 units.