Physiology of Gastric Secretion - An Overview

Introduction

Based on how the gastric mucosa makes gastric juice, the process of gastric secretion can be broken down into three stages: the celiac phase, the gastric phase, and the intestinal phase. Recent studies show a fourth phase called an ‘inter-digestive phase.’ It is unclear what the exact mechanism involved in this phase is. It is doubtful if it is associated with emotions.

What Is the Physiology of Gastric Secretion?

A stomach is a specialized muscular reservoir where food enters after being swallowed. While digestion begins in the oral cavity with the enzymes in the saliva, gastric juices do a significant bulk of digestion.

What Is the Antrum of the Stomach?

The lower or distal section above the duodenum is called the pyloric antrum. The pylorus is the orifice where the stomach and small intestine meet, and the pyloric sphincter is the strong sphincter that controls the flow of chyme into the duodenum. The stomach area that is mucosa-lined and does not produce acid is known as the gastric antrum.

The frontal antrum, ethmoid antrum, and maxillary antrum are names for the paranasal sinuses. Another name for it is the stomach antrum. This portion of the stomach is wider than the pylorus, which is the narrower region. It is located upstream of the duodenum and the pyloric sphincter's connection with the pyloric canal. Seventy-five percent of the stomach glands are the oxyntic glands, which release pepsinogen, mucus, intrinsic factor, and HCl.

What Are the Cells Present in the Stomach?

The cells in the stomach consist of two types of gland cells: oxyntic cells (also called parietal cells) and pyloric cells (also called G-cells). Oxyntic cells form a significant portion of the stomach cells and are mainly found in the fundus and the body of the stomach (the stomach is classified as having three parts: the fundus, body, and antrum). The pyloric cells are mainly found in the antrum. The parietal cells play an important role in the production of gastric juices.

The different cells in the stomach produce various secretions that have various functions.

For example, some of them include the following:

• Parietal cells secrete gastric acid, intrinsic factor (a glycoprotein that helps absorb vitamin B), and leptin (coordinates with the central nervous system with food intake and energy expenditure).

• Enterochromaffin-like cells (ECLs) secrete histamine, (an organic compound that helps with local immune responses and regulates gut functions).

• Chief cells secrete pepsinogen (an acid in the stomach that helps digest proteins), gastric lipase (which helps in lipids), and leptin.

• D-cells secrete somatostatin (also called growth hormone).

• Enterochromaffin cells produce ghrelin (a hormone that signals when the stomach is empty), obestatin (a hormone that signals satiety or hunger), serotonin, and atrial natriuretic peptide (a hormone that helps to reduce plasma volume).

• Mucous cells produce mucus and bicarbonate.

• G cells secrete gastrin (regulated secretion of gastric acid).

What Are the Phases of Gastric Acid Secretion?

When gastric acid is made, it goes through three stages: the cephalic, gastric, and intestinal. These phases depend on how the gastric mucosa produces gastric acid.

The gastric mucosa secretes an average of about 1.5 liters (of gastric juice) a day. The function of gastric juice is to convert the food into soluble components, initiate digestion, and convert the gastric components into chyme (a semisolid mass of partly digested food) so that it can facilitate digestion in the intestine. Gastric juice comprises water, hydrochloric acid (HCL), electrolytes like sodium, potassium, calcium, phosphate, etc., and organic components like mucous, pepsin, and protein. The stomach walls are protected from the highly acidic gastric juices by a membrane rich in lipoproteins lining the stomach's epithelium. The different phases of gastric secretion overlap, and the neural and humoral pathways (the brain and the endocrine system control the process of digestion) that control the acid secretion depend on each other.

The phases are:

1. Cephalic Phase

Stimuli initiate the cephalic phase from the senses, like taste, smell, sound, and light.

The vagus nerve (10th cranial nerve) mediates this phase. The senses, or electrical impulses, stimulate the vagus and initiate gastric acid secretion. Ivan Petrovich Pavlov demonstrated this using his famous experiment with dogs.

2. Gastric Phase

• It is also called the chemical or hormonal phase.

• Food presence in the stomach facilitates gastric acid secretion. It starts within 15 minutes after the food reaches the stomach. The vagus nerve and gastrin regulate this phase.

• The luminal amino acids and peptides released from the diet (by the action of pepsin) activate the G-cells to release gastrin.

• There is a direct mechanism by which gastrin is secreted. As the stomach receives food, it relaxes, stimulating the vagus nerve to release acetylcholine and stimulating the parietal cells to secrete gastric acid.

• In the indirect mechanism, acetylcholine stimulates the ECL to secrete histamine, stimulating the parietal cells to produce gastrin.

• Mechanical, chemical, and hormonal factors influence the gastric secretion process. This phase lasts until the food leaves the stomach.

3. Intestinal Phase

• In this phase, many mechanisms return the pH level to normal.

• A reduced sensory stimulus and the distension of the stomach reduce the stimulation of gastric acid secretion.

• The gastrin released in the initial phases sends negative feedback through the D-cells in the antrum.

• This stimulates somatostatin to stop the release of gastric acid.

• When the pH of the antrum falls below 2.5, gastrin release is stopped.

• Due to the complex mechanisms involved, the intestinal phase still needs to be fully understood. The intestinal phase takes about 2 hours to start.

4. Interdigestive Phase

• Even after the intestinal phase, a small amount of gastric juice is produced, called the inner digestive phase.

• The exact mechanism behind this needs to be clarified. Emotional conditions and conditioned reflexes may be due to this.

What Are the Factors Affecting Gastric Acid Secretion?

The different factors that affect acid secretion are:

• Nationality.

• Age.

• Sex.

• Body weight.

• The effect of another stimulus, like drugs.

• Electrolyte levels.

• Anemia.

• Deficiency of the iron-related enzyme system.

• Vagal continuity.

• Parietal cell mass.

What Are the Factors Stimulating Gastric Acid Secretion?

• Condiments (flavoring agents).

• Vegetable extracts are like soup.

• Alcohol is one of the most powerful stimulants in the stomach, increasing gastric acid secretion. This is probably because of the stomach stimulation of histamine and gastrin.

• Caffeine is a good stimulant for gastric acid.

• Parasympathetic agents, like acetylcholine, stimulate acid secretion.

• Cigarettes: Even small amounts of nicotine can stimulate the autonomic ganglia. The vagal stimulation increases acid secretion.

What Are the Factors Depressing Gastric Acid Secretion?

• Drugs like atropine, which acts against acetylcholine.

• Anti-H2 drugs like cimetidine.

• Antacids.

• Aluminum hydroxide.

• Sodium bicarbonate.

• Calcium bicarbonate.

Conclusion

In a nutshell, gastric secretion can be broken down into three stages. The gastric acid secretion phases depend on how the gastric mucosa produces gastric acid. There are many methods to study gastric secretion. However, studies are still going on to get a clear picture of the exact phases of gastric secretion.