The gastric glands, also known as the oxyntic glands, secrete hydrochloric acid (HCl), pepsinogen and intrinsic factor. These glands are made up of 3 types of cells :
- mucus neck cells which secretes mucus
- peptic/chief cells which secretes pepsinogen
- parietal/oxyntic cells which secretes HCl and intrinsic factor
All these cells empty their secretions into the canaliculi of the oxyntic gland and under appropriate stimulation, this is secreted into the stomach cavity.
Gastric Acid Production
Only the parietal cells secrete HCl. When the parietal cell is stimulated, the following occurs :
- Chloride ions are actively transported from the interior of the parietal cell (cytoplasm) into the lumen of the canaliculus of the oxyntic gland.
- As the chloride ions pass out of the cell, sodium are transported into the cell.
- The transport of sodium and chloride creates a negative potential and allows potassium ions to pass out of the cell into canaliculus.
- Due to the difference in charges, some of the potassium and sodium (positive) bind with some of the chloride ions (negative).
- The canaliculi of the gastric gland now contains potassium chloride and sodium chloride, potassium ions, sodium ions and chloride ions.
- Water is broken down (dissociated) into hydrogen and hydroxyl ions within the parietal cell.
- The hydrogen ions (H+) is passed out into the canaliculus while sodium and potassium enters the cell. This is catalyzed by the H+/K+ ATPase enzyme system.
- Hydrogen bonds with chloride and forms hydrochloric acid (HCl).
- Water also enters the canaliculus.
- The canaliculus of the gastric gland now contains water, large amounts of hydrochloric acid, moderate amounts of potassium chloride and a small amount of sodium chloride.
- To ensure that the cell has a constant supply of chloride ions, the remaining hydroxyl ions in the cell bind with carbon dioxide (from the blood or from the cell’s own metabolic process). This forms bicarbonate ions.
- The bicarbonate diffuses out of the cell in exchange for chloride ions which enters the cell from the extracellular fluid.
- The parietal cell now has more chloride to secrete into the oxyntic gland canaliculus for the production of HCl as and when required.
Other Gastric Enzymes
Two digestive enzymes known as pepsin and intrinsic factor are also secreted by the oxyntic glands and are worth mentioning because of their close relationship with HCl. Intrinsic factor is secreted by parietal cells which is also responsible for HCl secretion, while different types of pepsinogen are secreted by the chief cells.
Pepsinogen is activated into pepsin when it comes into contact with HCl. Pepsin needs an acidic environment to function as an enzyme and ideally this would be at a pH of 1.8 to 3.5. If the pH goes above 5, pepsin cannot function as an enzyme. Along with HCl, pepsin plays an important role in protein digestion.
Intrinsic factor is essential for the absorption of vitamin B12 from the small intestine and if the acid-producing parietal cells are not functioning normally or destroyed, intrinsic factor may also be disrupted.
Stimulation of Gastric Acid Secretion
The production and secretion of gastric acid is directly influenced by histamine. This chemical is secreted by the enterochromaffin-like cells (ECL) which are also located in the gastric (oxyntic) gland. ECL is stimulated by the endocrine and nervous system.
- Endocrine : Gastrin is the digestive hormone that is secreted by the gastrin (G) cells which are located in the pyloric glands towards the distal end of the stomach. This hormone is released into the stomach cavity when the presence of protein is detected in the stomach contents. Due to the vigorous churning in the stomach, the gastrin is able to make contact and act upon the ECL cells, stimulating it to secrete histamine.
- Nervous : Acetylcholine released by the vagus nerve and enteric system acts on the ECL cells to secrete histamine, which in turn stimulates HCl production and secretion.
The stimulation of gastric acid secretion is controlled in 3 phases :
- Cephalic Phase. The sight, smell, taste or even thought of food causes impulses from the cerebral cortex, amygdala and hypothalamus to send impulses through the vagus nerve. This triggers gastric acid production and secretion.
- Gastric Phase. Food in the stomach triggers reflexes and stimulates gastrin secretion. This then stimulates gastric acid production and secretion.
- Intestinal Phase. Food in the duodenum (small intestine) causes the duodenal mucosa to secrete gastrin and this continues to stimulate secretion of small amounts of gastric acid. However, food in the duodenum primarily inhibits gastric acid secretion through other mechanisms.
Gastric Acid Inhibition
- Nerve reflexes and hormones slow gastric emptying and inhibit gastric acid secretion due to :
- Acidic gastric contents in the duodenum.
- Distension of the small intestine (duodenum).
- Fat and protein breakdown products in the small intestine (duodenum).
- The hormones that inhibit gastric acid secretion includes :
- gastric inhibitory peptide (GIP)
- vasoactive intestinal peptide (VIP)
Refer to Digestive Hormones for more information on individual hormones.