Regulation of gastric acid secretion
Hydrochloric acid, often synonymously termed gastric acid, is a fundamental component of
gastric juice.
Hydrochloric acid has several functions: Not only is it involved in digesting food,
but it also ensures that microbes that enter the body with food are destroyed.
On an empty stomach, the pH of gastric juice is approximately 1.0 to 1.5.
It’s highly acidic and can cause damage to tissues.
As a result, the gastric epithelial cell lining regularly regenerates approximately every six days.
Also, the cell lining is protected by a thick layer of mucin.
Though, the production and secretion of hydrochloric acid requires strong regulation.
This Chalk Talk episode provides an overview of how hydrochloric acid production is adjusted to various digestive processes.
We’ll be placing our focus on the regulatory mechanisms directly associated with hydrochloric acid production.
The parietal cells of the stomach start to produce hydrochloric acid during the cephalic
phase of gastric secretion.
The term “cephal-” comes from the Greek word meaning head and implies that mental processes
play a role during this phase.
The smell, taste, or even the sight or thought of food results in parasympathetic stimulation.
In particular, the vagus nerve promotes the release of the transmitter acetylcholine,
which directly stimulates acid secretion.
Also, the vagus nerve acts on various enteroendocrine cells.
Upon vagal stimulation, G cells secrete gastrin, which also promotes acid secretion.
In addition, via the release of acetylcholine and gastrin,
the vagus nerve stimulates enterochromaffin-like
cells, in short, ECL cells.
These cells release histamine, promoting further acid secretion.
So, all three regulatory molecules that stimulate hydrochloric acid secretion, namely histamine,
acetylcholine, and gastrin, are made available before food intake.
Once food enters the stomach, the gastric phase begins.
The origin of this name is also Greek, and means stomach.
The ingested food causes the stomach wall to distend, which,
in turn, stimulates the vagus nerve.
If protein degradation products are present in the stomach,
G cell activity increases, resulting in the release of further gastrin.
Acid secreted during the gastric phase comprises approximately one-half of the total acid output.
However, if the gastric pH drops below 3, the digestive system begins to counter regulate.
A low pH indicates that there’s sufficient hydrochloric acid in the stomach.
Consequently, the pH level stimulates delta or D cells,
which secrete somatostatin to inhibit hydrochloric acid secretion.
The intestinal phase begins as soon as the liquefied food is emptied into the small intestine.
Small intestinal wall distension as well as the protein degradation products
continue to stimulate the G cells, resulting in gastrin secretion.
But the resulting amount of acid is low and accounts for approximately 5%.
In the intestinal phase, processes that inhibit HCl secretion predominate, since gastric acid
is no longer required for digestion after the food passes the stomach.
There are various stimuli that inhibit hydrochloric acid secretion,
signaling the movement of food from the stomach to the intestine.
These include lipids in the small intestine, an increase in the concentration of salt,
also termed hyperosmolarity, and a drop in pH levels to below 4 as a result of the highly acidic chyme.
This semifluid mass containing a mixture of digested food
and digestive enzymes is present in the stomach and small intestine.
Now, chyme stimulates D cells and increases somatostatin secretion.
Also, S cells promote the secretion of secretin, K cells gastric inhibitory peptide,
abbreviated as GIP, and I cells cholecystokinin.
Together, these chemical messengers inhibit acid secretion.
Now, let’s briefly recap this information:
Gastric acid secretion is stimulated by processes that signify its requirement,
such as the presence of food in the stomach.
These processes lead to the secretion of the chemical messengers histamine, acetylcholine, and gastrin.
Evidence of sufficient levels of hydrochloric acid is mainly signalized by food passing into the intestine.
This leads to the inhibition of hydrochloric acid secretion via the
chemical messengers somatostatin, secretin, GIP, and cholecystokinin.
Were you able to remember all the details of the regulation
process? Let’s see how you fare in the quiz.