Why do we produce stomach acid?
Stomach acid, or hydrochloric acid (HCl), is one ingredient of the gastric juices we produce. It aids in the digestion of protein, and prevents infection by killing ingested
microorganisms. Overall, stomach acid gets chyme (ingested food) ready for further digestion in the small intestine.
How do we produce stomach acid?
Acid production is a cycle that is triggered by the presence of food in the stomach. The reason food can have such an effect is that it raises the pH of the stomach, that is, makes it less acidic. Certain cells in the stomach lining detect this reduced acidity and start the process of acid secretion. In addition to food, antacids or the buffer found in immediate-release PPIs reduce the acidity of the stomach, therefore triggering acid production. Once the food has passed out of the stomach, acid is no longer needed and production is stopped.
So what about a more practical example? We could compare the cycle to central air conditioning. As the temperature rises in a house, the thermostat detects the change, and sends a signal to turn on the AC unit. As the house cools down, the thermostat stops sending the message to the air conditioner, causing it to turn off. In this example, the controlled temperature of the house. In acid production, the cycle is controlled by the pH of the stomach. Let’s get a little more specific on the details of the process. We have already established that food gets the cycle going by raising the gastric pH. Just as the thermostat detects the rise in temperature, the G cell in the stomach lining detects the rise in pH (brought about by food) (Figure 3). The G cell sends a message to let the body know what’s going on; it releases gastrin into the bloodstream. Gastrin then stimulates the ECL cell. The ECL cell in turn releases histamine, which tells the parietal cell to begin secreting stomach acid. After the food has moved out of the stomach, its pH returns to the low acidic level. No additional acid is necessary, so acid production stops (although some acid is always being produced). The G cell stops releasing gastrin; the ECL cell stops releasing histamine; the parietal cell stops secreting acid. These cells remain at rest until the next meal, when food again increases the pH of the stomach and the cycle starts over.
The parietal cell
Parietal cells are the producers of stomach acid. They are located in gastric glands, microscopic cavities in the surface of the stomach (Figure 5). There are many thousands of gastric glands and numerous parietal cells in each gland. They produce all stomach acid through tiny pumps on their surface. While a cell is producing acid, it is said to be in active or canalicular state, named after the characteristic shape of the cell membrane, the canaliculus.
While the cell is at rest, that is, producing minimal acid, it is said to be in basal (resting) or tubulovesicular state. It is so named because the canaliculus of the active state is replaced by tubulovesicles, storage pockets inside the cell.
The parietal cell cycles between active and basal states throughout the day, depending on the stomach’s demand for stomach acid.
The proton pump
Proton pumps are tiny pumps located in the parietal cell that produce stomach acid (Figure 6). Active pumps (those that are currently pumping out stomach acid) are found on the surface of the cell, in the canaliculus. From here the acid moves out into the stomach (Figure 5).
Inactive pumps are on hold inside the cell, in the walls of tubulovesicles (bubble-like holding areas) waiting to be called on to secrete acid. Inactive pumps become active when their tubulovesicle migrates to the cell membrane, and fuses with it. The once inactive pumps find themselves on the surface of the canaliculus, and begin pumping out acid.
When the stomach no longer needs more acid produced, the active pumps turn off and go back into storage, in tubulovesicles inside the parietal cell.