Why Doesn’t the Stomach Digest Itself?

With all that potent acid floating around the stomach, the question inevitably arises, why doesn’t the stomach digest itself? How is the thin, delicate lining of the stomach protected while the equally delicate lining of the esophagus is not?
In fact, the protection afforded the stomach lining is as simple as it is effective. It consists primarily of two substances: mucus and bicarbonate, which are secreted by cells in the epithelial lining (top cell layer) itself.
Mucus is a clear fluid that serves to lubricate and protect delicate epithelial tissue throughout the GI and other respiratory systems. Gastric mucus is not much different from the fluid that runs out of our noses when we have a cold. Consisting largely of water (95 percent) and a small portion of a sugar-protein substance called a glycoprotein (5 percent), it has a slighdy gel-like quality that helps it stick to the epithelial lining and serve as a physical buffer against stomach acid and other digestive juices.
Bicarbonate is alkaline and very effective at neutralizing potent acids. Bicarbonate ions are secreted by cells in the lining of the stomach and duodenum in response to contact with acid. Coating the gastric epithelium, along with mucus, they neutralize any acid they come in contact with. So effective is this mucus-bicarbonate barrier that when the pH is 2 (very acidic) in the lumen, or cavity, of the stomach, the pH at the stomach’s lining approaches 7, or neutral.
The esophagus is strictly a transport organ, not a digestive organ. Acid does not belong in the esophagus, because there are no cells in the esophageal lining to secrete protective mucus or bicarbonate. The esophagus is not totally at the mercy of refluxing acid, though. It gets some protection from saliva, which is slighdy alkaline. Thus, swallowing saliva not only helps wash refluxing acid out of the esophagus, it also neutralizes it to a certain degree. At the same time, acid in the esophagus triggers a series of wavelike peristaltic contractions designed to send the acid back to the stomach where it belongs.
An intact protective barrier in the stomach is essential to health. Inflammation (gastritis) or ulcers may occur in regions of the stomach where the barrier has been breached.Like heartburn, ulcers were, for many years, thought to be caused by too much acid. In fact, gastric ulcers often occur in people whose acid levels are low. It is now widely accepted that most ulcers are initiated by a disruption in the protective barrier, most frequently caused by the bacteria Helicobacter pylori but also by certain drugs (e.g., aspirin and other nonsterioidal anti-inflammatory drugs, NSAIDs), which are exploited by stomach acid. 
Hiatal Hernia

The chest cavity and abdomen are separated by a large sheet of muscle called the diaphragm. Above the diaphragm lie the heart and lungs. Below it lie the stomach, pancreas, liver, gallbladder, intestines, and the rest of the digestive organs. The esophagus passes through the diaphragm via an opening called a hiatus. It is not uncommon for a portion of the stomach to protrude though the hiatus into the chest cavity. This condition is known as a hiatal (or hiatus) hernia (see Figure 1).
haital hernia
figure 1:haital hernia

It is estimated that as many as 25 percent of people aged 50 years and older have a hiatal hernia. These hernias are more likely to occur in people who are overweight or pregnant. Normally, the LES aligns with the diaphragm, which helps keep the valve closed except when it is supposed to open following a swallow. However, when there is a hiatal hernia, the LES rises above the diaphragm, which helps weaken the valve and increase the chances of reflux. Also, the acidic gastric juice may accumulate in the herniated portion of the stomach and flow back into the esophagus. Hiatal hernias were once believed to be the primary cause of heartburn. It is known today, though, that they are only one factor. Most hiatal hernias, while troublesome, do not require any special attention. However, large ones can cause serious problems and may require surgery.

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