Hydrochloric Acid in the Stomach and Digestive Problems
Hydrochloric Acid in the Stomach
The inside of the stomach is a very acidic environment, especially after food has just been eaten. The acidic pH is created by hydrochloric acid, which is secreted by cells in the stomach lining. The acid is needed to activate a stomach enzyme that digests the proteins in food. Hydrochloric acid also kills many bacteria that enter the stomach, protecting us from harm.
Although stomach acid is a very helpful substance, we're normally unaware of its presence. The lining of the stomach is coated with mucus to protect it from acid damage. In addition, when the mixture of food and acid leaves the stomach and enters the small intestine, it's neutralized by the basic environment of the intestine. Sometimes, though, stomach acid causes problems, either because too much or too little is made or because it doesn't stay where it belongs.
A circular muscle called a sphincter closes the entrance to the stomach. If the sphincter doesn't work properly, food and acid may move up into the esophagus, creating a burning sensation. Stomach acid can also aggravate ulcers, making them more painful. Sometimes cells in the stomach lining are damaged by a factor other than an acid attack and are unable to make hydrochloric acid. Without enough acid, protein digestion in the stomach is difficult and bacterial overgrowth can occur.
Functions of the Stomach
Hydrochloric acid plays an important role in food digestion. The acid is made by parietal cells in the stomach lining and does its job in the stomach cavity, or lumen. The parietal cells also secrete intrinsic factor, which is necessary in order for vitamin B12 to be absorbed in the small intestine.
Other stomach lining cells known as chief cells make a substance called pepsinogen. Hydrochloric acid converts pepsinogen into an enzyme called pepsin. Pepsin begins the digestion of protein in our food, breaking up the long, folded chains of amino acids into shorter and simpler structures. The enzyme requires an acidic environment to do its job. Enzymes in the small intestine complete the breakup of the protein molecules, allowing individual amino acids to enter the bloodstream.
The pH in the stomach varies. If a person hasn't eaten for a long time, the pH of stomach fluid is generally around 4. When food enters the stomach, hydrochloric acid production increases and the pH may fall to as low as 1 or 2, a very acidic condition. Components of the food often raise the pH slightly as digestion proceeds. The acid not only provides a suitable environment for pepsin to work but also kills many potentially harmful microbes that enter the stomach in our food.
The parietal and chief cells are located in gastric glands in the stomach lining. The glands secrete a liquid called gastric juice. Around 2 to 3 litres of this liquid are secreted each day. Gastric juice contains water, a protein called mucin, hydrochloric acid, pepsinogen, intrinsic factor and other chemicals.
The Digestive Tract
Making Hydrochloric Acid - The Proton Pump
The production and secretion of hydrochloric acid is a complex process. A major contributor to the process is a proton pump in the membrane of the parietal cells. A proton pump is a special protein inside a membrane, which is either the cell membrane or the membrane of an organelle in the cell. The protein transports protons across the membrane by active transport, a process that requires energy.
A hydrogen atom is made of a positively charged proton and a negatively charged electron. When the atom loses its electron to form a hydrogen ion, all that remains is a proton. A hydrogen ion (H+) is therefore the same thing as a proton.
Hydrogen ions are moved through the membrane of a parietal cell and into the duct of a gastric gland by a proton pump known as H+/K+ ATPase. Chloride ions (Cl-) move through the parietal cell membrane by diffusion, a process that doesn't require a protein carrier or added energy. The hydrogen ions and chloride ions join in the duct of the gastric gland to make hydrochloric acid. The video below shows the process in more detail.
The Proton Pump in the Parietal (Oxyntic) Cells of the Stomach
Acid Reflux and GERD
The entrance to the stomach is protected by the lower esophageal sphincter, or LES. A sphincter is a circular muscle that closes or opens the entrance or exit of a tubular structure. Under normal circumstances, the LES closes the entrance to the stomach once food has entered its lumen. If the LES doesn't close or if it opens while food is still in the stomach, the churning, acidic mix in the stomach may be pushed up into the esophagus. The hydrochloric acid irritates the wall of the esophagus, producing pain and a condition known as heartburn. There may also be a sour taste in the mouth.
Acid reflux and GERD (gastroesophageal reflux disease) are closely related conditions, but GERD is more serious than acid reflux. Many people experience acid reflux occasionally. In GERD, the reflux is experienced regularly. The regurgitated acid produces heartburn, a sour taste and sometimes additional symptoms, including coughing, wheezing, chest pain and difficulty in swallowing.
GERD requires medical treatment. If the esophagus is exposed to acid frequently over a long period of time, its lining may be damaged by the acid. This can lead to further health problems, which may be serious.
Stomach Ulcers, H. pylori and Acid
An ulcer is a sore on the stomach lining. Ulcers in the stomach are known as stomach, gastric or peptic ulcers.
It used to be thought that a person under stress produced excess stomach acid and that this acid damaged the stomach lining and caused an ulcer. It's now known that stomach ulcers are usually caused by a bacterium called Helicobacter pylori, or H. pylori. They may also be caused by long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs), which reduce the amount of protective mucus made in the stomach. Aspirin is an NSAID. Although stomach acid doesn't cause an ulcer, it often makes it more painful.
H. pylori is an interesting creature, even though it may cause pain. It evades the acid that kills most bacteria and lives in the mucus coating on the stomach lining. Strangely, in some people the bacterium lives harmlessly in the digestive tract while in others it causes stomach ulcers or inflammation of the stomach lining (gastritis).
Medications for Reducing Stomach Acid
Doctors usually treat an H. pylori infection by prescribing antibiotics as well as medications that suppress acid production in the stomach. The most popular medication is a type known as a proton pump inhibitor, which blocks the production of hydrochloric acid. It joins to the protein pump, preventing the pump from attaching to hydrogen ions and transporting them to the duct of the gastric gland.
Sometimes other types of drugs called H2-receptor antagonists are prescribed to reduce the acid level in the stomach. These drugs block the receptor on the parietal cell membrane that normally binds to histamine. Histamine joins to parietal cells to trigger the process that makes hydrochloric acid. Proton pump inhibitors and H2-receptor antagonists are also prescribed for people who suffer from GERD.
Proton pump inhibitors are powerful reducers of stomach acid and are popular medications. They are often prescribed instead of H2-receptor antagonists because they are thought to be more effective, but their long-term use may increase the risk of osteoporosis and bone fractures. A limited amount of evidence suggests that proton pump inhibitors may block hydrogen ion transport in the bones as well as the stomach.
Some people take simple antacids to neutralize the acidity in their stomach. These aren't as effective as the more modern drugs because they don't prevent the acid from being made. They can provide quick, temporary relief from a mild case of acid reflux or pain caused by acid in the stomach, but anyone who finds themselves repeatedly taking antacids needs to visit a doctor for a diagnosis and treatment.
In some places, proton pump inhibitors and H2-receptor antagonists are sold over the counter without a prescription. If someone is tempted to use these because antacids aren't working, it's very important that they see a doctor to get his or her advice instead of performing self-treatment.
How a Proton Pump Inhibitor Works
Hypochlorhydria and Achlorhydria
Significant hypochlorhydria (low stomach acid) seems to be more common in elderly people than in younger people. Some evidence suggests that medications that are designed to reduce acid production in the stomach may cause hypochlorhydria. In severe cases, acid production stops and the condition is called achlorhydria.
Chronic hypochlorhydria may reduce bone density. A low level of hydrochloric acid in the stomach cavity can lead to an overgrowth of bacteria, including Helicobacter pylori. The bacteria may travel into the small intestine, where most nutrient absorption takes place. Here they may compete with human cells for the absorption of nutrients, including the calcium and other minerals needed to make strong, dense bones.
Without activated pepsin in the stomach, food containing protein will pass into the small intestine undigested. An enzyme called trypsin is present in the small intestine after we eat and does the same job as pepsin. Other enzymes in the small intestine complete the digestion of proteins into individual amino acids, which are then absorbed into the bloodstream. Nevertheless, without the action of pepsin, digestion may be reduced.
Symptoms of low stomach acid include stomach discomfort and bloating. Nausea and vomiting may also be present. In addition, there may be reflux through the lower esophageal sphincter. A doctor should be consulted if these symptoms are present, since they can indicate different disorders.
Autoimmune Atrophic Gastritis in the Stomach
There are many possible causes of gastritis, or inflammation of the stomach lining. These include the presence of H. pylori and the overuse of NSAIDs. Inflammation of the stomach lining is sometimes due to an autoimmune disorder. In this type of disorder, the immune system attacks and destroys the body's own cells.
Autoimmune atrophic gastritis is an "atrophic" disease because tissue is lost. The immune system damages the cells in the stomach lining, including the parietal cells and the chief cells. As a result, the amount of hydrochloric acid secreted decreases. In a serious case of gastritis the secretion may end altogether.
Autoimmune gastritis also reduces the amount of intrinsic factor secreted by the parietal cells, which in turn reduces the amount of vitamin B12 that can be absorbed. Vitamin B12 is needed to make red blood cells. The decrease in the vitamin B12 level may lead to a condition called pernicious anemia. The disease can be serious if it's untreated, since our body needs red blood cells to stay alive. Luckily, vitamin B12 injections can provide a person with the missing nutrient.
Hydrochloric Acid at Work
Like the myriad of other processes that occur in the amazing human body, hydrochloric acid production and action generally occur as required and without our awareness. Unfortunately, sometimes people are aware of the effects of too much or too little acid in the stomach. Luckily, there are ways to relieve or help these conditions, although a doctor's aid may be required.
References and Further Reading
- Food and Drug Administration (FDA) Recommendations for Proton Pump Inhibitors
- Autoimmune Atrophic Gastritis from Stanford University
© 2013 Linda Crampton
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