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The lower esophageal sphincter (LES), also referred to as the cardiac sphincter or gastroesophageal sphincter, is a specialized ring-shaped muscle located at the base of the esophagus that acts as a one-way valve between the esophagus and the stomach. Its primary function is to keep the contents of the stomach out of the esophagus and the trachea, or windpipe. The LES is made of smooth muscle tissue and its function is involuntary. Weakening of the lower esophageal sphincter can lead to a severe condition called gastroesophageal reflux disease(GERD).
At the top of the esophagus is the upper esophageal sphincter (UES), a valve which consists of striated muscle tissue and can be consciously controlled. When food is swallowed, the UES relaxes and the swallowed food passes into the lower esophagus. Then, the lower esophageal sphincter responds to esophageal contractions caused by swallowing and relaxes as well, allowing the swallowed food to pass into the cardia, which is the uppermost part of the stomach. Immediately after swallowing is complete, the LES closes again to ensure that the swallowed food remains in the stomach.
The lower esophageal sphincter is usually in its closed position. This is to prevent the mixture of stomach acid, digestive enzymes and bile, known as gastric juice, from rising into the esophagus or the windpipe. While the stomach is built to withstand the high acidity of gastric juice, the esophagus is not. Its lining is slowly eaten away by stomach acids, sometimes resulting in a pain sensation in the chest and throat commonly referred to as heartburn or acid reflux.
Sometimes, the lower esophageal sphincter will open and close at random. These openings and closings are referred to as transient LES relaxations and are not uncommon. One example of a transient LES relaxation is belching: air pressure in the stomach forces the lower esophageal valve to open, and the air is released. During a spontaneous opening of the LES, acid reflux symptoms can occur, but a healthy esophagus works hard to force stomach acids back down with contractions. Saliva in the esophagus helps reduce the damage done to the esophageal lining.
Experiencing acid reflux twice a week or more is a symptom of gastroesophageal reflux disease. GERD can lead to serious complications if left untreated, such as ulceration, scarring that leads to tightening of the esophagus, and even esophageal cancer. The specific causes of GERD remain unknown, but weight loss and diet changes are often effective in treating the condition, as are medications like antacids, histamine blockers, and proton pump inhibitors. Severe cases of GERD may require surgery on the lower esophageal sphincter.