Lower esophageal sphincter (LES) function

The esophagus is a tube that connects the mouth to the stomach. It is simply a conduit for food and does not aid in digestion. However, the esophagus does have another important job. It keeps stomach contents, (food, acid and bile) in the stomach, out of the esophagus and away from the airway (breathing tube). It does so by the aid of two muscles or valves called the upper and lower esophageal sphincters. The most important valve for keeping stomach contents out of the esophagus and airway is the lower esophageal sphincter, referred to as the LES.

The LES is located at the very bottom of the esophagus where the esophagus joins the top of the stomach. The LES is a high pressure zone between the esophagus and stomach. The LES is made up of muscles at the bottom of the esophagus as well as the muscles of the diaphragm (breathing muscle) that surround the bottom of the esophagus. When it is closed, the LES maintains a higher pressure than that of the stomach so that food and digestive juices cannot wash back into the esophagus.

The LES normally opens or relaxes (lowers its pressure) as food is moved down the esophagus by esophageal contractions. These esophageal contractions are started by swallowing. As soon as swallowing stops, the LES closes to keep the food in the stomach. A normally functioning LES prevents food and stomach acid from backing up into the esophagus and ultimately into the trachea or "windpipe".

In addition to the LES, the gastroesophageal flap valve may close when the stomach is full. The flap valve is an area in the top of the stomach into which the bottom of the esophagus and surrounding stomach tissue protrudes (extends into). The gastroesophageal flap valve can be seen on upper endoscopy.

Normal flap valve The endoscope can be seen at the top of the stomach. The stomach folds seen around the endoscope make up the flap valve. Normally, on endoscopic examination, the flap valve appears to "hug" the endoscope as seen here. Photo courtesy of the Seattle Barrett's Esophagus Research Program

Open flap valve The endoscope can be seen at the top of the stomach. The stomach folds seen around the endoscope make up the flap valve. The flap valve, which normally appears to "hug" the endoscope, is seen here to be lax creating an opening between the endoscope and the surrounding stomach folds. This opening leads to the hiatal hernia. Photo courtesy of the Seattle Barrett's Esophagus Research Program

Transient LES relaxations

It is normal for the LES to open occasionally when there is no swallowing. These spontaneous LES openings are called transient LES relaxations and usually occur after a meal when the stomach is full and distended with food and swallowed air. Many of these transient LES relaxations occur to let air out of the stomach and result in "belching or burping". A backwashing of food and stomach acid into the esophagus can occur at this time. Normally, transient LES relaxations do not occur while lying down.

In the normal esophagus, in response to transient LES relaxations and the backwash of gastric contents into the esophagus, strong esophageal contractions are initiated to force refluxed stomach contents back down into the stomach and clear the esophagus of acid. In addition, saliva neutralizes any remaining gastric acid so that it is less likely to injure the lining of the esophagus. As a result of these backup mechanisms, most transient LES relaxations do not cause heartburn symptoms.

Many people have occasional heartburn, but those who have frequent heartburn symptoms have more frequent transient LES relaxations as compared to those who have infrequent or no heartburn symptoms. In addition to increased transient LES relaxations, some people who have severe GERD, including those with Barrett's esophagus, may have a LES that has an abnormally low pressure. This constant low pressure allows free backwash of stomach contents, including acid, that far exceeds that of the normal transient LES relaxations. In many of these people, the gastroesophageal flap valve is open or lax as seen by upper endoscopy, rather than closed. Reflux in these people occurs not only after a meal or with burping, but also at night, and can be easily brought on by coughing, bending over, lying down, or wearing a tight-waisted garment.