How can acid flow backward from the stomach?

Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)
Normally, a valve between the esophagus and stomach keeps acid for digestion from flowing back up the esophagus towards the throat. If the valve is weakened by GERD, the acid can flow backward more easily and frequently. This animation shows how acid can flow backwards.

Joseph W. Beets, MD
When a person eats or drinks, under normal circumstances most of the material stays in the stomach, and then moves down into the intestine for digestion and absorption. Several circumstances can result in some of the material going back up into the esophagus/swallowing tube. This is called esophageal reflux. It often manifests as heartburn.
Everyday activities such as bending over or bearing down can increase pressure on the stomach and result in esophageal reflux.
There is a muscular barrier at the place where the esophagus meets the stomach that typically prevents or reduces the movement of material from the stomach back up into the chest. This barrier is not particularly effective in some people. A large percentage of people have a hiatal hernia (where some of the upper stomach may slide up into the chest and back down). Presence of a hiatal hernia may increase the likelihood of esophageal reflux. A hiatal hernia tends to reduce the effectiveness of the muscular barrier, though a hiatal hernia is not necessary to have esophageal reflux.

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