During acid reflux, stomach acid flows back into the esophagus, causing pain and irritation of the lining of the esophagus, notes Mayo Clinic. Unlike the stomach lining, the esophageal lining is not resistant to the corrosive effects of the acid, explains Healthline.
Between the esophagus and the stomach is a ring of muscles (the lower esophageal sphincter) that opens when food enters the stomach and closes to prevent food from flowing back into the esophagus. Sometimes, this ring does not close properly or opens when it should not, causing the stomach acid to flow up into the esophagus, notes WebMD. This action causes a painful burning sensation in the esophagus, dyspepsia (stomach discomfort) and regurgitation, characterized by bitter taste and nausea, according to Healthline.
A hiatal hernia is a primary cause of acid reflux. This occurs when the lower esophageal sphincter and upper part of the stomach move above the diaphragm. The diaphragm allows the acid to stay in the stomach, but in a patient with a hiatal hernia, the acid flows into the esophagus, causing a painful sensation. Other causes include overeating, overweight, lying on the back after eating a heavy meal and eating trigger foods such as garlic, citrus, onions and tomato, states WebMD.