The esophagus is a narrow muscular tube that carries food from the mouth to the stomach. At the site where it joins with the stomach (just underneath the tip of the breast bone), there is a circular band of muscles called the lower esophageal sphincter that separates the esophagus from the stomach and keeps the stomach contents from entering the esophagus.  Normally, this sphincter only opens to allow food and drink to pass from the esophagus into the stomach. But if these sphincter muscles are weak or otherwise fail to close entirely, the acidic contents of the stomach can seep up into the esophagus; this is called reflux. While the stomach has a thick layer of mucus to protect it from the acid produced there, the esophagus does not. So when acid contacts the lining of the esophagus, it can cause pain and irritation and lead to the symptoms of heartburn.

When this happens occasionally, it is usually not a problem. However, when the reflux is chronic, such as in GERD, the constant backwash of acid can irritate the lining of your esophagus, causing it to become inflamed (called esophagitis). This can cause erosions in the lining of the esophagus, bleeding, and narrowing of the esophagus tube which can make it difficult to swallow and may even cause breathing problems. While most people with inflammation of the esophagus have GERD, some people with GERD have normal-appearing esophagus despite having chronic symptoms.

So what causes the lower esophageal sphincter to malfunction? In many cases, the reason is not clear. Some cases of GERD are related to other anatomical problems such as a hiatal hernia. A hiatal hernia occurs when the uppermost portion of the stomach protrudes through the diaphragm muscle and enters the space in the thoracic cavity normally occupied by the lower portion of the esophagus. This can compress the site where the stomach and esophagus meet which, over time, can weaken the lower esophageal sphincter and cause reflux. Other factors that may contribute to GERD include obesity, pregnancy, and smoking.