Heartburn occurs when stomach acid backs up into your esophagus.

Normally when you swallow, your lower esophageal sphincter — a circular band of muscle around the bottom part of your esophagus — relaxes to allow food and liquid to flow down into your stomach. Then it closes again.

However, if the lower esophageal sphincter relaxes abnormally or weakens, stomach acid can flow back up into your esophagus, causing heartburn. The acid backup may be worse when you're bent over or lying down.

May. 21, 2011