As long as the container does its job, the acid isn't likely to cause any trouble. But the valve that connects the stomach to the esophagus sometimes leaks, allowing acid to splash upwards. Normally, everything that's in the stomach is kept from escaping upward by a band of muscle called the lower esophageal sphincter, or LES, located where the bottom of the esophagus intersects with the stomach. The LES valve opens when you're swallowing food, but otherwise is supposed to be clamped tight. The esophagus isn't designed to handle acid, so the leakage usually causes discomfort and a burning pain.
Most people have heartburn every once in awhile.
But if acid frequently escapes into your esophagus, the pain will be a regular part of your life. Heartburn is the most important symptom of GERD, but it's not the only one. Diet is a common culprit. Large meals, fatty foods, citrus or tomato-based foods, chocolate, peppermint, and caffeine can all make the LES valve open abnormally. Alcohol can irritate the valve as well as the mucous membrane of the esophagus.