Heartburn. Computer artwork showing stomach acid (green) erupting up the oesophagus (upper left) from the stomach. The acid burns the lining of the oesophagus, causing the pain known as heartburn. Normally the opening to the oesophagus is held closed by muscle. Frequent reflux of acid up the oesophagus can lead to oesophagitis, inflammation of the lining of the oesophagus. The condition is made worse by obesity, smoking and excessive alcohol consumption.