Acid reflux occurs when stomach acid gets into the oesophagus (the muscular tube which connects the throat to the stomach). The acid travels up towards the throat and can cause discomfort. The lining of the stomach has a mucus layer to protect itself from the acidic contents, but the oesophagus is unprotected from the acid. When acid leaks up into the oesophagus it can cause the symptoms of heartburn, which is the burning sensation in the middle of the chest.