HEARTBURN
The sensation of heartburn is caused
by exposure of the lower esophagus to the acidic contents of the stomach.
Normally, the lower esophageal sphincter (LES) separating the stomach from
the esophagus is supposed to contract to prevent this situation. If the sphincter
relaxes for any reason (as normally occurs during swallowing), stomach contents,
mixed with gastric acid, can return into the esophagus. This return is also
known as reflux, and may progress to gastroesophageal reflux disease (GERD)
if it occurs frequently. Peristalsis, the rhythmic wave of muscular contraction
in the esophagus, normally moves food down and past the LES and is responsible
for ultimately clearing refluxed stomach contents. In addition, gastric acid
can be neutralized by buffers present in saliva.(1)
source
(1) wikipedia