Acidity is caused due to excessive release of acid content in the stomach. This excess acid moves up to the oesophagus causing burning of heart and it has got nothing to do with the heart. The stomach contains a strong acid known as hydrochloric acid which helps in the breakdown of food and also protecting from pathogens. The inner lining of the stomach is adapted to this powerful acid but the oesophagus is not protected. This is the cause of acidity.