What is acid reflux?

Antegrade esophageal flow is achieved by the act of swallowing with the initiation of primary peristalsis. Gastroesophageal reflux is prevented by the physiologic lower esophageal sphincter (LES).
When the LES fails to function as an effective barrier to reflux, gastroesophageal reflux develops, with the associated complications of mucosal inflammation (reflux esophagitis).
Gastroesophageal reflux disease (GERD) refers to the varied clinical manifestations of reflux of stomach and duodenal contents into the esophagus and is preferable to the term "reflux esophagitis." Although GERD may be associated with a sliding hiatal hernia, the term "symptomatic hiatal hernia" tends to emphasize an anatomic entity and not the underlying pathophysiology. GERD can be characterized by any combination of symptoms and radiologic, endoscopic, or pathologic changes. In its milder manifestations, it is a common disease; its most florid state is uncommon but may be life-threatening.
HEARTBURN
Heartburn is the most common manifestation of esophageal disease and may occur in up to 20% of the population. The term "burning" rather than "pain" is usually used, although heartburn can increase in intensity until it is perceived as chest pain. Patients often illustrate heartburn with a movement of the open hand up and down the sternum, as compared with the stationary, tightly clenched fist of angina pectoris. Heartburn is usually relieved, even if only temporarily, by taking antacids. A constant burning unrelieved by antacids may well be of esophageal origin, but it does not represent heartburn. Heartburn is often worse after recumbency or lifting and may follow overeating or alcoholic indiscretion.
REGURGITATION
Regurgitation of fluid contents into the mouth often accompanies heartburn. Sometimes regurgitation is associated with eructation; often it accompanies bending over, lifting, or lying down at night. The bitter regurgitated fluid is often described as yellow-brown or green. Regurgitation at night may lead to stridor or to wheezing, a hoarse voice, and other respiratory symptoms from unrecognized reflux.
SPONTANEOUS ESOPHAGEAL CHEST PAIN
In addition to the discomfort from severe reflux, which can advance from heartburn into pain, abnormal contractile activity of the esophageal muscle can cause severe chest pain that is clinically indistinguishable from angina pectoris in terms of intensity, radiation, relationship to exercise, and even response to nitroglycerin. pain of esophageal origin can radiate directly through to the back and is often found in patients who also have dysphagia. Esophageal chest pain can last from several seconds to many hours.