**3. Esophageal anatomy and physiology**

A GER episode is diagnosed when esophageal pH drops below 4.0 for at least 30 seconds. The physiological GER occurs in normal individuals, typically postprandial. Under physiological conditions, there is a number of protective mechanisms, which prevent epithelial damage due to reflux contents. These include upper esophageal sphincter (UES), lower esophageal sphincter (LES), esophageal peristalsis, the squamous mucosal barrier, salivary production, and bicarbonate buffer. Esophageal sphincters work as physical barriers to the retrograde movement of stomach contents to the esophagus and upper airway spaces. The esophagus is a 25 cm hollow fibromuscular tube that allows the passage of solids and liquids from the pharynx to the stomach, with no metabolic, digestive or endocrine function. It makes continuation with pharynx with the upper esophageal sphincter, which measures 2–4 cm in length, and is composed of striated or skeletal muscle. The cricoid cartilage and the arytenoid and inter arytenoid muscles make up the anterior parts of the sphincter. The thyropharyngeus and the cricopharyngeus muscles form the majority of the sphincter's posterior and lateral walls, with the former accounting for the upper two-thirds of
