The normal antireflux mechanism.

The normal antireflux mechanism consists of several components, any one of which may malfunction and render an individual prone to the development of GERD. The LES is clearly the most important component because gastroesophageal reflux almost always occurs when the sphincter pressure equals that of the stomach. Usually, an LES pressure of just 2 to 3 mm Hg above intragastric pressure is sufficient to prevent reflux. Other factors certainly play significant ancillary roles in preventing reflux. In the absence of a hiatal hernia, the crural fibers of the diaphragm serve as an "extrinsic" sphincter. Furthermore, the unique anatomy of the proximal stomach (e.g., the angle of His, mucosal flap valve, posterolateral location of the fundus) serves to keep gastric contents away from the gastroesophageal junction, making it less likely for reflux to occur when the LES relaxes. When a hiatal hernia is present, these factors are lost, and the hernia sac provides a reservoir for gastric juices with ready access to the LES. Finally, some degree of reflux occurs in all individuals, but esophageal clearance and acid neutralization provide an important last line of defense.