Hiatal Hernia (cont.)

Normally, there are several mechanisms to prevent acid from flowing backwards (refluxing) up into the esophagus. One mechanism involves a band of esophageal muscle where the esophagus joins the stomach called the lower esophageal sphincter that remains contracted most of the time to prevent acid from refluxing or regurgitating. The sphincter only relaxes when food is swallowed, allowing food to pass from the esophagus and into the stomach. The sphincter normally is attached firmly to the diaphragm in the hiatus, and the muscle of the diaphragm wraps around the sphincter. The muscle that wraps around the sphincter augments the pressure of the contracted sphincter to further prevent reflux of acid.

Another mechanism that prevents reflux is the valve-like tissue at the junction of the esophagus and stomach just below the sphincter. The esophagus normally enters the stomach tangentially so that there is a sharp angle between the esophagus and stomach. The thin piece of tissue in this angle, composed of esophageal and stomach wall, forms a valve that can close off the opening to the esophagus when pressure increases in the stomach, for example, during strenuous exercise.

When a hiatal hernia is present, two changes occur. First, the sphincter slides up into the chest while the diaphragm remains in its normal location. As a result, the pressure normally generated by the diaphragm overlying the sphincter and the pressure generated by the sphincter no longer overlap, and as a result, the total pressure at the gastro-esophageal junction decreases. Second, when the gastro-esophageal junction and stomach are pulled up into the chest with each swallow, the sharp angle where the esophagus joins the stomach becomes less sharp and the valve-like effect is lost. Both changes promote reflux of acid.

Source: MedicineNet.com
http://www.medicinenet.com/hiatal_hernia/article.htm