There are two sphincter muscles in the oesophagus – the lower oesophageal sphincter (LES) and upper oesophageal sphincter (UES).
The LES is a muscle that prevents back-flow of food and acid from the stomach into the oesophagus. The UES is a muscle that prevents food and acid in the oesophagus from back-flowing into the larynx. When the LES is weak and does not close tightly, you will have gastrooesophageal reflux disease or GERD.
If acid (and digestive enzymes) from the stomach back-flows into the larynx, you will have laryngopharyngeal reflux (LPR).
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