At both the top end and the bottom end of the oesophagus is a special thickened and strengthened ring of muscle referred to as a sphincter. The muscle at the bottom end is called the lower oesophageal sphincter. This should work as a one-way valve, allowing swallowed food to pass down into the stomach but preventing stomach acid from coming back up into the sensitive oesophagus. Problems with the lower oesophageal sphincter can cause the common complaint of heartburn.