Diaphragm. The diaphragm is the dome-shaped muscle that separates the abdominal from the chest cavity. All people have a hole in the diaphragm thru which the esophagus passes; this is called the hiatus of the diaphragm. This hole can become enlarged, allowing the esophagus and stomach to yo-yo up into the chest, causing reflux (gerd).
Diaphragm . Hiatal hernias occur when the natural opening in the diaphragm stretches out, and allows the stomach to migrate into the chest. It is behind the lower third of the breast bone, and can cause pain, feeling full quickly, difficult swallowing, and pressure.