What is a hiatal hernia?

Hole in diaphragm: The diaphragm is the muscle that separates the chest- from the abdominal cavity. Many structures have to go thru the diaphragm in order to enter into the abdominal cavity, including the esophagus, the aorta, and the vena cava. The hiatus is the hole in the diaphragm thru which the esophagus passes. This can enlarge, allowing the stomach to "yo-yo" up into the chest: this is called a hiatal hernia.

Enlarged hiatus: A hiatal hernia is a hernia through the diaphragm. The natural opening where the esophagus comes through the diaphragm enlarges. Sometimes it gets big enough that a significant amount of stomach can get through the hole into the chest. This may or may not be related to reflux disease, which is the most common symptom associated with hiatal hernias.

See below: The esophageal hiatus is an anatomical feature traversed by the esophagus on its way from the chest to the abdomen. When muscles and ligaments that define the boundaries of the hiatus weaken or become lax, the upper stomach may transiently or permanently reside in the chest. This is a hiatal hernia. Uncommonly, the entire stomach or other organs (colon, small bowel, spleen) may herniate as well.