The diaphragm is a large, dome-shaped muscle located below the lungs that separates the chest and the organs in it from the abdomen and the body organs located there. It is the primary muscle used in respiration, which is the process of breathing. The diaphragm contracts rhythmically and continually, and most of the time, involuntarily, creating a vacuum that pulls air into the lungs when inhaling. When exhaling, the diaphragm relaxes and returns to its domelike shape, and air is forced out of the lungs.

In addition to its role in breathing, the diaphragm also has some nonrespiratory functions. It increases abdominal pressure to help the body get rid of vomit, urine, and feces, and places pressure on the esophagus to prevent acid reflux. The phrenic nerve controls the movement of the diaphragm. There are three large openings in the diaphragm that allow certain structures to pass between the chest and the abdomen.

Some conditions can affect how the diaphragm works, such as hernias and nerve damage from surgery or an accident, which can cause difficulty breathing, heartburn, and pain in the chest and belly. Neuromuscular disorders such as amyotrophic lateral sclerosis (ALS) can also weaken the diaphragm.