The diaphragm is a large sheet of muscle that separates the chest cavity (containing the lungs and heart) from the abdominal cavity (containing the digestive organs).
The diaphragm also serves as a muscle to help draw air into the lungs as a person breathes. It contracts to expand the lungs when breathing in (inhaling) and relaxes when breathing out (exhaling). If the diaphragm is not able to move as it should, breathing may become difficult.
WebMD Medical Reference from Healthwise