Diaphragm separates the "thoracic cavity" from the "abdominal cavity. "During inspiration, the "phrenic" muscles of Diaphragm contracts and move downwards due to which thoracic cavity also move downwards and air can enter the lungs. Also, lungs rest on Diaphragm, while during expiration, phrenic muscles of Diaphragm relaxes and pushes the thoracic cavity upwards and causes the lungs to release out the air. If Diaphragm gets ruptured, the downward push on abdominal cavity will not be there, thus lungs will not be able to expand and the breathing process would be greatly hindered which may even lead to the death of the person.