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The diaphragm is a thin muscular sheet that forms the floor of the chest cavity. The up and down movement of diaphragm causes change in the size of chest. If there is no diaphragm then it whole breathing mechanism stops and hence it can even lead to death. 
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Diaphragm is flat muscle which separates the chest cavity from abdominal cavity.It plays a vital role in respiration. During inspiration the diaphragm is pushed down so the space is occupied by air and during expiration the diaphragm comes to its original position.In this diaphragm is very important. Without it we can't breath. and it leads to death of the person