Diaphragm is a dome-shaped, muscular and membranous structure that separates the thoracic (chest) and abdominal cavities in mammals; it is the principal muscle of respiration.

Contraction of the diaphragm increases the internal height of the thoracic cavity, thus lowering its internal pressure and causing inspiration of air. Relaxation of the diaphragm and the natural elasticity of lung tissue and thoracic cage produce expiration.