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Pulmonary ventilation or breathing has two phases. Inspiration or inhalation moves air into the lungs and expiration or exhalation moves air out of the lungs. The lungs are enclosed within the thoracic cavity. Thus changes in the shape and size of the thoracic cavity result in changes in the air pressure within that cavity and in the lungs. This difference in air pressure causes the movement of air into the lungs. Air moves from an area where pressure is high to an area where pressure is low. Respiratory muscles are responsible for the changes in the shape of the thoracic cavity that cause the air movements involved in breathing.

The abdominal diaphragm is (or should be) the major muscle driving respiration.

Source(s):Structure and function of the body

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