To aid inhalation, both the diaphragm and intercostal muscles are used. By expanding the thoracic cavity, the action of these muscles creates a space with atmospheric pressure.

lower atmospheric pressure

Explanation:

During the inspiration process the diaphragm, and external intercostal muscles. The contraction of the diaphragm causes the increase in the size of the thoracic cavity this causes the contraction of the external intercostal muscles. As a result of this the ribs and the sternum get elevated.  

This causes the lungs to expand, and increasing the volume of the air passage. In this response the air pressure inside the lungs reduces below the air outside the body. Thus this facilitates the movement of the gases from a region of high pressure to low pressure, this way the air rushes into the lungs.

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