To create negative pressure in the thorax (chest cavity), the diaphragm contracts and moves _____________. This action expands the volume of the thorax, allowing air to rush into the lungs.

The diaphragm is a thin, tough muscle which separates the thorax and abdomen. The diaphragm is domed upward in its relaxed state, so when it contracts, the diaphragm flattens out, causing the volume of the thorax to increase. Since the pressure in the thorax is lower than atmospheric pressure, air will rush into the lungs and allow gas exchange. While the chest wall does expand, the diaphragm is the primary respiratory muscle. Chest muscles have a smaller role in regard to normal respiration.