The diaphragm separates the abdominal cavity from the chest cavity. Pulling the diaphragm down (it’s a muscle) pushes out the abdomen (stomach) and increases the size of the chest cavity, filling the lungs with air. The other way to breath in is to use the muscles between the ribs (intercostal muscles) to increase the size of the rib cage and thus the size of the chest cavity and the volume of lungs.
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