I think the chest expands and relaxes because when the diaphragm muscle contracts, its dome shape is flattened and the chest cavity gets bigger. This change in the volume of the chest cavity pulls air into the lungs, and they expand. When the diaphragm relaxes, it moves back into its dome shape. When this happens, the chest cavity gets smaller and you breathe out.
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Answer:
I think the chest expands and relaxes because when the diaphragm muscle contracts, its dome shape is flattened and the chest cavity gets bigger. This change in the volume of the chest cavity pulls air into the lungs, and they expand. When the diaphragm relaxes, it moves back into its dome shape. When this happens, the chest cavity gets smaller and you breathe out.