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During quiet inspiration, the diaphragm
Astays still while only ribs lift the chest
Bcontracts and flattens, expanding the thorax
Crelaxes and moves upward into the thorax
Dis pulled in by abdominal muscles only
Answer & Solution
Correct answer: B. contracts and flattens, expanding the thorax
The diaphragm is the principal muscle of inspiration. It contracts and flattens, moving downward, which increases the vertical dimension of the thoracic cavity. The external intercostals lift the ribs to add the side-to-side expansion. Both together drop intra-pulmonary pressure below atmospheric.
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