During inspiration the diaphragm
- A. contracts and flattens, increasing the volume of the thorax
- B. relaxes and domes upwards, increasing the volume of the thorax
- C. contracts and domes upwards, decreasing the volume of the thorax
- D. does not move at all
Explanation
A flattening diaphragm and the external intercostals swinging the ribs up and out together enlarge the thoracic cavity, so the pressure inside the lungs falls below atmospheric and air flows in. The diaphragm is a muscle, so contraction shortens and flattens the dome rather than raising it. Air movement is always the result of a pressure difference created by these volume changes.
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