Quiet expiration at rest is described as a passive process because
- A. no air actually leaves the lungs
- B. the internal intercostal muscles contract forcefully
- C. it results from the elastic recoil of the lungs and relaxation of the inspiratory muscles
- D. the diaphragm contracts to push air out
Explanation
When the diaphragm and external intercostals simply relax, the stretched elastic tissue of the lungs recoils and the thoracic volume falls without any muscle actively pulling it down, so no extra energy is spent. Forced expiration during exercise or coughing does use the internal intercostals and the abdominal muscles, which is the active case. Loss of this elasticity in emphysema makes breathing out difficult.
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