Ventilation Occurs because the thoracic cavity changes volume Insipiration uses external intercostals and dlaphragm Expiration is passive at rest, but uses internal intercostals and abdominals during severe respiratory load Breathing rate is 10-20 breaths/minute at rest 40-45 at maximum exercise in adults 88 • Occurs because the thoracic cavity changes volume • Insipiration uses external intercostals and diaphragm • Expiration is passive at rest, but uses internal intercostals and abdominals during severe respiratory load • Breathing rate is 10-20 breaths / minute at rest, 40 - 45 at maximum exercise in adults Ventilation