Figure 31.13: Resting Human Ventilation When a person is at rest, inhalation (A) is an active process driven by contraction of the diaphragm and some of the intercostal muscles. However, exhalation (B) is a passive process that occurs by elastic recoil; the thoracic cavity and lungs elastically return to their relaxed volume as soon as muscle contractions stop maintaining them in an expanded state.