Anatomy and Physiology II Practice Test

Normal inspiration is characterized primarily as an active process due to the involvement of muscular contractions. During inhalation, the diaphragm, which is the primary muscle of respiration, contracts and moves downward. This contraction increases the volume of the thoracic cavity, leading to a decrease in pressure within the lungs compared to the atmospheric pressure. As a result, air flows into the lungs to equalize the pressure.

Additionally, accessory muscles of inspiration, such as the external intercostals, may also assist in elevating the ribs and expanding the thoracic cavity during deeper or more forceful breathing. This coordinated muscular action fundamentally distinguishes normal inspiration as an active process.

In contrast, expiration, particularly at rest, is typically a passive process because it relies on the elastic recoil of the lung tissue and thoracic cavity rather than muscular contraction. However, this distinction does not apply to normal inspiration itself, reinforcing why it is best characterized as primarily active.