The Respiratory System

Both statements are scientifically accurate. During forced expiration, the internal intercostals and abdominal muscles contract to push air out. Simultaneously, contraction of the diaphragm (controlled by the phrenic nerve) increases the thoracic cavity’s volume antero-posteriorly during inspiration.

The respiratory system involves a branching network of airways. Trachea branches into bronchi, which further branch into smaller bronchioles. The bronchioles terminate in clusters of tiny, thin-walled air sacs called alveoli (singular: alveolus). These alveoli are the primary sites of gas exchange (oxygen and carbon dioxide) between the air in the lungs and the blood in the surrounding capillaries. "Air spaces" is a general term. Bronchi and bronchioles are conducting airways. The functional units for gas exchange, which appear as clusters of sacs, are the alveoli.