The movement of air into and out of the lungs requires a pressure gradient.

A. Respiratory bronchioles: Respiratory bronchioles are part of the respiratory zone, not the conducting zone. They have alveoli budding from their walls and participate in gas exchange, marking the transition from air conduction to respiration.

B. Alveolar ducts: Alveolar ducts are elongated passages lined with alveoli, located within the respiratory zone. They conduct air to alveolar sacs and are primarily involved in gas exchange rather than simple conduction of air.

C. Terminal bronchioles: Terminal bronchioles are the smallest airways of the conducting zone. They branch from the larger bronchioles, lack alveoli, and primarily serve to transport air to the respiratory bronchioles while maintaining airway resistance and airflow distribution.

D. Segmental bronchi: Segmental (tertiary) bronchi are larger branches within the conducting zone that supply bronchopulmonary segments. They are significantly larger than terminal bronchioles and are involved in bulk air conduction rather than fine distribution.

E. Alveolar sacs: Alveolar sacs are clusters of alveoli at the ends of alveolar ducts within the respiratory zone. They are the primary sites of gas exchange and are beyond the conducting zone, making them functionally distinct from terminal bronchioles.

A. Accessory muscles: Accessory muscles of respiration, including the sternocleidomastoid, scalene, and pectoralis minor, are recruited when increased ventilatory effort is needed, such as during exercise, respiratory distress, or lung diseases like COPD. They assist in elevating the thoracic cage, increasing lung expansion, and enhancing air movement into the lungs beyond what the primary muscles can achieve.

B. Vocal cords: Vocal cords are structures within the larynx that produce sound and regulate airflow through the glottis. They do not participate in the mechanics of breathing or provide supplemental respiratory power.

C. Bronchioles: Bronchioles are small airways that conduct air to the alveoli and regulate airflow through smooth muscle constriction or dilation. They are not muscles and do not contribute mechanical force to ventilation.

D. Primary muscles: The primary muscles of respiration, mainly the diaphragm and intercostal muscles, perform the bulk of normal, restful breathing. While essential for ventilation, they are supplemented by accessory muscles only when additional respiratory effort is required.