Final answer:
The bronchial tubes undergo structural changes as they become finer, transitioning from cartilage to elastic fibers, increasing in smooth muscle, and losing cartilage in the smallest bronchioles, which affects their role in airflow regulation and ends in alveolar ducts and alveoli for gas exchange.
Step-by-step explanation:
As the respiratory tubes in the lungs branch into finer structures, they undergo significant changes in structure before ending in the alveoli. The main changes include the transition from cartilage-supported bronchi to the elastic-fiber-supported bronchioles, an increase in the amount of smooth muscle, and the lack of cartilage in the smallest bronchioles. The process begins as the primary bronchi branch into secondary and tertiary bronchi, each decreasing in diameter.
As they continue to split, these tubes become bronchioles, which are less than 1 mm in diameter and eventually lead to terminal bronchioles. The terminal bronchioles then give way to gas exchange structures in the lungs. Unlike the larger bronchi, bronchioles do not contain cartilage; instead, they have elastic fibers and more smooth muscle. This muscle allows the diameter of the bronchioles to change, regulating airflow in the lungs. At their ends, bronchioles branch into alveolar ducts, which conclude in clusters of alveoli.