Final answer:
The airways subject to smooth muscle activity due to the lack of supporting cartilage are the bronchioles. These structures, which are less than 0.5 mm in diameter, rely on muscle contractions to regulate airflow and maintain patency.
Step-by-step explanation:
At which level of the airways does the lack of supporting cartilage mean that the airways are subject to the activity of the smooth muscles to remain patent?
The airways that lack supporting cartilage and therefore rely on the activity of smooth muscles to remain open are the bronchioles. In the respiratory system, air enters the lungs through the primary bronchi, which branch into secondary and tertiary bronchi, decreasing in diameter and eventually becoming bronchioles.
Beyond the trachea and larger bronchi, which have cartilage, the smaller bronchioles (typically less than 0.5 mm in diameter) lack cartilage and are composed of elastic fibers and smooth muscle.
The smooth muscle in the walls of these bronchioles responds to the autonomic nervous system, with the parasympathetic nervous system causing bronchoconstriction, and the sympathetic nervous system promoting bronchodilation. This regulation of airway size is essential for controlling airflow and for the respiratory system to function effectively, especially during activities such as exercise or in response to irritants.