Final answer:
The narrowest segment of the human airway is the respiratory bronchioles, which have a diameter smaller than 0.5 mm and lack cartilage, relying on air to maintain their shape.
Step-by-step explanation:
The narrowest segment of the human airway is the bronchiole, specifically the respiratory bronchioles that are less than 0.5 mm in diameter. As the airways make their way through the lungs, they divide into smaller and smaller passages, starting with the primary bronchi, then to secondary and tertiary bronchi, and finally into bronchioles. In conditions such as asthma, these airways can become inflamed and narrowed, leading to breathing difficulties.
In comparison to the trachea, which is approximately 2.5 cm wide, and the primary bronchi, the bronchioles are significantly smaller and lack cartilage in their walls, making them susceptible to collapse if not supported by inhaled air. The extremely small diameter of respiratory bronchioles represents the narrowest air passages in the respiratory tract. Their reduced size, along with an increase in smooth muscle, allows for greater control over airway resistance and airflow, especially during an asthma attack when airways constrict and may be clogged with mucus.