Bronchiole

A bronchiole is a small airway in the respiratory tract that branches from the bronchi and lacks cartilage in its walls, distinguishing it from the larger bronchi. Bronchioles are typically less than 1 mm in diameter and are lined by ciliated columnar epithelium that transitions to cuboidal epithelium in the smaller terminal bronchioles.

Key features:

• Structure: Bronchioles have walls composed primarily of smooth muscle and elastic fibers, without the cartilaginous support found in bronchi. This smooth muscle allows for regulation of airflow through bronchoconstriction and bronchodilation.

• Classification: Bronchioles are subdivided into:

  • Primary (lobular) bronchioles - initial branches from the segmental bronchi entering the pulmonary lobules

  • Terminal bronchioles - the smallest conducting airways, representing the final generation of the conducting zone

  • Respiratory bronchioles - arise from terminal bronchioles and contain scattered alveoli in their walls, marking the transition to the respiratory zone where gas exchange begins

• Function: In the conducting zone, bronchioles serve to distribute air to the gase-xchanging regions of the lung. Respiratory bronchioles participate in both air conduction and gas exchange.

• Clinical significance: Bronchioles are the primary site of airway obstruction in diseases such as asthma (smooth muscle spasm), bronchiolitis (inflammation, particularly in infants), and chronic obstructive pulmonary disease (small airway remodeling and mucus plugging).