bronchiole

Bronchioles are small airways in the lungs that branch from the bronchi and progressively divide to form the respiratory units of the lung. They are part of the lower respiratory tract and play a key role in directing air to the alveoli. The bronchioles include two main categories: terminal bronchioles, which are purely conducting airways, and respiratory bronchioles, which mark the beginning of the gas exchange region.

Anatomy and histology vary along the bronchial tree. Bronchioles lack cartilage and glands, and their walls contain smooth muscle that can regulate airway resistance. The epithelium changes from ciliated pseudostratified in larger airways to simple cuboidal in the terminal bronchioles. In terminal bronchioles, goblet cells are scarce or absent, and club cells (Clara cells) are present, secreting protective and detoxifying substances. In respiratory bronchioles, alveolar outgrowths begin along the walls, and the epithelium becomes thinner to accommodate gas exchange.

Physiology involves conducting air and controlling airflow. The smooth muscle layer can constrict or relax, influencing airway resistance and ventilation efficiency. The terminal bronchioles help clear mucus and particulates through mucociliary transport, while respiratory bronchioles transition toward the gas-exchanging regions of the lung, with alveoli beginning to be involved in gas exchange.

Clinical relevance includes diseases that affect small airways, such as bronchiolitis and asthma, where inflammation or bronchoconstriction narrows the bronchioles and impairs ventilation. Treatments often target smooth muscle relaxation to improve airflow, including bronchodilators and anti-inflammatory therapies.