bronchioles

Bronchioles are small airways in the lungs that branch from the bronchi and form part of the lower respiratory tract. They measure less than 1 mm in diameter and lack cartilage, glands, and goblet cells. The bronchioles are divided into terminal bronchioles, which constitute the last portion of the conducting airways, and respiratory bronchioles, which begin to participate in gas exchange as they give rise to alveolar ducts.

Histologically, the walls of bronchioles are supported by smooth muscle and a sparse lamina propria. The epithelium transitions from pseudostratified ciliated columnar in larger bronchi to simple tall cuboidal or squamous in the distal bronchioles. Goblet cells are scarce toward the distal end, and club cells (Clara cells) appear in the terminal bronchioles, secreting detoxifying enzymes and contributing to airway repair. Cilia are present proximally and diminish distally, reducing mucociliary clearance as you move toward the respiratory bronchioles.

Functionally, smooth muscle in the bronchiolar walls allows dynamic regulation of airway caliber in response to neural and chemical stimuli, thereby influencing airway resistance and ventilation distribution. The mucociliary apparatus clears inhaled particles; the distal bronchioles contribute to gas exchange through the respiratory bronchioles and subsequent alveolar ducts.

Clinical significance includes bronchiolitis, an inflammation of the bronchioles often caused by viral infections in infants. Bronchiolitis obliterans is a more severe fibrotic condition that can follow injury. In asthma and chronic obstructive pulmonary disease, bronchiolar smooth muscle hyperreactivity and remodeling can reduce airflow and contribute to symptoms.

Bronchioles arise from the conducting airway tree during lung development. They receive blood supply from the bronchial arteries, branches of the systemic circulation, and drain to the pulmonary and bronchial circulations via capillaries; lymphatics accompany the airways.