bronchioles
Summary
Bronchioles are small airways (<1mm diameter) distal to bronchi that lack cartilage and glands but contain smooth muscle. Terminal bronchioles are the smallest purely conducting airways, while respiratory bronchioles participate in gas exchange and mark the beginning of the respiratory zone.
Detail
Bronchioles are subdivisions of the bronchial tree that differ structurally and functionally from larger bronchi. They are classified into terminal bronchioles (purely conducting, 0.5mm diameter) and respiratory bronchioles (0.2-0.5mm diameter with alveolar outpouchings for gas exchange). The walls contain smooth muscle controlled by autonomic innervation - beta-2 adrenergic stimulation causes bronchodilation while parasympathetic (muscarinic) stimulation causes bronchoconstriction. Unlike bronchi, bronchioles lack cartilage support and submucosal glands, making them more susceptible to collapse and obstruction. Clara cells (club cells) in terminal bronchioles secrete surfactant-like protein and detoxifying enzymes. Clinically, bronchioles are the primary site affected in asthma (reversible bronchoconstriction), chronic bronchitis (mucus hypersecretion and inflammation), and bronchiolitis (viral inflammation in infants). Their small diameter makes them the major site of airway resistance, and they're critical in conditions like COPD where structural changes lead to airflow limitation.
Sources
- Robbins Basic Pathology
- First Aid for the USMLE Step 1
- Ganong's Review of Medical Physiology
- West's Respiratory Physiology
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