lowerairway

The lower airway refers to the portion of the respiratory tract from the trachea to the alveoli. It includes the trachea, main bronchi, lobar and segmental bronchi, bronchioles (terminal and respiratory), and the alveolar ducts and alveoli. The lower airway conducts air to the gas-exchange region, conditions it by warming and humidifying, filters particles, and participates in immune defense and mucociliary clearance.

Anatomy and histology: The trachea bifurcates into right and left main bronchi, which branch into progressively smaller airways. Cartilage supports the trachea and large bronchi; cartilage becomes absent in most distal bronchioles. The airway wall contains smooth muscle and elastic tissue; bronchiole diameter can change with autonomic input. The respiratory zone begins at the respiratory bronchioles, with gas exchange in the alveolar ducts and alveoli, lined by type I and II pneumocytes; alveolar macrophages provide defense.

Physiology: The lower airway is a conduit and conditioning system, filtering, warming, and humidifying inspired air, while maintaining mucociliary clearance. Gas exchange occurs in alveoli, aided by surfactant. Airway smooth muscle tone regulates resistance; inflammation or constriction (as in asthma or COPD) narrows airways and impairs ventilation.

Clinical significance: Diseases of the lower airway include acute and chronic bronchitis, asthma, COPD, bronchiectasis, bronchiolitis, pneumonia, and other infections. Evaluation uses history, spirometry showing obstructive patterns, imaging, and sometimes bronchoscopy. Treatments focus on reducing inflammation and bronchoconstriction, treating infections, and supporting ventilation.