Chapter 28: Structure and Function of the Pulmonary System

The respiratory pathway begins with conducting airways including the nasopharynx, oropharynx, larynx, trachea, and progressively smaller bronchi, which condition inspired air through warming, humidification, and filtration before it reaches the gas-exchange regions. The terminal structures of the respiratory tract, comprising respiratory bronchioles, alveolar ducts, and alveoli, facilitate oxygen and carbon dioxide transfer across the respiratory membrane. Specialized alveolar cells perform distinct functions: type I pneumocytes provide structural support for gas diffusion, type II pneumocytes synthesize surfactant to minimize alveolar surface tension, and resident macrophages provide immune surveillance. Multiple pulmonary defense mechanisms including nasal filtration, mucociliary clearance, irritant receptors, and alveolar immune cells protect against inhaled pathogens and particles. The dual blood supply to the lungs includes low-pressure pulmonary circulation dedicated to gas exchange and higher-pressure bronchial circulation supplying nutritive blood to airway tissues. Alveolar oxygen tension critically influences pulmonary vascular tone through hypoxic pulmonary vasoconstriction, a mechanism that can lead to pulmonary hypertension and secondary right heart disease when chronically activated. Ventilation is regulated by brainstem respiratory centers and chemosensitive neurons that monitor blood pH, carbon dioxide, and oxygen levels, while airway diameter is modulated through autonomic innervation. The work of breathing depends on lung compliance and elastic recoil properties, both of which can be compromised in disease states like acute respiratory distress syndrome or pulmonary fibrosis. Oxygen transport requires coordinated ventilation, diffusion across the alveolar-capillary membrane, and hemoglobin binding characterized by the oxyhemoglobin dissociation curve, which shifts in response to pH, carbon dioxide, temperature, and 2,3-diphosphoglycerate. Carbon dioxide transport is enhanced by the Haldane effect and depends on appropriate ventilation-perfusion matching. Age-related changes including decreased chest wall compliance, loss of elastic lung tissue, and reduced alveolar surface area significantly impact respiratory function in older adults.