Chapter 5: Pulmonary System

The pulmonary system functions as the primary interface between atmospheric air and the body's circulatory system, enabling the critical exchange of oxygen and carbon dioxide while maintaining proper acid-base homeostasis. Embryologically, the respiratory tract develops through four distinct phases beginning in the fourth gestational week, with particular clinical significance attached to surfactant production by alveolar type II pneumocytes during the terminal sac phase; premature infants lacking adequate surfactant develop respiratory distress syndrome characterized by hyaline membrane formation and severely compromised gas exchange. Oxygen transport occurs predominantly through hemoglobin binding in red blood cells, while carbon dioxide circulates largely as bicarbonate ions, with the Bohr effect and Haldane effect working synergistically to facilitate oxygen unloading in metabolically active tissues and carbon dioxide release in pulmonary capillaries. The structural organization of airways relies on pseudostratified ciliated columnar epithelium and mucus-secreting goblet cells that comprise the mucociliary clearance mechanism, protecting deeper lung tissue from inhaled pathogens and particles. Gas exchange efficiency depends fundamentally on appropriate ventilation-to-perfusion matching, wherein regional pulmonary vasoconstriction redirects blood flow away from poorly ventilated alveoli, preventing physiologic shunting and dead space complications. Breathing mechanics involve the coordinated action of the diaphragm and intercostal muscles under the influence of negative intrapleural pressure, with expiration occurring passively through elastic recoil of lung tissues. The chapter addresses major pulmonary pathologies including obstructive diseases such as asthma, chronic obstructive pulmonary disease, and emphysema that obstruct airflow and elevate residual volumes, contrasting with restrictive diseases like pulmonary fibrosis that reduce overall lung compliance and capacity. Infectious processes range from bacterial pneumonia causing consolidation to mycobacterial tuberculosis producing characteristic granulomatous lesions and cavitation, while viral and opportunistic fungal infections present additional clinical challenges. Additional pathological conditions covered include pneumothorax with subsequent lung collapse, pulmonary embolism creating life-threatening dead space ventilation, acute respiratory distress syndrome involving diffuse alveolar damage, and primary lung malignancies with strong associations to smoking and environmental exposures.