Chapter 22: The Respiratory System

The respiratory tract divides functionally into the conducting zone, which includes the nose, nasal cavity, pharynx, larynx, trachea, and bronchi that filter, warm, and humidify incoming air, and the respiratory zone, comprising the bronchioles and alveoli where actual gas exchange occurs. The lungs occupy the thoracic cavity and are enclosed by pleural membranes that facilitate movement during ventilation. Pulmonary ventilation, governed by pressure gradients explained through Boyle's Law, occurs when the diaphragm contracts to increase thoracic volume during inspiration and relaxes during expiration. The chapter connects breathing mechanics to measurable lung volumes and capacities—including tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume—which clinicians assess using spirometry to evaluate respiratory function. External respiration involves oxygen and carbon dioxide diffusion across the alveolar membrane following concentration gradients, while internal respiration describes gas exchange between blood and tissues. Gas transport relies on hemoglobin binding oxygen in red blood cells, while carbon dioxide travels through multiple pathways including plasma dissolution, hemoglobin binding, and bicarbonate ion formation catalyzed by carbonic anhydrase. Respiratory control emerges from neural centers in the medulla and pons that receive chemoreceptor input monitoring arterial carbon dioxide, oxygen, and pH levels to adjust breathing rate and depth accordingly. The chapter concludes by examining pathological conditions including chronic obstructive pulmonary disease, emphysema, chronic bronchitis, asthma, tuberculosis, and lung cancer, demonstrating how structural or functional impairment of any respiratory component disrupts gas exchange, compromises cellular metabolism, and threatens physiological stability.