Chapter 17: Mechanics of Breathing

Mechanics of Breathing examines the anatomical structure of the respiratory system, distinguishing between the upper and lower respiratory tracts, and highlights the critical function of the pleural sacs and fluid in coupling the lungs to the thoracic cage. The summary applies fundamental physics to physiology, using Dalton’s Law to explain partial pressures in gas mixtures and Boyle’s Law to demonstrate how the inverse relationship between pressure and volume drives pulmonary ventilation. It breaks down the respiratory cycle, detailing how the contraction of the diaphragm and intercostal muscles creates pressure gradients for active inspiration, while expiration remains largely passive due to elastic recoil unless forced by accessory muscles. The text also covers pulmonary function tests using spirometry, defining essential lung volumes and capacities such as tidal volume, inspiratory reserve volume, residual volume, and vital capacity. Furthermore, it discusses the physical factors affecting breathing work, including lung compliance and elastance, and how these properties change in restrictive and obstructive diseases like pulmonary fibrosis and emphysema. The vital role of surfactant, secreted by Type II alveolar cells, is explained in the context of reducing surface tension and stabilizing alveoli according to the Law of LaPlace. Finally, the chapter addresses airway resistance, the distinction between total pulmonary ventilation and alveolar ventilation due to anatomic dead space, and the local control mechanisms that match airflow and blood flow (perfusion) to maximize gas exchange efficiency.