Breathing is governed by an elegant integration of mechanics, gas exchange, and neural control — knowledge that underpins respiratory care and critical medicine. This text explains pulmonary ventilation, perfusion, diffusion, and the determinants of oxygenation and CO₂ elimination using concise physiology and clinical examples. It covers lung mechanics, airway resistance, alveolar–capillary interface, acid–base balance, and the control of breathing during health and disease. Practical chapters translate physiology to bedside tools: interpretation of blood gases, ventilator settings, and exercise testing. Clinical scenarios illustrate physiologic reasoning in COPD, ARDS, sleep-disordered breathing, and pulmonary vascular disease. Designed for students, respiratory therapists, and clinicians who need clear, applied respiratory physiology.