Nose/nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.
At alveoli, oxygen diffuses into pulmonary capillary blood and carbon dioxide diffuses toward alveolar air according to partial-pressure gradients.
During quiet inspiration the diaphragm contracts and descends, increasing thoracic volume; external intercostals assist rib expansion. Quiet expiration is largely passive elastic recoil. Forced breathing recruits additional muscles.
Brainstem respiratory centers regulate breathing rhythm and respond strongly to carbon dioxide/pH-related signals, with oxygen also important under specific conditions.
Position clients for easy breathing. Acute respiratory distress, severe shortness of breath, cyanosis or concerning chest symptoms require medical response rather than massage. Respiratory infection may require postponement based on illness/transmission risk.
Be able to identify the major structures, state their functions, connect the system to relevant pathology and explain how the information changes massage safety or treatment planning.