Respiratory System
The airway and lungs — nose/mouth, pharynx, larynx, trachea, bronchi, bronchioles, alveoli — where oxygen enters the blood and carbon dioxide leaves by diffusion across the alveolar walls.
Air follows the pressure gradient: when the diaphragm and rib muscles expand the chest cavity, pressure inside drops below atmospheric pressure and air flows in. Rising blood carbon dioxide — not low oxygen — is the main trigger that speeds breathing.